intersection design
TRANSCRIPT
CONTENTS
I. Introduction
1.0 General
1.1 Intersection Problems
1.2 Intersection Types
1.3 Selection of Solution
1.4 Principles of Intersection Design
1.5 Data Collection
II. Design of at-grade intersections
2.0 General
2.1 Design Elements
2.2 Channelization
2.3 Intersection Design Using Templates
III. Rotaries and Mini-Roundabouts
3.1 Rotary Intersections
3.2 I R C Guidelines for selecting Rotary Intersection
3.3 Design of Rotary
3.4 Capacity of Rotary Intersection
3.5 Mini – roundabouts.
IV. Traffic Signals
4.0 General
4.1 Benefits and Drawbacks
4.2 Warrants for Signal Installation
4.3 Basic Features of a Traffic Signal
4.4 Definitions of Terms
4.5 Design of Isolated Signal Timing
4.6 Signal Co-ordination
V. Intersections with grade separations
5.0 General
5.1 Warrants
5.2 Types of Grade Separations
5.3 General Guidelines of Design
5.4 Geometric Design
VI. Planning in relation to traffic safety
6.1 Treatment for Pedestrians
6.2 Treatment for Cyclists
6.3 Use of Traffic Signs
6.4 Use of Markings
6.5 One-way streets
6.6 Re routing of Right Turns
6.7 Safety Related Improvements.
INTRODUCTION
1.0 GENERAL
Intersections are an inevitable part of any street system. A road intersection can be defined as the
general area where two or more roads join or cross, including the roadway and roadside facilities for
traffic movement.
Intersection is the place shared by vehicles from all the approaches and permit a change in travel
route. Therefore, the intersection needs to be designed with great care so that the driving through the
intersection is a simple and safe task. The basic forms of intersections are given in Fig. 1.1. The actual
traffic movement and its sequence at these intersections can be handled by various means, viz.,
application of channelization, traffic control devices, or grade-separation. The main factors that influence
the choice of a particular intersection layout are cost, capacity, delay to vehicles, aesthetics and most
important safety.
1.1 INTERSECTION PROBLEMS
The present system of street network operates in a most complex manner. It becomes still
complex when elements of the system come together at intersections. It is further complicated by the
change in nature and composition of traffic. Normally it is the intersection that fails first to handle the
traffic demand. This is reflected by the increased congestion, delays, accident situation and in nutshell
reduced level of service. The intersection thus becomes a bottleneck and unless improvements are carried
out the system gets disturbed.
1.2 INTERSECTION TYPES
The improvements are based on the nature and degree of complications involved and can be by
way of spot improvements or system improvements. The spot improvements can be divided into minor
and major improvements. The minor improvements can be done at relatively less cost and can be
accomplished by traffic signs, Channelisation, provision of Right and Left turn lanes, parking controls,
traffic signals, restriction of turning movements, etc.
Major improvements generally require the acquisition of additional land and considerable amount
of money. Some of these measures are
- Reconstruction, widening, straightening
- Vehicular/pedestrian grade separations.
The intersections based on the types of controls can be classified as follows
Uncontrolled Intersections
Intersections with sign control
Intersections with channelization
Rotary and mini roundabouts
Intersections with signal control
Grade separated Intersections.
Generally the first five categories are known as at grade intersections. Fig. 1.2 illustrates these
intersections.
1.3 SELECTION OF SOLUTION
Although it is difficult to set a definite procedure for selecting an appropriate solution, the
diagrams recommended in U.K. and rest while USSR may be of some help in decision making. These
diagrams, presented in fig. 1.3 & 1.4 respectively give the sphere of application of different design
solutions at intersections. The diagrams can be used if volume of traffic on the two roads of the
intersection are known.
For example, let the major road volume, V1, be 3000 veh/day. The point corresponding to these
flow values fall in zone 3, which suggests channelising islands on both the roads, speed change lanes and
road markings, etc.
1.4 PRINCIPLES OF INTERSECTION DESIGN
In practice, the development of intersection layouts is very much influenced by governmental
recommendations and standards, as well as of course the previous experience and knowledge of the
designer. Underlying most recommendations and standards relating to safety are the following basic
design principles with which the designer should be familiar.
1.4.1 Minimize the carriage way area of conflict:
Excessive intersection area causes driver confusion and potentially dangerous movements. Large
areas are inherent in skewed and multiple approach intersections. When intersections have excessive
areas of conflict, channelization should be employed. Fig. 1.5 (a) shows how the large area, in which
possible conflicts can take place at a conventional y- intersection, can be modified by placing the
elongated islands appropriately in the intersection.
1.4.2. Control the angle of potential vehicle conflict:
Most authorities are of the opinion that the roads intersecting at about 90 degrees are the most
desirable, and when the angle of intersection is reduced to less than 60 degrees, the accident hazard is
considerably increased. Besides reducing the potential accident severity, a right angle arrangement also
provides the entering motorist with favorable conditions for estimating the speed and position of traffic on
the through road, while the time and distance required by the entering vehicle to cross the conflicting
traffic stream are both reduced.
Fig. 1.5(b) shows the very dangerous situation where the traffic streams on two opposing
roadways meet at about a 30 degree angle. There are obvious risks here; increased chance of conflict due
to the difficulty in seeing to left and high risk of fatality due to high relative speed.
1.4.3. Control relative speed:
This applies particularly to merging traffic streams. Relative speed is the rate of convergence of
vehicles in intersection. A small difference in the speeds of intersecting vehicles and a small angle
between converging paths allow intersection vehicular flow to operate continuously. Also, in the event of
an accident occurring, the relative damage caused should be significantly less. Low relative speeds
require elimination of both speed differences and large angles between intersection flows through design.
High relative speeds result in interrupted flow and so traffic should be controlled by traffic control
devices.
Fig. 1.5(c) shows how a flat intersection angle enables merging vehicle to accelerate easily and
run parallel to a major traffic stream until a safe gap is available.
1.4.4 Reduce the number of conflict points:
The number of conflict points among vehicular movements increases significantly as the number
of intersection legs increases. For example, an intersection with four two way legs has 24 conflict points,
but an intersection with six two-way legs has 172 conflict points. The number of conflict points can be
reduced by limiting the number of legs, by restricting certain movements or by changing the layout.
Fig. 1.5(h) shows how the number of conflict points at a right angled intersection can be reduced
by declaring one of the roads as one way.
1.4.5. Provide protection for vehicles leaving or crossing the main traffic:
Vehicles leaving a main road normally decelerate and, if necessary, stop and so the space
provided, separate from that used by through traffic, for these vehicles minimizes the potential for rear
end collisions. Further, vehicles crossing high volume traffic streams should be able to complete the
manoeuvre in stages. Shadowed area in Fig. 1.5 (f) provides decelerating and /or storage space for
vehicles waiting to turn to the right and cross the opposing traffic stream. This layout also has the safety
feature that a motorist attempting to cross the main highway only has to be concerned with taking one
decision and obtaining one safe gap in each traffic stream at a time.
1.4.6. Clearly define the travel paths to be followed:
The objective of this principle is to minimize the free choice of the driver in respect of the path to
be followed. Fig. 1.5(a) illustrates this principle. Fig. 1.5(g) shows another example, where a prohibited
movement is discouraged. The triangular island is deliberately shaped to encourage motorists entering a
one-way traffic stream on the main road to turn left only and follow correct direction of travel.
1.4.7. Favour predominant or high-speed traffic flows:
Priority of movement should normally be given to the major traffic movement. This results in
increased capacity of the intersection and improved safety. For example, drivers who travel for long,
uninterrupted distances at high speed will often be slow to react to a sudden change in alignment or to the
entry of a high-speed vehicle from a minor road.
Fig. 1.5(b) illustrates how the minor traffic movement is deflected so that it is required to stop
before it enters the major traffic stream.
1.4.8. Control high speed:
Traffic carrying cutting manoeuvres should do so at low speeds. Fig. 1.5(e) will illustrates how
the speed of entering minor road traffic stream can be reduced by funnelling traffic into a gradually
narrowing opening, as well as causing the motorist to feel hemmed in, so that his or her response is to
reduce speed. Funnelling also prevents overtaking from taking place in the potential conflict area.
1.4.9. Avoid multiple and compound merging & diverging manoeuvres:
Multiple merging or diverging requires complex driver decisions and creates additional conflicts.
This multiple or compound manoeuvres, happen when intersection manoeuvre areas are too close
together or when they overlap. These manoeuvres can be eliminated by providing drivers with sufficient
time (or distance) between successive manoeuvres so that they can cope up with the traffic situation.
1.4.10. Segregate non homogeneous flows:
Separate lanes should be provided at intersections when there are appreciable volumes of traffic
travelling at different speeds.
1.4.11. Co-ordinates the design and control:
Intersection design and controls should be properly coordinated. For example, if the intersection
layout permits high relative speeds then traffic controls such as top signs or traffic signals should be
provided to ensure safe movement of vehicle.
1.4.12. Use the highest feasible crossing method:
Vehicle crossing manoeuvres can be accomplished in four ways.
(i) Uncontrolled crossing at-grade.
(ii) Traffic sign or signal controlled crossing at-grade.
(iii) Weaving
(iv) Grade separation.
In general, both operational efficiency and construction cost increase in this order. The highest
type of facility should be used consistent with the numbers and the types of vehicles using the
intersection.
1.4.13. Consider the needs of pedestrians and cyclists:
For example, when there are pedestrians crossing the wide streets, refuge islands should be
provided so that the crossing of the street can be carried in separate movements.
1.4.14. Provide advance warning of change:
Motorists should never be suddenly faced with the unexpected. Thus advance signing of
intersections ahead is particularly important on minor roads.
1.4.15. Illuminate intersections for night-time use:
Priority for lightning should always be given to intersections with heavy pedestrian flows, heavy
vehicular flows and where raised channelization islands are introduced into, what might otherwise be
considered the natural vehicle paths.
1.5. DATA REQUIREMENTS
The following data is essential for the design of intersections.
A base plan of the intersection site showing the existing roads and salient features like road
land boundary, location of structures, trees, service lane etc. The details should be shown for
a length of 200 meters for each road merging at the intersections. If the terrain is not plain
and/or there is too much of variation of ground level at the site, contours at 0.5 metre interval
should also be marked on the base plan and additional longitudinal sections along the centre
line of intersecting roads be given.
Traffic data, such as volume on each leg mode wise, turning movements from each leg
modewise approach speeds.
Accident data at intersection should be collected and should be reduced to diagrammatic form.
This information will indicate the necessary engineering measures required at the
intersections.
The details of physical features of the intersection area and surroundings can be obtained by the
standard methods of land surveying, like plain table surveying and leveling or by making use of aerial
photographs of the area.
The traffic volume data by modewise for each of turning movements and from each of the legs
can be obtained by conducting traffic volume counts. Traffic volume counts can be done by manual
methods or mechanical methods.
1.5.1 Manual method
The manual method employs a field team to record traffic volume on the prescribed data sheets.
A typical format for collection of traffic volume data is given in Table 1.1. By this method, it is possible
to obtain data such as vehicle classification and turning movements. However it is not practicable to have
manual count for all the 24 hours of the day and on all days round the year.
1.5.2 Mechanical Methods
This method employs the use of mechanical devices, which may be either fixed (permanent) type
or portable type. These devices automatically record as and when a vehicle crosses a section of the road
at which these devices are installed. These devices consists of two parts, viz., the counters and the
sensors. Different types of sensors, like pneumatic tube, piezo-electric cells, magnetic loops, radar
detectors, etc., are available. The counter or recorder is activated by the impulse caused by the passage of
vehicle on the detector. The main advantage of mechanical counter is that it can work continuously for
the desired period. The main drawback of the mechanical counter is that it is not possible to get the
traffic volume by type of vehicle and turning movement generally.
Daily volume counts are taken for selecting suitable control method and peak-hour counts are
taken for design of traffic signals. The traffic volume data needed for intersection design is presented in
the form of Intersection Flow Diagrams, as shown in Fig. 1.6. These diagrams give the volume for all
movements through the intersections.
DESIGN OF AT-GRADE INTERSECTIONS
2.0 General
As with the highway, a number of geometric design elements are needed to be considered at an
intersection to ensure that its final layout is both functional and safe. However, the intersections require
a simultaneous treatment of all these elements on two or more traffic streams.
2.1 Design Elements
The intersection design elements are:
Sight distance or visibility
Turning radii
Auxiliary lanes
Tapers
Carriage way widths
Gaps in the central reserve
Channelization
The design of these elements is influenced by the dimensions of the vehicles that use the facility.
As a wide variety of vehicles are in use in our country, it will not be possible to design the facilities so
that the requirements of all the vehicles are met. Therefore, the dimensions of the Design Vehicle are
used in geometric design. The Design Vehicle is the one having largest dimensions and largest turning
radius of all the vehicles in that class. The dimension of the Design Vehicles of different classes of
vehicles are given in Table 2.1
Table 2.1 Dimensions & Turning Radii of Design Vehicles
S.No Vehicle Type
Overall
Width
(m)
Overall Length
(m)
Overhang Turning
Radius
(m)
Front
(m)
Rear
(m)
1. Passenger Car(p) 1.4 –2.1 3 –5.74 0.9 1.5 7.3
2. Single Unit Truck (S.U.) 2.58 9 1.2 1.8 12.8
3. Semi Trailer and Single unit Bus
(WB – 12 m)
2.58 15.0 1.2 1.8 12.2
4. Large Semi- Trailer (WB – 15 m) 2.58 16.7 0.9 0.6 13.71
5. Large Semi- Truck Trailer (WB-
18m)
2.58 19.7 0.6 0.9 18.2
2.1.1 Visibility
The layout of the intersection should be visible, clear, simple and understandable by the driver.
For example, at a major/minor priority intersection it should be possible to distinguish between a major
route and minor rout. On minor route, the design should warn the drivers about the manoeuvres awaiting
them and permit the reduction of speed turning vehicles. Also the minor road drivers should have
unobstructed visibility to the left and right along main road for a distance depending on major road traffic
speed. The major road traffic should be provided with the desirable minimum stopping sight distance on
the approaches to and through the junction. Parking should not be permitted in the sight triangle, as this
will obstruct visibility. Whenever, it is necessary to erect essential traffic signs within such visibility
areas , great care should be taken to minimize their obstructive effect. Sight distance requirements are
illustrated in fig 2.1 and 2.2 and values are given in Tables 2.2 and 2.3.
Table 2.2 Safe Stopping Sight Distance of Intersections
SpeedSafe stopping Sight Distance
(m)
20
25
30
40
50
60
65
80
100
20
25
30
45
60
80
90
130
180
Table 2.3 Visibility Distance on Major Roads
Design Speed
(km/h)
Minimum Visibility Distance
along major road (m)
100
80
65
50
270
180
145
110
2.1.2 Turning radii
The turning radii or corner radii is governed by many factors. The two considerations, viz., the
land area must be minimized and the pedestrian must cross the street safely, dictate the maximum
possible radii. The intersection design should enable the selected design vehicles to negotiate the
intersections, without requiring backing up. The minimum radius depends on the design speed of
operation and turning path of vehicles. Table 2.4 provides the turning radii information for various
turning speeds. For example, residential streets, which have low truck volumes, can be designed with 4-8
meters curb radius while collector and arterial streets should have a minimum radius of 12 m.
Table 2.4 Design Speed & Minimum Radii
Design Speed
Km/hr
Minimum inner radii
(m)
18.5
15
20
30
40
50
75
100
125
150
Straight
18
23
27
32
37
41
50
57
62
64
__
Turning, particularly for larger vehicles and angles of turn greater than 90O can be made easier
and safer by providing compound curves on the corners instead of circular curves. This will reduce the
rear wheels hitting the curb. Three centered compound curve is widely used and it consists of three
smoothly connected simple curves of different radii.
Design of Three-centered compound curve
Design of 3-centered curve require calculations of offsets and co-ordinates and centre of curves.
This can be derived mathematically as presented below:
Let R1 = First Radius X1 = First Offset
R2 = Middle Radius X2 = Second offset
R3 = Third Radius = Angle of turn
Fig 2.3(a) shows the curve and its centres and Fig. 2.3(b) is
Table 2.5 Centered Compound Curve (symmetrical) Without Channelising Island
Design VehicleAngle of
TurnRadii (Metres) Offset
Co-ordinates of
Centre of Middle
arc in metres
WB – 15.0 45o 61.0 – 30.5-61.0 0.91 30.955
WB – 15.0 60o 61.0-22.9-61.0 1.68 23.949
P
SU
WB 12.0
WB 15-0
75o
30.5-7.6-30.5
36.6-13.7-36.6
36.6-13.7-36.6
45.7-15.2-45.7
0.61
0.61
1.52
1.83
8.055
14.082
14.651
16.421
P
SU
WB-12.0
WB-15.0
90o
30.5-6.1-30.5
36.6-12.2-36.6
36.6-12.2-36.6
54.9-18.3-54.9
0.76
0.61
1.52
1.83
6.708
12.607
13.213
19.520
P
SU
WB-12.0
105o
30.5-6.1-30.5
30.5-10.7-30.5
30.5-10.0-30.7
0.76
0.91
1.52
6.700
11.291
11.687
P
SU
WB-12.0
WB-15.0
120o
30.5-6.1-30.5
30.5-9.1-30.5
36.6-9.1-36.6
54.9-12.2-54.9
0.76
0.91
1.83
2.59
6.708
9.738
10.475
14.214
P
SU
WB-12.0
WB-15.0
135o
30.5-6.1-30.5
30.5-9.1-30.5
36.6-9.1-36.6
48.8-10.7-48.8
0.45
1.22
1.98
2.74
6.460
9.956
10.588
12.839
P
SU
WB-12.0
WB-15.0
150o
20.9-5.9-20.0
30.5-9.1-30.5
30.5-9.1-30.5
48.8-10.7-48.8
0.76
1.22
1.83
2.13
6.445
9.956
10.384
12.363
P
SU
WB-12.0
WB-15.0
180o
(U Tum)
15.2-4.6-15.2
30.5-9.1-30.5
30.5-6.1-39.6
39.6-7.6-39.6
0.15
1.37
2.90
2.90
4.705
10.061
8.420
9.943
Table 2.6 3-Centred Compound Curve (Asymmetrical) Without Channelising Island
Design
Vehicle
Angle of
Turn
Radii
Metres
Offset
metre
Co-ordinates of centre
of middle arc
WB-150 60o 61.0-0.22.9-83.8 0.61-1.83 23.281 24.230
WB-12.0
WB-15.075o
36.6-13.7-61.0
45.7-15.2-68.6
0.61-1.98
0.61-3.05
14.082
15.607
15.235
17.574
WB-12.0
WB-15.090o
36.6-12.2-61.0
36.6-12.2-61.0
0.61-1.83
0.61-3.05
12.607
12.607
13.664
14.640
WB-12.0
WB-15.0105o
30.5-10.7-61.0
45.57-12.2-64.0
0.61-2.44
0.61-3.05
11.096
12.647
12.712
14.669
WB-12.0
WB-15.0120o
30.5-9.1-54.9
45.7-10.7-67.1
0.61-2.74
0.61-3.66
9.523
11.167
9.069
13.776
WB-12.0
WB-15.0135o
30.5-7.6-54.9
39.6-9.1-56.4
0.91-3.05
0.91-4.27
8.283
9.301
10.228
12.681
WB-12.0
WB-15.0150o
27.4-7.6-48.8
36.6-9.1-54.9
0.91-3.35
0.91-4.27
8.258
9.784
10.428
12.662
WB-12.0
WB-15.0180o
25.9-6.1-45.7
30.5-7.6-54.9
1.83-3.96
1.83-3.96
7.499
8.974
9.531
11.012
Table 2.7 3-Centred Compound Curve (Symmetrical)-With Channelising Island
Angle of
Turn
Radii
Metres
Offset
Metres
Co-ordinate of centre
of middle arc (in metres)
SU
WB-15.075o
45.7-22.9-45.7
54.9-27.4-54.9
1.52
1.07
23.658
27.936
SU
WB-15.090o
45.7-15.2-45.7
54.9-19.6-54.9
1.52
1.03
16.214
20.262
SU
WB-15.0105o
30.5-10.7-30.5
54.9-13.7-54.9
1.52
2.44
11.687
15.531
SU
WB-15.0120o
30.5-10.7-30.5
54.9-13.7-54.2
0.91
2.59
9.738
14.207
SU
WB-15.0135o
30.5-9.1-30.5
48.8-10.7-48.8
1.22
2.74
9.956
12.839
SU
WB-15.0150o
30.5-9.1-30.5
48.8-10.7-48.8
1.22
2.13
9.956
12.363
An enlarged plan of the required portion to make illustration more clear.
Determination of the position of ‘O’, the centre of radius R2’ the middle radius:
P1Q is an arc drawn with radius R1’ QR with radius R2 and RS with radius R3 having offset X1 at point Q
and offset X2 at point R.
P1O1 P2O2 or O2D
P1O1Q = PO2Q =Q1
QE = C1P1 =X1 =First offset
C1P1 = R1 – R1 Cos 1
or x1 = R1 – R1 Cos 1
or Cos 1 = (1-1
1
R
x) =1-
souterRadiu
offset
O2D = O2 C2 + C2D
= R2 Cos 1 + x1
O2D = R2 (1-1
1
R
x) + x1
Therefore distance between AO2 and the line AO.
Y1 = R2 (1-1
1
R
x) + x1
= R2 + x1 –x1 (1
2
R
R)
or middle radius + offset-fraction of offset which is equal to offset multiplied by the ratio of
middle.
Similarly distance between OB and OB
Y2 = R2 (1-3
2
R
x) + x2
= R2 + x2 – x2 (3
2
R
R)
Procedure of Curve tracing:
Refer to Fig 2.3.
1. Draw line AO and OB at an angle = angle of turn
2. Draw line AO and OB parallel to A O and O B at distance Y1 and Y2 respectively.
3. Knowing the position of O2 draw an arc of Radius R2 with centre at O2.
4. Draw line C1C2 parallel to AO at distance X1cutting the arc drawn with centre O2 at Q.
5. Extend QO2 up to O1such that O1Q=R1and with Radius R1and centre O1 draw arc P1Q touching
AO at P1 (Tangent)
6. Similarly draw the other curves RS.
P1Q R S is the required 3 centred curve with offset x1at Q and x2 at R.
Tables 2.5, 2.6 and 2.7 give values of R1, R2 and R3 for various angles of forms and design of
vehicles depending upon whether the compound curve is symmetrical or asymmetrical. With the help of
these tables, the 3-centred compound can be drawn.
2.1.3 Auxiliary Lanes
Auxiliary lanes are provided at intersections in order to allow through vehicles to proceed
relatively unhindered by turning vehicles. Also, the extra carriage way widths serve to reduce accident
severity by enabling turning vehicles to merge with, and diverge from, the main traffic streams at low
relative speeds. Thus, the auxiliary lanes improve the performance of intersections.
The auxiliary lanes are essential at all intersections on high-volume, high-speed roads. On other
roads they should justify additional costs and right of way requirements. The auxiliary lanes can be
classified as Deceleration lanes, Acceleration lanes and Storage lanes.
The deceleration lanes allow the gradual separation of turning traffic from the through traffic.
Without these lanes, the vehicles leaving the through carriage-way would have to slow down within a
high speed traffic lane, a well recognized cause of rear-end collisions. The deceleration lanes are
provided for both left turning and right turning traffic. Fig 2.4 shows different shapes of left turn lanes.
The shape down in fig.2.4 (C) is well adapted. The length of a left turn lane is dependent upon the speed
at which the vehicles can manoeuvre on to it from the main carriage-way, the rate of safe deceleration and
the turning speeds of vehicles after traversing the lane.
The right turn lane provides space for right turning traffic, while at awaits crossing gaps in the
opposing traffic stream. These storage lanes are provided by having a recess in the median. The length
of the lane is dependent on the volumes of opposing and turning traffic, in addition to those of left turn
lanes.
Acceleration lanes permit entering vehicles to increase speed in order to enter the main carriage
way at the speed of the traffic. If the main road traffic is very heavy, a long acceleration lane also
provides entering traffic with space to manoeuvre while it awaits merging gaps in the main traffic stream.
Fig.2.5 shows different shapes of Acceleration lanes. Fig.2.5(b) illustrates the continuous turn type of
acceleration lane most favoured by motorists. The length of acceleration lane is dependent on the speed
of the traffic on the through carriage-way, the rate of acceleration of merging vehicles, the design speed
of merging minor road and the volumes of through and entering traffic.
The auxiliary lanes should be provided with a taper. This is a wedge shaped area designed to
encourage and ease movements. It is designed based on the lateral transition time of 1 sec/m of
displacement. The design lengths of deceleration and acceleration lanes are given in Table 2.8.
2.1.4. Carriageway widths at junctions:
The width of carriage way needs to be widened at curves to ensure adequate clearance for vehicle
overhangs, off tracking of rear wheels.
Table 2.9 gives the details of carriageway widths. Compound curves with transitional approaches
should be used wherever possible on sharp bends.
Table 2.8 Minimum Acceleration Lane Lengths
Highway
Acceleration Length (m)
for entrance curve design speed (kmph)
Stop
conditions
25 30 40 50 60 65 75 80
Design
Speed
(kmph)
Speed
Reached
(kmph)
and initial speed (kmph)
0 20 30 35 40 50 60 65 70
50
65
80
100
110
40
50
60
75
85
60
120
230
360
490
-
100
210
340
470
-
75
190
330
460
-
70
180
300
430
-
40
150
280
400
-
-
100
240
380
-
-
50
160
310
-
-
-
120
250
-
-
-
50
180
Minimum Deceleration Lane Length
Highway
Design
Speed
(kmph)
Average
Running
Speed
(kmph)
Deceleration Length (m)
For Design Speed of Exit Curve
Stop
condition
25 30 40 50 60 65 75 80
for Average Running Speed of Exit Curve
0 20 30 35 40 50 60 65 70
50
65
80
100
105
110
45
60
70
85
90
95
70
95
130
160
175
190
60
90
120
150
165
180
50
80
120
150
160
175
40
70
110
140
150
170
-
60
100
130
150
160
-
50
90
125
130
150
-
-
70
100
120
130
-
-
50
90
100
120
-
-
-
70
85
100
Table 2.9 Width of Lanes at Intersections
Inner Radius Design Speed
Km/h
Single Lane
width, m
Single lane
width with
space to pass
stationary
vehicles, m
Two lane width
for one or two
way traffic, m
(1) (2) (3) (4) (5)
10.5
15
20
30
40
50
75
100
125
150
-
18
23
27
32
37
41
50
57
62
64
-
5.50
5.50
5.00
4.50
4.50
4.50
4.50
4.50
4.50
4.50
4.50
10.53
9.50
9.00
8.00
7.50
7.00
7.00
7.00
6.50
6.50
6.00
11.5
10.5
10.0
9.0
9.00
8.00
8.00
8.00
8.00
8.00
7.00
These widths are applicable for longer slip roads (over 60m length) and should be used only if
vehicles are allowed to park.
2.1.5 Gap in median at junctions:
To ensure that large vehicles can turn right without difficulty, to or from a major road, the gap in
the median should be extended 3m, beyond the continuation of both curb lines of minor road to the edge
of major road, and should also be determined by 12 to 15m radii control circles tangential both to centre
line of the minor road (or to the sides of any refuge or island) and the side of the central reserve away
from the minor road. To facilitate easy turning, the end of the median should be bullet nosed. Fig. 2.6
illustrates this.
2.2 CHANNELIZATION
Channelization is the separation or regulation of conflicting traffic movements into definite paths
of travel by the use of pavement markings or raised islands, to facilitate the safe and orderly movement of
both vehicles and pedestrians. Proper channelization increases capacity, improves safety, provides
maximum convenience and instills driver confidence. Channelization serves the following purposes:
Separation of conflicts
Control of angle of conflict
Control of speed
Protection of vehicles leaving or crossing main traffic stream
Protection of pedestrians
Elimination of excessive intersectional areas
Blockage of prohibited movements
Location of traffic control devices.
The channelizing islands are generally classified based on purpose, for which they are meant:
Pedestrian refuge islands: These are designed for the use of and protection of pedestrians.
A loading island is an island provided especially for the protection of transit vehicle users.
Traffic divisional island: These are usually elongated and narrow, which follow the course
of roadway to separate traffic moving in same or opposite directions.
Traffic channelizing islands: These are located in a roadway area to confine specific
movements of traffic usually turning movements, to definite channels.
2.2.1 General Guidelines of design of channelising islands
The channelising islands should be so planned that the shape will confirm to the natural vehicle
paths, and the raised island will not constitute a hazard in the roadway. There are no standard criteria
regarding the shape and size of the channelising of the intersection. Fig 2.7 illustrates the shapes of some
of channelising islands. To enable the traffic islands to be seen clearly, they should preferably be
bordered by raised curbs, have an area of at least 4.5 sq.m. and or bollards at suitable places. Islands
smaller than about 4.5 sq.m. are often defined by paint markings alone, known as ghost islands.
The following are some of the rules that govern the design of islands.
The islands should be arranged so that the driving paths seem natural and easy to follow.
There should be only one path for the same intersection movement.
Points of crossing of the paths of vehicles should be separated as much as possible.
Sudden and sharp reverse curves should be avoided.
The intersection areas not used by vehicles should be marked as a reserve zone and thus areas of
vehicle conflict should be reduced.
Traffic streams should diverge or merge at small angles so that the process of vehicles leaving or
entering through traffic stream is quick.
Traffic streams that cross without merging and weaving should intersect at or near right angles.
A few cell placed, large islands are better than many small islands.
Adequate approach end treatment should be given to warn drivers and to permit gradual changes
in speed and path.
Turning roadway should have width and radii adequate for the governing design vehicle.
Islands should be offset 0.3-0.6 from the edge of the normal traveled way as shown in Fig. 2.8.
Over channelization should be avoided, as it has proved to be counter productive.
Approach end treatment
The approach end of island need to be designed to warn the approaching traffic of the presence of
the island and to indicate the proper vehicle path to be followed. This is done by:
Pavement marking
Object marking
Traffic signs
Reflecting hazard markers and delineator.
Flashing yellow beacons
Contrasting pavement colour texture
Raised bollards, buttons or blocks.
Illumination devices
Fig. 2.10 shows the markings used at islands.
Development of Islands
Islands are delineated by any one of the following methods:
They may be raised and outlined by curbs and filled with pavement construction material or other
material. Fig 2.11 shows the different types of curbs.
They may be formed by pavement marking, buttons or raised bollards on all paved areas, when
speeds are low and space is limited.
Since the behaviour of human-machine combine is very complex, it is generally advisable to install
temporary channelisation by traffic cones, sand bags, etc and study its effect on traffic operations before
installing permanent curbing and marking.
Fig. 2.12 to 2.18 illustrate the design of intersections as worked out for real case studies.
2.3 INTERSECTION DESIGN USING TEMPLATES:
In the design of intersections the dimensional and operational characteristics of vehicles have a
pronounced effect. Turning configurations are particularly important. The ‘design vehicle’ for each class
of vehicles provides some of the criteria and basis for the design of highway intersections. To facilitate
the application of the controlling features of design vehicles, appropriate turning paths are produced in
graphic from and used as templates the quality and productivity of intersection designs increases. These
templates are also useful in designing the car parks, truck terminals and bus terminals.
2.3.1 Use of Templates:
Turning vehicle templates may be used for checking plans of intersection or in designing the new
facilities. In the former, appropriate templates are placed over the plan with the vehicle paths properly
superimposed and this reveals whether the path is proper or not. If necessary, suitable modifications, like
widening of roadway or changing the shapes of channelising island may be proposed. In the latter case,
the appropriate templates are laid in desired location over the framework or base of the intersection and
the turning paths are transferred to the plan. This is accomplished by placing pencil dots through
perforations along the wheel paths. The turning templates provide the wheel paths of vehicles turning
through angles from 30 degrees to 180 degrees with equal increments of 30 degrees as shown in Fig. 2.19.
The same template can be used for both left and right turns by reversing the template.
These wheel paths will give a clear picture of the area required for the movement of the vehicles.
Rest of area will be enclosed by constructing the channelizing islands, which separate the traffic and thus
minimize the conflicting area.
These templates can be used directly for the intersections when the angles between the legs are
multiple of 30 degrees as marked on templates.
2.3.2 Use of templates for odd angles
In reality all roads may not intersect at an angle that is multiple of 30 degrees as indicated on
template. In such cases the design can be achieved by manipulating the template to conform to the
required angles.
Consider an intersection whose left turning angle is 70 degrees and right turning angle is 110
degrees. According to template configurations this angle falls between the paths of 90 and 120 degrees.
In general, the sharper of the two angle is used as a guide. However, if the flatter angle is close, it may be
used for the design. To obtain the turning path of vehicle to the required angle, the template is first
placed with the beginning portion of the path centred on the entry of the lane and at the same time the
outer circular arc of the path is placed so as to permit proper entry, at the completion of the path, into the
designated lane of the intersecting roadway. Then holding the template firmly at centre, the far end of the
path is swung in to position by revolving the template. This results in the path as intended. The vehicle
path is transferred to the lay out by marking through perforations in both the positions. Thus the turning
path of vehicle for the odd angle is obtained.
2.3.3 Illustration
The technique of using templates in intersection design is demonstrated in following design
problem.
The basic conditions of intersection are presented in Fig.2.20. The functional plan of the
intersection is shown to a small scale in inset. A layout is to be developed using a SU-9 design vehicle, to
determine complete channelization, including curb returns, islands, median openings, etc..
As a first step, the controlling width of divisional island on the main road is located on the base
plan, shown hatched, to serve as a guide for the two right turning movements. Wheel paths for these two
turns are established by positioning appropriate selected templates over the base plan, placing the pencil
dots through the perforations, and sketching the turning configurations. (Fig. 2.21 & 2.22)
After the establishment of two right- turning paths, the median opening and the cross road
divisional island are sketched to fit these paths (Fig. 2.23). The sketching is done free hand for the most
part with frequent scaling to maintain proper dimensions. The edges of traveled ways and islands, except
along lanes of standard width, are positioned not less than 1.0m outside the wheel paths, and offset
several feet more at approach points of island. The radii of island ends at the face of curb are generally
set at 0.6 to 0.8m on the approach nose and 0.3 to 0.5m at the merging point.
Then the left turning paths are added to the plan utilizing appropriate templates in a similar
manner. The remainder of the plan is sketched in by adding the triangular island and the outer edges of
pavement along the paths of left turning vehicle (Figs. 2.24 & 2.25). The plan is complete except for
refining the layout and providing firm dimensions, curvature, etc.
ROTARIES & MINI ROUNDABOUTS
3.1 ROTARY INTERSECTIONS
In one sense, Rotary intersections (or Roundabouts) can be considered as a from of channelised
intersection in which vehicles are guided onto a one-way roadway and required to move in a clockwise
direction about a central island. At one time, the rotary intersection was considered to be the answer for
all the problems associated with intersections. In fact, the rotary intersections have particular advantages
and disadvantages, and the decision as to whether a rotary should be used at any individual location
requires an understanding of these.
Where roundabouts are properly used and designed, the efficient flow of traffic is promoted by
the orderly movement of vehicles about the central island. There is only minor delay to traffic due to
speed reductions and no delay, at all, due to stopping. Further more the possibility of having vehicle
conflicts is considerably reduced. Since, all traffic streams merge and diverge at small angles, accidents,
if occur, rarely have fatal consequences, damage being usually confined to vehicle only.
The rotary design is more suited to intersections with four or more approach roads and or where
there are very heavy right turning movements. These intersections usually require greater land area and
cost more than other at –grade intersections, capable of handling same traffic flow. These are nor
adaptable to locations with difficult top graphic conditions. These can not be properly used at locations
with large traffic volumes in built up areas. Similarly they can not be adapted for high speed roads as
long weaving lengths are required to ensure low relative speeds.
Roundabout intersections are not readily adaptable as are traffic signal controlled intersections to
the long term stage development of highway. If constructed to meet the long term needs, they usually
result in over-design when compared with immediate traffic requirements. For traffic control reasons,
round about should not be provided in areas with an existing or proposed area-wide urban traffic control
systems.
3.2 IRC GUIDELINES FOR SELECTING ROTARY INTERSECTION
Lowest traffic volume for which rotary treatment should be considered is about 500 veh/hr.
The volumes entering from different intersection legs are almost equal.
The maximum volume, that a traffic rotary can efficiently handle, can be taken as 3000 veh/hr.
entering from all intersection legs.
A rotary is advantageous at locations where the proportion of right turning traffic is high.
A rotary is preferable if there are other junctions so near, that there would be insufficient space
for the formation of queues.
3.3 DESIGN OF ROTARY
A traffic rotary is a specialized form of at grade intersection where vehicles from the converging
arms are forced to move round an island in one direction in an orderly and regimented manner and
‘weave’ out of the rotary movement into their desired direction. The following steps outline the design of
rotary intersections.
3.3.1 Shape of Central Island:
The shape and disposition of central island (control island) depend upon various factors such the
number and disposition of intersecting roads and traffic flow pattern. Fig 3.1 illustrates the common
shapes of rotary islands. The conditions under which a particular shape is favoured are discussed in Table
3.1
Table 3.1 COMMON SHAPES OF ROTARY ISLANDS
Type Remarks
a) Circular Equal importance to all the roads meeting.
b) Squarish with rounded edges Suitable for predominantly straight ahead
flows.
c) Elliptical, elongated oval or rectangular
shapes
To favour through traffic/to suit the geometry
of the intersecting legs/ to provide longer
weaving lengths.
d) Irregular Shape is dictated by the existence of large
number of approaches
3.3.2 Design speed
The design speed of a rotary governs the various elements such as radii and weaving lengths.
Vehicles approaching a rotary have to reduce the speed to keep the dimensions of the rotary elements
within a practical limit. Following are the speeds recommended by IRC for design of rotaries.
Urban areas = 30 kmph
Rural areas = 40 kmph
The various elements of rotary intersection are shown in Fig.3.2
3.3.3 Radius at Entry
Radius of curve at entry is designed based on design speed, super elevation and co efficient of
friction. The radius should result in the drivers reducing their speeds to a safe value, which is the design
speed of the rotary. The following values are adopted.
Rotary design speed (kmph) Radius at entry (meters)
40
30
20 to 35
15 to 25
3.3.4 Radius at Exit:
The radius of the curve at exit should be larger than that of central island and at entry so that the
drivers pick up speed and clear away from the rotary expeditiously. The general practice is to keep the
radius of the exit curve 1 ½ to 2 times the radius at entry. If, however, there is large pedestrian traffic
across the exit road, radius similar to that at entrance may be provided, to keep the exit speeds reasonably
low.
3.3.5 Radius of Central Island:
Theoretically, the radius of central island should be equal to radius at entry. In practice, the
radius of central island is kept slightly larger than that at entry, this being an attempt to give a slight
preference to traffic already on rotary and to slowdown the approaching traffic. A value of 1.33 times the
radius of entry curves is generally recommended.
3.3.6 Weaving Length:
The weaving length determines the ease with which the vehicle can manoeuvre through the
weaving section and thus determines the capacity of the rotary. The weaving length is decided on the
basis of the factors, such as, the width of weaving section, average width of entry, total traffic and
proportion of weaving traffic in it. It is desirable to prevent direct traffic cuts and this can be achieved by
making the ratio of weaving length to weaving width large enough. A ratio 4:1 is regarded as minimum.
The minimum values of weaving lengths as recommended by IRC are given below:
Design speed (kmph) Minimum weaving length (m)
40
30
45
30
In order to discourage speeding in weaving sections, maximum weaving lengths should be
restricted to twice the values given above.
3.3.7 Width of Carriageway at entry and exit:
The carriageway width of intersection legs is governed by the design year traffic entering and
leaving the intersection. A minimum width of 5m with provision for extra widening due to curvature,
may be provided at entry and exit. Table 3.2 summarizes the current Indian practice.
Table 3.2 Width of Carriageway at entry & exit
Carriageway width of the
approach road
Radius at entry (m) Width of carriageway at
entry & exit (m)
07.0 m (2 lanes)
10.5 m (3 lanes)
14.0 m (4 lanes)
21.0 m (6 lanes)
07.0 m (2 lanes)
10.5 m (3 lanes)
14.0 m (4 lanes)
21.0 m (6 lanes)
25-35
15-25
06.5
07.0
08.0
13.0
07.0
07.5
10.0
15.0
3.3.8 Width of Rotary Carriageway:
The width of non-weaving section should be equal to the widest single entry into the rotary, and
should generally be less than the width of weaving section.
The width of weaving section should be one traffic lane wider than the mean of the width at entry
and of non weaving section. The details are shown in Fig. 3.3
3.3.9 Entry and Exit angles:
Entry angles should be larger than exit angles. It is desirable that the entry angles should be
about 60 degrees. The exit angles should be small, if possible, even tangential.
3.3.10 External curb line:
The external curb line of weaving sections should not be normally re-entrant, but consist of
straight or curve of large radius and of the same sense as the entry and exit curves. Such an arrangement
eliminates waste of area, which is not likely to be used by traffic. Curb line configurations are shown in
Fig. 3.4.
3.3.11 Super elevation and camber:
Since the rotary curvature is opposite to that of entry and exit, vehicles, especially, heavily loaded
buses and trucks, experience difficulty in changing over from one cross slope to another in the opposite
direction. It is, therefore, recommended that the algebraic difference in the cross slope be limited to about
0.07. The super elevation should be limited to the least amount consistent with design speed. The crown
line where the opposite cross slopes meet, should, as far possible, be located such that the vehicles cross it
while travelling along the common tangent to reverse curve. Channelising island should be on the peak
with the road surface sloping away from them to all sides. The details of cross slope and cumber are
shown in Fig. 3.5.
3.3.12 Other features:
Stopping sight distance should be provided for the design speed adopted. On the approaches to
the rotary, the sight distance available should enable the driver to discern the channelising and rotary
islands clearly.
3.4 CAPACITY OF ROTARY INTERSECTION:
The capacity of a rotary is governed by the capacity of each weaving section. The capacity of
weaving section is determined by geometric layout and the percentage of weaving traffic. The following
formula, a modification of well known Wardrop formula, gives capacity of rotary:
QP = lw
pwew
/1
3/1)(/1(280
Where, QP : Practical capacity of weaving section of rotary in Passenger Car Units (PCU) per hour.
w : Width of weaving section in metres.
e : Average entry width of rotary.
l : Length of weaving section between the ends of channelising islands in metres.
P : Proportion of weaving traffic.
: dcba
cb
(Fig. 3.6 illustrates this)
The passenger car units of different vehicles as recommended by IRC for use in rotary design are
given in Table 3.3.
Table : 3.3
Sl. No. Vehicle Type PCU
1 Cars and light commercial vehicles including
3- wheelers
1.0
2 Buses and Medium, Heavy Commercial
Vehicles
2.8
3 Motorcycles and Scooters 0.75
4 Pedal cycles 0.50
5 Animal or Hand drawn vehicles 4 - 6
3.5 MINI-ROUND ABOUTS:
In mini-round about the large diameter central island is replaced by small diameter island. This
increases the capacity of the intersection and provides continuous flow of traffic through intersection. In
case of conventional roundabout if the total incoming flow is greater than the capacity of the intersection,
the vehicles get locked in the intersection resulting in large delays and unlocking can be achieved only by
means of police help. The principles incorporated in mini-roundabout design include:
Wider entries by narrowing median strips or islands and advanced give-way lines.
Smaller central island to provide more space for circulation, compatible with entry width and
turning movements.
Deflection of traffic passing through the roundabout, so that the vehicles traverse the intersection
at a safe speed.
Layout should be simple, clear and conspicuous, so that important features of design are easy to
see and use.
Fig. 3.7 illustrates the layout of a mini-round about. It has been observed that there is an increase in
the capacity of intersections, wherever the mini-round about are in use. The number of fatal and serious
injury accidents are less at these junctions. However, mini- roundabouts work well only in situations
where “priority from the right” rule is strictly followed. Non-observance of this rule can only lead to
chaos and serious accidents.
3.5.1 Designs procedure of Mini- Roundabouts:
The general method of design of Mini-Roundabout is as follows:
The central island should normally be about 1/3 of the diameter of a circle inscribed within the
outer carriageway boundaries or 8m, which ever is larger. Smaller islands can be tried but in that
case it will be necessary to ensure that traffic is deflected well to the left on entry into the
roundabout.
The number of traffic lanes should be increased at the give-way line by either advancing the give-
way lane into existing junction space or by flaring approaches. Flare of 1 in 10 to 15 is usual so
that a single lane entry could be increased to 3 lanes over a distance of 30 to 50 m.
A minimum stopping distance of 25 m between the give-way line and the point of conflict with a
vehicle from left should be available.
The width between traffic islands and the central island should not be less than the total width at
the preceding entry.
Exit tapers of 1 in 12 are suitable, but may be reduced if traffic is light or site conditions are
difficult. The entry taper should normally be twice as sharp as the exist taper, that is 1 in 6.
A deflection island has to be provided to ensure that straight through movements do not occur.
An improved mini-roundabout design is given in Fig.3.8. This is a compact design where by the
usual one-way circulation about a single central island is replaced by two-way circulation about a central
core, with a three arm roundabout at the mouth of each arm. This is known as ring design and has a
number of advantages: it results in high capacity, all opposing right-turns are non-hooking. The path
lengths of the right turn movements are shortened, and the crossing conflicts between various traffic
streams are separated and therefore made easier. Its major disadvantage is that the layout is complex
causing difficulty to drivers.
TRAFFIC SIGNALS
4.0 GENERAL
A traffic control signal is defined as any power operated traffic control device, by which traffic is
alternately directed to stop and permitted to proceed. For traffic control signals to serve any useful
purpose their indications should be clearly understood and clearly observed. To achieve these objectives,
traffic signals should be uniform and their compliance legally forcible.
4.1 BENEFITS AND DRAWBACKS OF TRAFFIC SIGNALS
Well-designed traffic signals, when properly located and operated, usually have one or more of
the following advantages:
Provide for orderly movement of traffic
Provide for the continuous flow of a platoon of traffic through proper co-ordination at a definite
speed along a given route.
Reduce the frequency of right-angled and pedestrian accidents.
Permit other vehicles and pedestrians to cross a heavy traffic stream.
Control traffic more economically than by manual methods
Assign right of way impartially
Increase the handling capacity of a congested intersection.
Unjustified, i11 designed, improperly operated or poorly maintained traffic signals have following
disadvantages.
They can increase total delay at intersections
Increased frequency of rear-end collisions
Failure of the installation, although infrequent, may lead to serious and wide spread traffic
difficulties.
Thus the traffic engineer should recognize that a signal should be installed only if the net effect,
balancing benefits versus draw backs, is to the public’s advantage.
4.2 WARRANTS FOR SIGNAL INSTALLATION:
Traffic control signals should not be installed, unless one or more of the signal warrants specified
herein are met. Information should be obtained by means of traffic and engineering studies and compared
with the requirements set forth in the warrants. If these requirements are not met, a traffic signal should
neither be put into operation nor continued in operation (if already installed).
4.2.1 Warrant 1: Minimum Vehicular Volume:
The minimum vehicular volume warrant is intended for application where the volume of
intersecting traffic is the principal reason for consideration of signal installation. The warrant is satisfied
when for each of any 8 hours of an average day, the traffic volume given in Table 4.1 exist on major
street and on the higher volume or minor street approach to the intersection.
Table 4.1 Minimum Vehicular Volume For Warrant 1.
Number of lanes for moving traffic on
each approach
Motor vehicles per
hour on major
street (total both
approaches)
Motor vehicles per
hour on higher
volume minor street
approach(one
direction only)
Major street Minor street
1
2 or more
2 or more
1
1
1
2 or more
2 or more
650
800
800
650
200
200
250
250
These major street and minor street volumes are for the same 8 hours. Each traffic lane are
marked at the intersection, shall be minimum 2.8 metre wide. During the 8 hours mentioned above, the
direction of higher volume on the minor street may be on one approach during same hours and the
opposite approach during other hours.
When the 85the percentile speed of major street traffic (or average approach speed) exceeds 50
kmph or when the intersection lies within the built-up area of an isolated community having a population
of less than 2.5 lakhs the minimum vehicular warrant is 70 per cent of the requirements above (in
recognition of differences in the nature and operational characteristics of traffic in urban and rural
environments and small municipalities).
4.2.2 Warrant 2: Interruption Of Continuous Traffic:
The interruption of continuous traffic warrant applied to operating conditions where the traffic
volume on a major street is so heavy that traffic on a minor intersecting street suffers excessive delay or
hazard in entering or crossing the major street. The warrant is satisfied when, for each of any 8 hours of
an average day, the traffic volume given in Table 4.2 exist on the major street and on the higher volume
minor street approaching the intersection, and the signal installation will not seriously disrupt progressive
traffic flow.
Table 4.2 Minimum Vehicular Volume For Warrant 2.
Number of lanes for moving traffic on
each approach
Motor vehicles per on
major street (total of
Motor vehicle hour
on higher volume
Major Street Minor Street
higher both approaches) minor street
approach (one
direction only)
1
2 or more
2 or more
1
1
1
2 or more
2 or more
1000
1200
1200
1000
100
100
150
150
These traffic lanes at intersection shall be minimum 2.8 metre wide.
During the 8 hours, the direction of higher volume on the minor street may be on one approach
during some hours and 85th percentile speed (or average approach speed) of major street traffic exceeds
60 kmph or when the intersection lies within the built-up area of an isolated community, having a
population of less than 2.5 lakhs, the interruption of continuous traffic warrant is 70 per cent of the
requirements above.
4.2.3 Warrant 3 – Minimum Pedestrain Volume
The minimum pedestrian volume warrant is satisfied when for each of any 8 hours of an average
day, the following traffic volumes exist:
(i) On the major street, 600 or more vehicles per hour enter the intersection (total of both
approaches) or where there is a raised median island 1.5 metre or more in width,
1000 or more vehicles per hour (total of both approaches) enter the intersection on
major street, and
(ii) During the same hours as in para (i) above, there are 150 or more pedestrians per
hour on the highest volume or cross-walk crossing the major street.
When the 85th percentile speed (or average approach speed) of major traffic exceeds 60 kmph, or
when the intersection lies within the built-up area of an isolated community having a population
of less than 2.5 lakhs the minimum pedestrian volume warrant is 70 per cent of the requirements
above.
A signal installed under this warrant at an isolated intersection or mid-block, should be of the
traffic actuated type with push buttons for pedestrians crossing the main street. If experience
proves that it is being tampered with or misuse anticipated, they can be of the usual fixed time
cycle and phases. If such a signal is installed at an intersection, it should be equipped and
operated with control devices, which provide proper co-ordination with other signals in the
system.
Signal installed according to this warrant shall be equipped with pedestrian indications
conforming to requirements set forth in previous sections. Signals may be installed at non-
intersection locations (mid block) provided the requirements of this warrant are met, and provided
that the related cross-walk is not closer than 300 metre to another established cross-walk. Kerb
side parking should be prohibited for 75 metre before and 75 metre beyond the cross-walk.
Phasing, co-ordination and installation must conform to standards. Special attention should be
given to the signal head placement and the signs and markings used at non-intersection locations,
to be sure that drivers are aware of this special application.
4.2.4 Warrant 4 – Accident Experience
The accident experience warrant is satisfied when:
(i) Adequate trial of less restrictive remedies with satisfactory observance and enforcement have
failed to reduce the accident frequency, and
(ii) Five or more reported accidents, of types susceptible of correction by traffic signal control
have occurred within a period of 12 months, each accident involving personal injury or
property damage to an apparent extent of Rs 2000 or more.
(iii) The signal installation will not seriously disrupt traffic flow.
4.2.5. Warrant 5 – Combination Of Warrants
In exceptional cases, signals may be justified occasionally where no signal warrant is satisfied but
where two or more of warrants 1,2 and 3, are satisfied to the extent of 80 per cent or more of the stated
volume.
Adequate trial of other remedial measure which causes less delay and inconvenience to traffic
should precede installation of signals under this warrant.
4.3 BASIC FEATURES OF A TRAFFIC SIGNAL
Traffic signal is a complete installation which includes signal heads (containing different
coloured lanterns), poles, interconnecting cables, controlling mechanism is normally determined by the
degree of desired sophistication and the need to ensure uniformity and standardization.
4.3.1 Signal Head
A typical main signal is composed of three lanterns arranged vertically above one other, with a
red lens at top, amber in the middle and a green lens at the bottom. The lenses are normally 203 mm in
diameter and each illuminated from behind by an independent light source.
The standardization of lantern location is important as it ensures that colour-blind drivers are
always aware of which lamps are burning at a given time.
The normal sequence of indications is red, red-amber, green and amber. The function of red-
amber aspect is to indicate to stopped motorists (or pedestrians) that the lights are about to change green;
they are therefore be prepared to enter the intersection as soon as the lights change, and so wasted cycle
time can be kept to minimum. The amber period similarly warns so that they can slow down safely; at the
same time, it acts as clearance interval for vehicles or pedestrians, within the junction and also for those
moving vehicles that are so close to the stop line that to halt suddenly could be dangerous.
Filter signals, normally mounted alongside main signal heads, permit the movement of vehicles in
the direction shown by the green arrow even though the main signal is showing red.
4.3.2 Traffic Signal Controller:
This is the complete timing mechanism, which controls the operation of the traffic signals. The
basic function of a controller unit is to switch the appropriate signal indications on and off according to a
fixed or variable plan, thus assigning the correct and safe right-of-way at a location. Some of the
controllers are as follows:
1. An ‘automatic controller’ is a self operating mechanism which operates the traffic signals
automatically.
2. A ‘pretimed controller’ is an automatic controller for supervising the operation of traffic signals
in accordance with a pre-determined fixed time cycle.
3. A ‘traffic-actuated controller’ is an automatic controller for supervising the operation of traffic
signals in accordance with varying demands of traffic. The controller receives data from
detectors located on one or more approaches to the junction, and on the basis of these data,
allocates green time by predetermined methods.
4. A ‘master controller’ is an automatic controller for supervising a system of secondary controllers,
maintaining definite interrelationships or accomplishing other supervisory functions.
Traffic Detector:
A traffic detector is any device by which vehicles or pedestrians presence is detected and
informed to traffic-actuated controller. The various types of detectors include pneumatic tube, magnetic
loops, piezo electric, radar, ultrasonic, photoelectric, infrared devices.
DEFINITIONS
Definitions of some of the terms used in the design of to Traffic signals are as follows:
1. Cycle (Cycle length): Any complete sequence of signal indications.
2. Phase (signal phase): The part of the cycle allocated to any combination of traffic
movements receiving right-of –way simultaneously.
3. Interval : The part or parts of the signal cycle during which signal indications do not
change.
4. Offset : An offset is the time lapse, in seconds between the beginning of a green phase at
the intersection and the beginning of green phase at the next intersection.
5. Intergreen period: The time between the end of the green of the phase loosing the right-
of –way and the beginning of the green period of the phase gaining the right-of-way. It
includes amber time plus any all-red time and the and the overlapping red/red-amber time
between the two green periods. Fig. 4.1 illustrates this.
6. All-red interval : The period during which the indication is red for all approaches. This
is illustrated in Fig. 4.1
7. Lost Time : This is the total time during the cycle, which is not effectively used for
vehicle movement.
8. Peak-Hour Factor : The ratio of number of vehicles entering the intersection during the
peak hour to four times the number of vehicles entering during the peak 15 min. period.
9. Saturation Flow : It is the flow that would be obtained if there were a continuous queue
of vehicles and they were given 100% green time.
10. Passenger-car equivalents : To account for the effects of different types of vehicles and
turning movements on start up time (or average headway), it is customary to convert
actual flows (given in mixed vehicles per hour) to an equivalent volume in straight-
through passenger cars. The PCE values for traffic signal computations are given in
Table 4.3.
Table 4.3 PCU Equivalents for traffic signal Computations
Type of Vehicle PCU Equivalent
Heavy or medium goods vehicle 1.75
Light goods vehicle 1.00
Bus 2.25
Motor Cycle, moped or scooter 0.33
Pedal cycle 0.20
TRAFFIC SIGNALS AT ISOLATED INTERSECTIONS
An isolated signal is one at which the timing is independent of any signal control in the vicinity.
The signals at isolated intersections can be classified into pretimed and traffic actuated groups.
Pretimed Signals
In pretimed operation, the cycle length, phase, green times and change intervals are all preset.
The signal rotates through this defined cycle in a constant fashion: Each cycle is the same, with the cycle
length and phase lengths constant. Depending on the equipment available, several preset timing patterns
may be used, each being implemented automatically at fixed times of the day.
Traffic actuated signals
This signals can be subdivided into fully- and semi- traffic actuated signals.
Semi actuated signal operation
In semi actuated operation, the designated main street has a green indication at all times until
detectors on the side street determine that a vehicle or vehicles have arrived on one or both of minor
approaches. The signal then provides a green phase for the side street, after an appropriate change
interval, which is retained until all the vehicles are served or until a preset maximum side street green is
reached.
Fully actuated signal operation
In fully actuated operations all signal phases are controlled by detector actuations. In general,
minimum and maximum green times are specified for each phase, as is the phase sequence. In this form
of control, cycle lengths and green times may vary considerably in response to demand. Certain phases in
the cycle may be optional or may be skipped entirely if no demand is sensed by detectors.
Number of methods are available for estimating the appropriate green time for each phase at
individual intersection. The widely used Webster’s method is illustrated in the following articles:
4.5.1 Phasing
In the control of traffic at intersections the conflicts between streams of vehicles are prevented by
a separation in time. The procedure by which streams are separated is known as phasing. The number of
phases depend upon the conflicting of traffic flows and intersection layout.
It is desirable to have the number of phases as minimum as possible so that the signal will work
satisfactorily. The most commonly employed system is the two-phase system, in which the right-of-way
is alternately assigned to each of the two cross movements. The traffic movements during each of the
phases are illustrated in Fig. 4.2. Intersections having more than two major conflicting flows, or heavy
pedestrian movements require more than two phases. A typical situation is at normal cross roads where
there is a heavy right turning movement on one of the approaches as illustrated in Fig. 4.3. The traffic
movements permitted during each of the phases of a two phase system at a T-section are shown in Fig.
4.4.
4.5.2 Saturation Flow
When the green period commences, vehicles take some time to start and accelerate to normal
running speed but after a few seconds, the queue discharges at a more or less constant rate which is called
Saturation Flow. This concept is explained in Fig. 4.5. Saturation flow depends on the layout of the
intersection and the number of right turning vehicles and the composition.
The saturation flow(s) expressed in terms of Passenger Car Units (PCUs) per hr. and with no
parked vehicles present is given by
s = 525(w) PCUs per hr.
Where w = width of approach road in metres measured from kerb to the inside of the central median or
centre line of the approach whichever is nearer.
This expression is valid for widths from 5.5 m to 18 m. For a lesser width the values may be
taken as given below.
Approach
Width w in
metres
3.0 3.5 4.0 4.5 5.0 5.5
Saturation
flow (s)
(PCUs) per
hr.
1850 1890 1950 2250 2550 2990
Effect of Gradients:
For each 1 per cent of uphill gradient, the saturation flow decreases by 3 per cent and for each 1
per cent downhill gradient, saturation flow increases by 3 percent. The gradient shall be the average slope
between the stop line and a point on the approach 60m before it.
Effect of right turning traffic:
If the right turning movements from opposite directions cause the intersection to lock, then the
capacity of the intersection cannot be easily assessed. Under non-locking conditions, the effects of right
turning traffic depend on whether or not conflicting traffic moves on the same phase and on whether or
not right turning traffic is given exclusives lanes. There are four possibilities.
(i) No opposing flow, no exclusive right turns: The saturation flow can be obtained as explained
earlier.
(ii) No opposing flow, exclusive right-turning lanes: The saturation flow (s) depends on the
radius of curvatures (r) and is given by:
Sr =
r
52.11
1800
PCUs/m for single file streams
And Sr =
r
52.11
3000
PCUs/m for double file streams
Where r is radius of curvature in metres of the right turning steam through a right angle.
(iii) Opposing flow, no exclusive right turning lanes: The maximum number of right turning
vehicles per cycle that can take advantage of gaps in the opposite stream can be determined
from the following equations.
nr = Srqs
qxcgxs
)()(
Where Sr = right turning saturation flow
q = flow in opposing arm
s = saturation flow for opposing arm
g = green time
c = cycle time
1.75 times the straight ahead vehicles.
(iv) Opposing flow, exclusive right lanes: There should be no delay to the straight ahead traffic
using the same approach as the same approach as the right turners, but there will be an effect
on the cross phase and this should be calculated using the same method as in (iii) above.
Effect of mixed traffic condition:
The saturation flow gets reduced due to presence of slow moving vehicles in the stream. This
effect is accounted for by converting the flow in mixed vehicles per hour to equivalent passenger cars.
4.5.3 Total Lost Time:
It is the time during which no flow takes place. It may be
(i) The total lost time per cycle (L) - The sum of lost time in each phase and the period
when all signals show red on red-amber. It can be expressed by,
L = nI + R
Where n = number of phases
I = average lost time per phase (excluding any all red periods or sequent
R = all red period time during each cycle where all signals display red or red with amber.
(ii) the lost time for a single phase (I) - The amount of time in a cycle which is
effectively lost to traffic movement in phase because of starting delays and the falling
of the discharge rate during amber period. No flow takes place in lost time as would
be seen in Fig.4.5
For the average signal cycle, the lost time caused by starting delays and reduced flow during the
amber period amounts to about 2 seconds per phase.
4.5.4 Cycle Time:
The total time period required for one complete sequence of signal indications is the cycle time
with minimum delay and could be represented by
Co = (1.5L + 5)/(1-Y) seconds
Where L = total lost time per cycle in seconds, generally taken as the sum of total amber and all
red clearance per cycle in seconds. Y = Y1+Y2 +…..Yn
Where Y1, Y2,……Yn
are the maximum ratios of flow to saturation flow for the phases 1,2…n ie y = q/s for a given phase.
The maximum cycle length recommended is preferably 120 seconds, being the maximum
acceptable delay for drivers of vehicles and pedestrians.
4.5.5 Calculation of effective green times:
The total effective green time per cycle is equal to the cycle time less the total lost time. The total
effective green time is split among the different phases in the ratio of ‘y’values. For example, for a two
phase signal, the effective green time for each phase is obtained as
g1 = &)(1 LCoy
y
g2 = )(2 LCoy
y
4.5.6 Determination of Signal Settings
The Controller setting time, the interval of green aspect, for each phase is determined as the sum
of effective green time and amber time less the time lost due to starting delays.
i.e., (g + a – 1)
Finally, the duration and sequence of different aspects during the various phases are presented by
means of time – phase diagram as shown in Fig. 4.1.
4.6 Signal Coordination
To achieve smooth flow on a street or highway system, it is not sufficient to ensure that each
point of local conflict is efficiently controlled. Unless the controls at each of the locations are
coordinated in some fashion, continuous and smooth flow on the street system will be impossible. It is
particularly important that signals in close proximity to each other be coordinated to prevent inefficient
stop and go flow from developing.
There are 4 major areas of engineering considerations for signal coordination.
1. Benefits
2. Purpose of the signal system
3. Factors reducing benefits
4. Exceptions to coordinated scheme
Different system of signal coordination are as follows.
1. Simultaneous System: All signals in the system have a common indications at all times.
Alternatively all offsets are zero and all splits are equal.
2. Alternate System: In this system, alternate signals or group of signals give opposite indications
to a given street at the same time. This system is an improvement over simultaneous system, in
that continuous movements can be achieved at more reasonable speeds.
3. Simple progressive system: In this system all offsets are so arranged that a vehicle entering the
system in the progressed direction, just after the green initiation of that signal. Unlike the
previous system there is no restriction on the individual speeds at several intersections. It does
not provide the full flexibility necessary at many locations to meet variations in traffic flows.
INTERSECTIONS WITH GRADE SEPARATION
5.0 GENERAL
A grade separation is a crossing of two or more highways at different levels. The intersections
with grade separations become more advantageous when the difficulties at at-grade intersections become
unduly great. Grade separated structures generally have very large initial costs, compared to single level
intersections.
5.1 WARRANTS
The main situations, which justify the very high cost, can be summarized as follows:
1. At intersections on high speed roads- with complete control of access automatically justifies the
use of grade separated structures in order to ensure the free movement of high-speed traffic.
2. To eliminate existing traffic bottlenecks- the inability of an important at-grade intersection to
provide the necessary capacity is in itself a justification for a grade separation. This situation
may arise when the total traffic of all the arms of intersection is in excess of 10,000 PCU’s per
hour.
3. Safety considerations- The large number of accidents at at-grade intersections justify the
construction of grade separations. For example, many lightly travelled rural roads, having high
vehicle operating speeds, have relatively large number of accidents at certain intersections. In
these locations, land is relatively cheap and so it may be possible to construct fairly low-cost
grade separations and so eliminate these accidents.
4. Economic considerations- The intersectional frictions and the resultant delays to traffic at at-
grade major junctions cause very considerable economic loss. If these intersections are converted
to ones with grade-separation, the very considerable long-term economic gain to the community
may by far outweigh the burden of initial costs.
5. Topographical difficulties- At certain sites, the nature of the topography or the cost of land may
be such that the construction of an at-grade intersection is more expensive.
TYPES OF GRADE SEPARATIONS
The grade-separations can be broadly classified as with slip roads and without slip roads.
Grade – separation without slip roads: This is a bridge or series of bridges which enable the traffic
streams on the intersecting highways to cross each other without any vehicle conflicts and without any
interconnecting ramps.
Grade – separation with slip or link roads: This is a system of interconnecting slip or link roads in
conjunction with a grade-separation or grade-separations which provide for the interchange of traffic
between two or more roads at different levels.
As with the at-grade intersections, there are many types of junctions with grade-separation, which
are used in various situations. Since, however, their basic purpose is to provide an easy and safe means
by which vehicles may transfer from one roadway to another, it is possible to classify the many types
according to the manner in which they perform this function. The simplest way of so doing is to classify
them according to the number of approach roads. Thus they may be classified as three-, four-, and
multiway. Fig. 5.1 illustrates several forms of junctions with grade-separation.
5.2.1 Three-way Junctions:
The general forms of the three-way interchanges are T-and Y-interchanges. The traffic
movement at these three-way interchanges are shown in fig.5.1. Both utilize a single bridge structure and
illustrate the situation where the greater volume of interchange traffic is given preferential turning
treatment at the expense of the lower turning volume, which has to use semi-directional loop. If the loop
movement is heavy, then extra bridge structures may have to be constructed so that both turning
movements are favoured equally. Where possible, three-way interchanges should be designed to enable
future conversion to four-way ones without alteration.
5.2.2 Four-way Junctions
The simplest type of four-way, two-level junction with grade-separation is the diamond.
Consisting of a single bridge and four one-way slip roads, it has the particular advantage that it can be
located within a relatively narrow land area. As fig.5.1 illustrates, this junction has direct high-speed
entrance and exit slip roads on the main road and at grade terminals on the minor road. Thus its use is
confined to intersection of major and minor highways.
A major advantage of diamond configuration is that it is very easily understood by motorists.
Further more, a greater slip road capacity can be relatively easily obtained. The main disadvantage is the
conflicts that can occur at the intersection, where, the slip roads meet the crossing minor road. These
junctions may need a pair of closely spaced traffic signals with relatively long cycle if the minor road
carries fairly heavy volumes of traffic.
The cloverleaf is a fourway, two-level single structure interchange having no terminal right-turns
at-grade, i.e., right turning traffic is handled instead by loop roads, thereby obviating the need for traffic
signals on the crossing road. Internationally, it is often regarded by motorists as the ultimate answer to
intersection problems. It has the great advantage of being very uncomplicated to use, while the right
turning movements are accomplished with no direct vehicle conflicts. Nevertheless, there are a number
of features about this type, which limit its usefulness. Firstly, if a cloverleaf is used at the junction of two
high-speed, heavy volume highways, an excessively large area of land may be required. A second
undesirable feature of cloverleaf is that vehicles desirous or right turning movements must negotiate a
270-degree semi-direct turn. The loop roads do not operate very satisfactorily when widened to more
than one lane. Furthermore, from a safety aspect, pedestrian movements along cross-streets in urban
areas are more difficult to handle with this type of interchange.
The Turban, the Braided and the All –directional multi level interchanges are particular types
with all right turning as well as left-turning movements made directly. Thus they fall into the category
known as directional interchanges. Directional interchanges are the highest type of interchanges that are
most suitable at the intersections of highways carrying high volumes of traffic. When compared with the
semi-direct loop types of interchange, they reduce vehicle travel distance, increase the speed of traffic
operation, have greater capacity, and more often than not eliminate weaving problems. Directional
interchanges are very expensive to construct because of the type and number of bridge structures
required. However, they need relatively small land areas and thus certain types can be used in urban
areas, provided the aesthetics of a multilevel bride structure are environmentally acceptable.
5.2.3 Multiway Junction:
The most common types of multiway grade-separated junction is the two-bride round-about. In
this type of grade separation, the main highway underpasses or overpasses an at-grade round-about
intersection. Vehicles enter and leave the main road on diagonal slip roads. The grade separated
roundabout has the great advantage that it can be adapted from an existing at-grade roundabout that is
overloaded. Roundabout interchanges require relatively small overall areas (eg. a three level roundabout
requires one-fifth the area of fully-directional, free-flow interchange) and requires less carriage way area
than many types of interchanges. However, the safety of the junction as a whole is limited by the manner
in which the intersections of slip road with the minor road are handled.
5.3 GENERAL GUIDELINES FOR DESIGN OF INTERCHANGES
The design of a junction with grade separation is a complex matter, and each has a unique
combination of problems arising from the need to satisfy traffic demand, existing highway networks,
topography, land use, and the physical, social and political environments. The problems are at their most
complex in urban areas. Stock solutions to major junction design needs are rarely possible and hence, an
in-depth investigation is essential for the design of intersections with grade separations. The following
points should be considered in the design of a junction with grade separation.
Requirements of traffic flows through the junction
General siting, depending on engineering, topographical, environmental, land acquisition and
operating and capital costs.
The minimum area of land required for grade-separation structure.
Of these the determination of the siting and configuration is probably the most complicated. The
points to be considered for deciding the location and configuration include.
Non-free-flow solutions should be considered before alternatives, which incorporate less
restraint. Non free flow means that the vehicles in the design year peak hour at the junction with
grade separation can expect to ‘yield’ and ‘hesitate in groups’ without extensive queuing. Non
free flow solutions generally have lower capital costs and land requirements.
Junctions with grade-separations should be designed to cater primarily for the major traffic flows.
Connectors for turning movements should normally be designed safely, economically and to a
lower speed than main line routs.
All turning movements of a minor nature should be accommodated, if it is reasonable to do so, to
retain flexibility.
Difficult sites should be avoided.
Learn from existing designs. It is recommended to visit the different types of interchanges and
get first hand information about the factors that influenced their selection.
5.4 GEOMETRIC DESIGN GUIDELINES:
The general requirements of design of intersections with grade separations are as follows:
5.4.1 Spacing:
Interchanges should be located as needed to discharge and receive local traffic effectively. The
spacing of arterial roads usually governs the spacing. To provide optimum frequency operation with
adequate weaving distance and sign placements, the average spacing of urban interchanges should not be
less than 3.2 km in sub urban areas not less than 6.4 km; and in rural areas not less than 12.9 km.
However, individual spacing of adjacent interchanges may vary considerably. In urban and suburban
areas the minimum distance between adjacent interchanges should desirably not less than 1.6 km and
never less than 0.8 km.
5.4.2 Ramps:
The term ramp includes all types, arrangements, and sizes of turning roadways that connect two
or more legs at an interchange. The components of a ramp include a terminal at each leg and a central
connecting roadway. Ramps can be classified broadly as one of the five types, viz., diagonal loop, semi
direct (jug handle), direct and cloverleaf, shown in Fig. 5.2. A diagonal ramp, shown in Fig 5.2 (a), may
be largely tangent or wishbone-shaped, depending on the angle of intersecting roadways. With the loop
pattern shown in Fig.5.2 (b), the right turning movement is made without an at-grade crossing of the
opposing through traffic. Instead, drivers making a right turn travel beyond the highway separation,
swing left, and turn through approximately 270 degrees to enter the other highway. With a semi direct
connection, Fig 5.2 (d), drivers making a right turn first swing away from the intended direction,
gradually reverse, and then directly-follow around and enter the road on the left. Each ramp is generally a
one-way roadway. Each ramp type can have a different shape according to traffic pattern, traffic volume,
design speed, topography, development, intersection angle and type of ramp terminal.
It is rarely feasible to provide ramps on which turning traffic can travel in the same range of
speeds as on through roads. Nevertheless, there should not be a great difference between the design speed
of the highway and the design speed of the connecting ramp. Table 5.1 gives the minimum and desirable
values for ramp design speeds as related to highway design speeds. Minimum stopping sight distances
should be available the ramps.
Ramp grades should be as flat as possible to minimize the driving effort in manoeuvering from
one road to another. The grades on ramps should be limited to a maximum of 4
Table 5.1 Speed, Horizontal Curvature and Sight Distance for Ramp Design
Notes: 1. The major highway design speeds of 80 km/ his appropriate for highways in urban areas.
2. The radius of curvature values have been worked out for a maximum super elevation of 7 per
cent.
Table 5.2 Length of Vertical Curve
Sl.
Design speed
(km/h)
Safe
stopping
sight
Length of vertical curve for
safe stopping sight distance
(m)
Absolute
minimum
length of
Particulars
Design values for major highway design speed of
For loop ramps80 km/hr 100 km/hr
Minimum Desirable Minimum Desirable Minimum Desirable
Ramp design
speed (km/h)
Radius of
curvature (m)
Stopping sight
distance (m)
40
60
45
50
90
60
50
90
60
65
155
90
30
30
25
40
60
45
No. distance
(m)
Summit
curve
Valley
curve
vertical
curve (m)
1 2 3 4 5 6
1.
2.
3.
4.
5.
6.
30
40
50
65
80
100
30
45
60
90
120
180
2.0A
4.6A
8.2A
18.4A
32.6A
73.6A
3.5A
6.6A
10A
17.4A
25.3A
41.5A
15
20
30
40
50
60
Notes: 1. ‘A’ in columns 4 and 5 is the algebraic difference in grades expressed as percentage.
2. Where the length given by columns 4 or 5 is less than that given in column 6, the latter
value should be adopted.
percent and in no case should it exceed 6 percent. The length of vertical curves for design speeds of 30
to 100 kmph are given Table 5.2.
Cross-section:
The ramp may be for one-way or two-way operation. If for two-way, divided type of cross-
section should be used with a minimum width of 1.2 m for the median.
The width of pavement to be provided for each way will depend on the design hour traffic
volume expected to use the ramp. The capacity for unidirectional flow given below will be helpful in
choosing the appropriate pavement width. The minimum width of shoulders should be 2 m of which at
least one metre should be paved. The shoulders should be properly delineated by means of pavement
markings different surfacing material, etc.
Pavement width Capacity, PCU’s/hour
1. Single lane, 3.75m wide
2. Intermediate lane, 5.5m wide
3. Two-lanes, 7.0m wide
1500
2000
2500
Note: The above capacity figures are for roads provided with one metre wide paved shoulders on either
side.
Rates of cross slope on ramps where super elevation is not required would be the same as for
though roadways. A common direction of cross slope is generally provided on ramps. Super elevation
on ramps should be the same as for through roadways.
5.4.3 RAMP TERMINALS:
The terminal of a ramp is that portion adjacent to the through traveled way, including speed
change lane, taper, approach nose, merging end and island. Ramp terminals may be at grade
intersections, as at diamond or partial clever leaf interchanges, or directional, where ramp traffic merges
or diverge from through traffic at flat angles. Speed change lanes can be of either the straight line taper or
the parallel type. The length of speed change lanes are given in Table 5.3. The taper for acceleration lane
is more flat.
Table 5.3 LENGTH OF SPEED-CHANGE LANES
Type of lane
Length including taper (m)
Desirable Minimum
Acceleration lane
Deceleration lane
250
120
180
90
The deceleration approach nose should be offset from the through lanes at least 3.6m. and the
gore area between the through lane and the ramp should be surfaced. The surfaced area should be tapered
from the nose to the edge of the traffic lane down stream from the nose. The nose should also be offset
from the travelled way of ramp a distance equal to the width of shoulder of the ramp. Fig 5.3 illustrates
exit and entrance terminals.
Distance between ramp terminals: Serving the numerous traffic generators along an urban freeway
may require frequent ramp terminals in close succession. In order to provide sufficient manoeuvering
distance is required between successive terminals. When the distance between the end of the taper on the
exit terminal is less than 300 m the speed change lanes should be connected to provide a continuous
auxiliary lane. When an exit ramp is followed by an entrance ramp, there should be 500 ft. between
terminals so that the drivers are not confronted with merging vehicles too soon after passing an exit area.
5.4.4 Control of access:
Access should be controlled throughout all portions of interchanges, just as for the highway
between interchanges. When interchange is on non controlled access highways, it is desirable to extend
the control of access for some distance beyond the ramp terminal.
5.4.5 Clearance:
Lateral clearance: For underpass roadways, desirably the full roadway width at the approaches should
be carried through the underpass. This implies that the minimum lateral clearance (ie the distance
between the extreme edge of the carriageway and the face of nearest support, whether solid abutment pier
or column) should equal the normal shoulder width. For overpass structures, the clearances are not that
critical as in the case of underpasses since the drivers do not generally get the feeling of obstruction. A
cross section with 225mm wide kerb construction, open type parapet will generally be suitable for most
cases.
Vertical Clearance:
Vertical clearance at underpass should be minimum 5.5m in urban areas, after making allowance
for any future raising/strengthening of the underpass roadway.
PLANNING IN RELATION TO TRAFFIC SAFETY
6.1 TREATMENT FOR PEDESTRIANS
Since pedestrians are more vulnerable to being involved in accidents, it is essential that adequate
consideration is given to their safety in the design of junctions. The main principle in the design of the
pedestrian facilities is to reduce pedestrian conflicts with vehicular traffic to the minimum. Some of the
pedestrian facilities are illustrated below.
6.1.1 Footpath (Side-Walk)
In order to be effective, the side-walks should be provided on both sides of the road and above the
level of the carriageway separated by non-mountable kerbs. Height of the kerb at the edge should,
however, not exceed the height of non-mountable kerbs, as this might otherwise detract pedestrians from
getting on to the side-walks.
The width of side-walks depends upon the expected pedestrian flows and could be fixed with the
help of guidelines given in Table 6.1, subject to a minimum width of 1.5 m.
Table 6.1 CAPACITY OF SIDE-WALKS
Width of side-walk (metre) Capacity in number of persons per hour
All in one direction In both directions
1.50
2.00
2.50
3.00
4.00
1,200
2,400
3,600
4,800
6,000
800
1,600
2,400
3,200
4,000
For side-walks in shopping areas, the width should be increased by 1 m, which is treated as the
“dead width”. In other situations where side-walks pass adjacent to buildings and fences, the dead width
can be taken as 0.5 m. For areas of heavy pedestrian activity such as bus stops, railway stations and
recreational area, the width of side-walk should be suitably increased to account for accumulation of
pedestrians.
6.1.2 Pedestrian crossings (cross-walks)
When the flow becomes heavier it is very difficult for the pedestrian to cross the street. In such a
situation it is very much necessary to provide formal pedestrian crossing. The crossings define the
pedestrian walking area. These can be either At-grade crossings or Grade-separated crossings.
Pedestrian cross-walks should be provided at all important intersections. Wherever possible the cross-
walks should be at right angles to the carriageway and properly marked so that the pedestrians are
subjected to minimum inconvenience. Cross-walks should not substantially increase the walking distance
of pedestrians. Adequate visibility, freedom from obstructions and sufficient space for waiting are other
important requirements for location of cross-walks.
AT-GRADE PEDESTRIAN CROSSINGS
At grade pedestrian crossings could be uncontrolled or controlled. Uncontrolled crossings are
those where the pedestrian cross-walk is marked by studs or paint line but not controlled by any system of
signals or a zebra form of crossing. The use of uncontrolled crossings should be limited to location where
both the vehicular and pedestrian volumes are extremely low and accident records do not indicate any
need to segregate vehicular traffic from cross pedestrian traffic.
CONTROLLED CROSSINGS
Controlled form of crossing is achieved normally through provision of zebra crossings. Once a
pedestrian is on a zebra crossing, he gains priority of movement with respect to vehicular traffic.
ZEBRA CROSSING
A zebra crossing is a clearly specified pedestrian track across the carriageway and is delineated
with the help of alternate black white strips. A zebra crossing must always be accompanied by ‘STOP’
line.
The width of zebra crossing must be adequate and should generally lie within a range of 2.0-4.0
m. For divided carriageways, the crossing should, as far as possible, proceed uninterrupted through the
median strip. In the event of the median strip being used as pedestrian refuge, adequate width of median
must be provided. In case of raised medians, such portion could be suitably depressed with curb height
not exceeding 15 cm.
Guard-rails in the vicinity of zebra crossing should be for sufficient length to deter pedestrians
from crossing the road at any arbitrary point along the road. Night tine visibility of zebra crossing is of
vital importance and this can be achieved through proper lightning of intersection area. Fig. 6.1 & 6.2
give layouts of zebra crossing at intersections controlled by channelisation and traffic signals
respectively.
Provision of zebra crossings at short intervals adversely affects the overall efficiency of traffic
operation in the area. The distance between two crossings should not be less than 150 m.
GRADE SEPARATED PEDESTRIAN CROSSINGS
Grade separated crossings are those where the pedestrians are required to cross the carriageway at
a level different from that of vehicular movement. Thus, the grade separated crossings may be in the
form of a pedestrian subway of a foot-over bridge across the road.
Provision of a grade separated pedestrian facility may be warranted at locations where one or
more of the following conditions exist.
Volumes of pedestrian and vehicular traffic are so large that insertion of an exclusive pedestrian
phase will increase the cycle time for traffic signals beyond 120 sec.
Vehicular traffic demands uninterrupted flow as associated with major arterial roads and
expressways.
Control at at-grade pedestrian crossing decisively fails to mitigate the problems of pedestrian-
vehicle collision.
The successful utilization of grade separated crossings depends, in order of importance, on directness
of route, ease of negotiation, interest of specific features, general environmental appeal and safety. From
this, it can be seen that a segregate crossing that is justified solely on the basis of being, for example, safe
and environmental appealing may be relatively little used. Even if the facility is clean, well designed and
well-illuminated, pedestrians will use provided that the route via the crossing is quicker than the surface
route. Thus before constructing a segregated crossing, proper studies should be carried out to ensure that
it meets the basic directional movements of potential users.
GENERAL DESIGN GUIDE LINES:
Foot-over bridges:
A foot over bridge is usually a straight bridge across the carriageway. Such bridges are provided
not far away from the intersection and across a straight portion of any approach arm. Where both land
and adequate funds are available, the layout geometry could be suitably designed to improve the
aesthetics of such a structure. The vertical clearance required below such over bridges is a minimum of
5m.
Subways
Even though subways are more costlier both in terms of initial construction cost and recurring
cost of maintenance than foot over bridges, they are more acceptable to users. The reason for this could
be that the energy expended in negotiating a subway is much less. Subways, if properly maintained,
could be source of revenue.
It is desirable to adopt a rectangular subway section. The minimum width of pedestrian subway
is 2.5 m and vertical clearance should not be less 2.5 m. It is preferable to increase the width, if possible,
up to 6 m. At locations of large volumes of pedestrian traffic, the width should be fixed on capacity
criteria. Capacity of grade separated crossing can be taken 50 persons per minute per metre width on the
level or up to 1 in 20 gradient; and 35 persons per minute per metre width on steps or ramps over 1 in 20
gradient. If display boards, etc; are provided, the width of subway should be increased suitably to
account for dead-width.
The floor of a subway should normally be cambered to fall to channels on each side at a slope of
1 in 30. Gullies should be provided to trap water entering from the ramps or steps. The entire subway
must be kept properly illuminated and care should be taken that it is not misuse, particularly in the night.
6.2 TREATMENT FOR CYCLISTS:
The presence of cycles in the traffic presents additional problems at intersections. About two-
thirds of accidents involving cyclists occur at or near intersections. So it is desirable to segregate cycle
traffic at intersections also. Some of the methods of segregating cycle traffic are given below:
6.2.1 No Segregation
In this method the approaches to an intersection do not have separate cycle tracks but keep to the
left while reaching the junction. Fig. 6.3 shows the conflict area created when taking right turn.
6.2.2 Lane System
About 80 metres before the intersection, cyclists are required to take proper lanes and travel to cross
the junction. However, the cyclists usually mix with fast moving traffic and move with them thus
loosing the effectiveness Fig. 6.3 (b) illustrates this.
6.2.3 Pocket Lane System
Separate pocket lanes are provided next to the respective lane vehicular traffic and marked
exclusively from them as shown on Fig. 6.3 (c).
6.2.4 Separate Track System
In this method cycle tracks are provided on either side of the roads right up to the intersection.
The conflict however, remains same as in no segregation method Fig. 6.3 (d).
Sometimes, the right turning cyclists are required to move to right turning traffic lane 80 m ahead
of the intersection as in the case of lane system, or as in case of pocket lanes.
6.2.5 Pedestrian Track System
In this method cycle tracks are provided parallel to the pedestrian crossings. The right turning
cyclists will first move across the road A and after waiting, if necessary in the storage space ‘A’ provided
at the corner of the intersection cross road B. Fig. 6.3 (e) illustrates this system.
The cyclist avoid conflict with left turning traffic at many places. This method has gained
considerable popularity.
6.2.6 Exclusive Cycle Tracks
When the cycle traffic becomes excessive all the above methods do not give good results. In
such cases, cycle traffic is separated by grade separation. Two such forms are shown in Fig. 6.3 (f) & 6.3
(g)
6.2.7 DESIGN CRITERIA
If the volumes of bicycle and moped traffic exceeds 250/day and motor vehicles traffic reaches
2000 a day, special tracks are always desirable. The following data may be used for the design.
Design speed 25 km/hour
Clearance 2.5 m
Width of path 1.2 m
Maximum grade 4%
6.2.8 Advanced Stop-Lines for Cyclists:
At signalized intersections, a separate lane for cycle traffic along with a reservoir space between
pedestrians crossing zone and stop line for motorized vehicles provides an efficient regulation. Fig. 6.4
shows the details of such layout. A separate cycle track of 1.2 m width should be provided in the
approaches of the intersection. There would be two stop lines, one for the motor vehicles and the other
for cyclists, such that the cyclists can wait in the reservoir space, till the signal turns green. The signs to
be provided and their locations are illustrated in Fig 6.4.
6.3 Traffic Signs at Intersections
Traffic signs are very much useful in guiding and controlling the traffic approaching the
junctions. Some of the signs useful at intersections are given in Fig. 6.1. While providing traffic signs at
intersections the following principles should be followed to ensure the effectiveness of signs.
should be conspicuous and easily visible both during day and night.
should contain only essential information.
should be placed so that they are obscured as little as possible.
should be located so that the drivers have sufficient time to take necessary action with safety.
sign lettering should be legible.
too many signs with inadequate spacing become totally ineffective.
should be installed only by the authority of law.
should not cause obstruction to pedestrian or vehicular traffic.
In urban areas the lowest edge of any traffic sign should not be lower than 2.1m from the pavement
when posted on footpaths or sidewalks. In rural areas the clear height of sign from pavement should be
1.5m. The nearest edge of sign should be at least 1.2m away from the edge of the road. When posted on
raised footpaths the same should be away by at least 30 cm from the edge of kerb.
6.4. Markings at intersections:
Carriageway markings within and in the neighborhood of an intersection restrict vehicles from
using areas other than those actually meant for them and thus ensure orderly movement of traffic.
Depending on the actual intersection conditions, markings of intersections may consist of one or more of
the following:
(i) Centre line markings
(ii) Lane markings
(iii) Turn markings
(iv) Direction markings
(v) Approach to island markings
(vi) Kerb markings.
Some of the markings of intersections are illustrated in Fig. 6.6. In choosing the type of marking,
discretion should be exercised judiciously, taking into consideration the conditions at site.
6.5 ONE-WAY STREETS
With one-way street operation motor vehicle movement on any given carriage-way within the
system is limited to one-direction.
6.5.1. Advantages of one-way streets:
The following are the advantages of one-way operation:
Simple and economical tool for relief of traffic congestion.
Reduction in conflict points and thus accidents.
Capacity of the street can be increased from zero to 100 per cent.
More convenient and less confusing for motorists to drive.
Odd lanes, which could not be utilized under two-way working, can be fully utilized under one-
way operations.
Slow moving or stationary vehicles are easily overtaken.
Traffic congestion at busy intersections is greatly reduced due to elimination of some right
turning movements and any extra road width can now be used efficiently to speed vehicle
movements.
Results in efficient signal timing giving rise to smoother traffic flow, reduction in stops and air
pollution.
Results in economic saving due to reduction of need for police control at congested intersections
and streets.
Accidents due to bad road lighting and glare problems are reduced.
Reduction in parking problem
6.5.2 Disadvantages of One-way Streets:
Increased travel distance, since it is necessary for motorist to travel further to reach their
destinations.
It may some times involve vehicles using residential streets with consequent loss of immunity to
the surroundings.
Introduction of the system is often opposed by local trading interests who fear that business will
be detrimentally affected.
There will be an increase in severity for accidents due to heavy impact of faster moving vehicles.
It may result in confusion to motorists, particularly just after the scheme has been introduced.
6.6 REROUTING OF RIGHT TURNING TRAFFIC:
Many times right turning vehicles cause a disproportionate loss of capacity at intersection and
result in inordinate delays. Usually the heavy right turning traffic is accommodated by adjusting signal
timings to provide more green time either by an extra phase in the cycle or early cut-off or late start
facility. However, in many instances it is better to ban and reroute the right turning traffic, rather than
providing directly. Such alternative routing procedures which allow right –turn traffic to complete their
man oeuvre without actually making a right turn are given below:
6.6.1 T-Turn:
Diversion of right turning traffic to an intersection further along the road where there is more
capacity. The routing is more useful for dealing with a difficult right turn from a minor road on to a
major road; then right turning movement thus takes place at a minor intersection. This illustrated in Fig.
6.3 (a)
6.6.2 G-Turn
In this, the right turning traffic is diverted to the left before the congested intersection. This is
particularly suitable for a right turn from a major road. Fig. 6.3 (b) illustrates this.
6.6.3 Q-Turn
This is a diversion to the left beyond the intersection, which requires 3 left turns. This is
illustrated in Fig. 6.3 ©
6.6.4 Limitations of Rerouting
1. Difficulty for finding alternate routes, which are suitable for traffic movements.
2. Improper signing of the diversions causes confusion to drivers.
6.7 Safety Related Improvements
Presented in the following pages are some of the safety conscious design principles collected by
Ross (1986). He had drawn material from his direct experience on a number of safety related projects in
many developing countries Bahrain, Jordan, Korea, Nigeria, Kenya, Trinidad and others.
Illustrations of Safety Conscious Design
Undesirable practice Desirable practice Comment
By-passes reduce congestion
and improve Traffic Safety
Grid layouts should be
avoided wherever possible so
that road functions can be
separated
Rural accident rates are
lower on roads with large
radius curves.
Reduce the number of U-turn
facilities and consolidate into
a limited number of protected
turns.
Reduction in the number of
accesses onto main roads
reduces accidents.
Reduce “gore” area to
simplify decisions.
Simplification of
complicated traffic situations
will tend to increase traffic
safety.
Traffic islands at crosswalks
increase pedestrian safety by
simplifying crossing task.
Staggered crossroads allied
to pedestrian railings give
average accident reductions
of around 25%.
Maintaining shoulders in
good condition to avoid
dangerous edges between the
carriageway and the
shoulder.
Introduction of stop sign and
priority.
Adjusting, intersection layout
to achieve right angled
intersections is an effective
countermeasure. If necessary
traffic island can be placed in
the secondary road.
Channelisation with small
island in the secondary roads
will help to focus on the
give-way situation.
Channelization of the main
road at intersections with
high right turning traffic
volumes will reduce right
turn and rear end accidents.
Reconstruction of a 4-way
intersection to 2 3-way
intersections will reduce
conflict movements and
increase safety.
Small roundabouts with
priority give-way to entering
traffic reduces accidents and
increases capacity.
Average accident reduction
30-40%
Improving visibility by
removing trees and bushes
Road lighting at locations
with high pedestrian and/or
bicycle traffic, or road
section with a high
proportion of night-time
vehicle accidents.
Bus-stops located beyond the
intersection will avoid
pedestrian crossing in front
of the bus
SELECTED BIBLIOGRAPHY
Babkov, V.F, “Road Conditions and Traffic Safety”, Mir Publishers, Moscow, 1975.
George, R (1988), Junction improvement Proposals for Calicut City - A viable T S M Strategy,
Unpublished M.Tech thesis submitted to University of Calicut, Calicut.
Homburger, W.S. (Ed.), “Transportation and Traffic Engineering Handbook”, 2nd Edition, Prentice Hall,
N.J. 1982.
“Hyderabad Area Transportation Study, (HATS)”, Progress Report No:2, Centre for Transportation
Engg., Regional Engineering College, Warangal.
Indian Roads Congress (IRC), “Recommended Practice for the Cite Design &Layout of Cycle Tracks”
IRC: 11-1962, New Delhi, 1975.
Indian Roads Congress (IRC), “Recommended Practice for Traffic Rotaries”, IRC: 65-1976, New Delhi
1976.
Indian Roads Congress (IRC), “Guidelines on Design and Installation of Road Traffic Signals”, IRC: 93-
1985, New Delhi 1985.
Indian Roads Congress (IRC), “Guidelines for the Design of Interchanges in Urban Areas”, IRC: 92-
1985, New Delhi, 1985.
Indian Roads Congress (IRC), “Guidelines for Pedestrian Facilities”, IRC: 103-1988, New Delhi 1989.
Indian Roads Congress (IRC), “Guidelines for the Design of At Grade Intersections in Rural and Urban
Areas”, Special Publication 41, IRC, New Delhi 1994.
Kadiyali, L.R. “Traffic Engineering & Transport Planning”, Khanna Publishers. Delhi.
Khanna, S.K. & Justo, C.E.G. “Highway Engineering”, Nemchand & Brothers, Roorkee, 1991.
Khisty, C.J. “Transportation Engineering – An Introduction” Prentice Hall, N.J, 1990.
O. Flaherly, C.A, “Highways - Traffic Planning & Engineering”, Edward Arnold. U.K, 1986.
Papacostal, C.S. “Fundamentals of Transportation Engineering”, Prentice Hall of India, New Delhi, 1990
Ross, A (1986) “Improving Road safety via Traffic Engineering in developing countries”, proceeding of
International Conference on Transportation System studies, 1986, Tata Mc-Graw Hill Publishing
Company Ltd., New Delhi. PP. 502-509.
Salter, R.S. “Highway Traffic Analysis & Design”, ELBS/ Macmillan, U.K. 1990.