chapter 1 - traffic volume

7/22/2019 Chapter 1 - Traffic Volume

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Chapter 1

Traffic Volume and CapacityThe design of a road should be based on traffic volume that the road will have to

accommodate.1.1 Design Volume: The usual design control is the design volume, which is the

estimated traffic volume at a certain future year, the design year, taken as 10 to 20

years. For geometric design of roads, Road Development Authority adopts a design

year of 20 years to the future.

The measures of traffic volume on a road are:

(1)Average Annual Daily Traffic (AADT), which is the total

traffic volume for the year divided by 365.

(2)Average Daily Traffic (ADT), which is the total volume in

both directions during a given time period divided by the

number of days in that time period

Traffic counts: RDA carries out traffic counts using automatic recorders over a 7 day

period to obtain ADT volumes and hourly traffic volume during this period.

Manual traffic counts are carried out during the daytime for 12 hours from 6.00am to

6.00pm or during a 16 hours period from 6.00am to 10.00pm to obtain the vehicle

composition of the traffic volume obtained from automatic recorder counts.

Origin Destinations Surveys: O-D Surveys are carried out to determine the travel

pattern so that when a new road is constructed or a road improvement is planned, a

traffic assignment could be done to estimate the traffic that will be diverted to the newor improved facility.

Traffic forecasting: An accurate forecast of future traffic during the design life is

necessary to determine the standards that should be adopted for a particular design.

The forecasting of future traffic is based on:

1. Normal traffic growth on a road due to the increase in

population and resulting socio-economic activity of the

population served by the road

2. Generated traffic, that is the traffic generated as a result of new

development planned for the area or the development that takes

place as a result of provision of new or improved road

3. Diverted traffic, that is traffic that is diverted from other roads

to the new or improved road

1.2 Design Hour Volume (DHV) is the projected hourly volume used for design.

This volume is taken as a percentage of the expected ADT on the highway. On major

roads carrying relatively heavy traffic volumes throughout the year, hourly traffic has

to be used to determine the design volume. However, it will not be economical to use

the maximum peak hour traffic in the design year because this traffic volume will

occur only during a few hours of the year.

Fig 1.1 shows the variation of hourly volume, expressed as a percentage of the ADTthroughout the year. It can be observed that the highest hourly volume occurred was


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about 18% of the ADT. Examination of the curve between 0 and about 30 thhighest

hours, shows that a small increase in the number of hours results in a significant drop

in the percentage of ADT. Beyond 30thhour, a large increase in the number of hours

results in only a small change in the ADT. This characteristic of the curve has led to

the decision of using the 30thhighest hourly volume as the design hourly volume.

In motorized countries it has been found that the 30 thhighest hourly volume is 12% to

15 % of the ADT. However, analysis of traffic in Sri Lanka has confirmed that the

30thhighest hourly volume is 8% to 9% of ADT. Therefore, for design purpose DHV

is taken as 8% - 9% of ADT.

It has also been shown that the 30thhighest hourly volume as a percentage of ADT

varies only slightly from year to year, even when significant changes of ADT occur.

However, the 30th hour volume should not be indiscriminately used as the Design

Hourly Volume on roads having unusual or high seasonal fluctuation in the traffic

flow

Variation of two-way hourly volume

During the year

0

5

10

15

20

25

0 100 200 300

Hours of the year

Hourly

volume

(%ofADT)

%PHV

Fig. 1.1 Variation of Hourly Volume

1.3 Equivalent Passenger Car Units (PCU)

Traffic on a road is composed of different type of vehicles such as passenger cars,

trucks, buses, vans, motorcycles, bicycles and bullock carts etc. For design purpose,

the various types of vehicles are converted to equivalent passenger car units (PCU).

PCU for a particular type of vehicle is the equivalent number of passenger cars that

will produce the

same effect as that produced by that particular vehicle. It depends on the type of

vehicle, type of terrain, type of the carriageway, the level of service and speed.

30thhighest hourly volume

30


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Equivalent Passenger Car Unit values used by the Road Development Authority are

given in table 2.1 and 2.2

Table 1.1 Equivalent Passenger Car Units for two-lane two-way roads

PCU FactorVehicle typeFlat terrain Rolling terrain Mountainous

terrain

Passenger Car 1.0 1.0 1.0

Small Bus 2.0 3.4 6.0

Large Bus 2.2 5.0 10.0

Light Truck 2.0 4.0 7.0

Medium truck 2.2 5.0 10.0

Heavy truck 2.2 5.0 10.0

Motorcycle 0.5 0.5 0.5

Bicycle 1.0 1.0 1.0

Animal drawn cart 4.0 10.0 24.0

Source: Road Development Authority, Planning division

Table 1.2 Equivalent Passenger Car Units for Multi-lane Roads

PCU FactorVehicle type

Flat terrain Rolling terrain Mountainous

terrain

Passenger Car 1.0 1.0 1.0

Small Bus 1.5 3.0 5.0Large Bus 1.7 4.0 8.0

Light Truck 1.7 4.0 8.0

Medium truck 1.7 4.0 8.0

Heavy truck 1.7 4.0 8.0

Motorcycle 0.5 0.5 0.5

Bicycle 1.0 1.0 1.0

Animal drawn cart 4.0 10.0 24.0

Source: Road Development Authority, Planning division

1.4 Capacity Analysis

Capacity Analysis is a set of procedures used to estimate the traffic carrying capacity

of facilities over a range of defined operational conditions.

Capacity is defined as the maximum hourly rate at which vehicles (or persons) can

reasonably be expected to traverse a point or a uniform section of a lane or roadway

during a given time period under prevailing roadway, traffic and control conditions.

Highway Capacity Manual published by the Transport Research Board, USA, is used

by many road authorities around the world to design and plan highways. The

recommendations in this manual should be judiciously applied for designing newhighways or improvement to existing highways in Sri Lanka as the conditions


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prevailing in Sri Lanka with respect to roadway, traffic and driver behaviour are

significantly different to assumptions made in the Highway Capacity Manual.

1.5 Level Of Service(LOS)

Level of Service is defined as a qualitative measure describing operational conditionswithin a traffic stream and their perception by the motorists and/or passengers. A level

of service generally describes these conditions in terms of such factors as speed and

travel time, freedom to maneuver, traffic interruptions, comfort, convenience and

safety, defined in terms of density.

Six Levels of Service have been defined from LOS-Athrough LOS-F

LOS-A: Free flow, unaffected by other users. Free to select desired speed, excellent

level of comfort

LOS-B: Stable flow, but the presence of other vehicles begins to be noticeable.

LOS-C: Stable flow, but operation of individual user is significantly affected by the

interaction with othersLOS-D: Represents high density but still stable flow. Speed and maneuverability is

severely restricted, poor level of comfort and convenience

LOS-E: Operating conditions are at or near capacity level, all speeds are reduced to a

low but relatively uniform value, freedom to maneuver within the traffic stream is

extremely difficult

LOS-F: Forced or breakdown flow, queues form, stop and go situation, inflow

exceeds outflow

Level of Service, A to F apply to uninterrupted flow.

Note: Uninterrupted flow facilities have no fixed elements such as traffic signals

external to the traffic system that cause interruption to traffic flow. Traffic flow is

influenced only by the presence of other vehicles in the traffic stream and the

geometric and environmental characteristic of the highway.

1.6 Service Flow Rate

Service Flow rate is the maximum rate of flow which can be accommodated by

various facilities at each level of service A to E. It is the maximum hourly rate under

prevailing conditions while maintaining the designated level of service.

Prevailing Roadway, Traffic and Control Conditions

Roadway Conditions: refer to the geometric characteristics of the highway or street

including the type of facility (e.g. freeway, multi-lane, two-lane highway, signalized

intersection, un-signalized intersection etc), its development environment (e.g. urban,

suburban or rural), the number of lanes, lane widths and shoulder widths, lateral

clearance from obstructions (e.g. lamp posts, road signs and guard rails), design speed

and horizontal and vertical alignment.


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Traffic Conditions: refer to characteristics of the traffic stream using the facility. This

is defined by the distribution of the vehicle types (e.g. trucks, buses, passenger cars)

and amount and distribution of vehicles in lanes and direction.

Control Conditions: refer to type and specific design of control devices and traffic

regulators present (e.g. traffic signals, no passing zones, STOP and YIELD signs, turnrestrictions etc.)

Note: Capacity refers to rate of vehicle or persons flow during a specified period of

interest, which is taken as a peak 15 minutes period.

1.7 Capacity Design of Two-Lane Highways

A two-lane highway is a two-lane road having only one lane for use by traffic in each

direction. Passing of slower vehicles require the use of opposing lanes when sight

distances and gaps in opposing traffic stream permit.

As volume increases the ability to pass decreases and results in formation of platoons

in the traffic stream.

Three parameters are used to describe service quality for two-lane highways.

1. Average Travel Speed

2. Percent Time Delay

3. Capacity Utilization

Average Travel Speed: reflects the mobility function of two-lane highways and is the

length of the highway segment under consideration divided by the average travel time

of all vehicles traversing the segment over some designated interval of time.

Percent Time Delay: reflects both mobility and access functions and is defined as the

average percent of time that all vehicles are delayed while traveling in platoons due to

their inability to pass. Percent time delay is difficult to measure directly in the field.

The percent of vehicles traveling at headways less than 5 seconds can be used as a

surrogate measure in field studies.

Capacity Utilization: reflects the access function and is defined as the ratio of the

demand flow rate to the capacity of the facility.

Design computations cannot be readily performed for two-lane highways because the

number of lanes is fixed. Modifications to grade and alignment however could

improve the operational efficiency.

Two traffic stream characteristics, the average travel speed and percent time delay are

used as operational measures describing quality of service provided to motorists on a

two-lane highway.

Speeds of about 80 kmph are usual on rural two-lane highways in level terrain. But

due to restrictions of road widths, shoulder widths and other deficiencies in road

geometry, safe speeds are much lower on Sri Lanka roads.


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1.8 Ideal Conditions for Two-Lane Highways

1. Design speed is greater than or equal to 96 kmph

2. Lane width greater than or equal to 3.7 m

3. Clear shoulder width greater than or equal to 1.8 m

4. No no passing zones in the highway5. Only passenger cars in the traffic stream

6. A 50/50 directional distribution of traffic

7. No impediments to through traffic due to traffic control or

turning vehicles

8. Level terrain

The capacity of a two-lane highway under ideal conditions is 2800 pcph total in both

directions.

A no passing zone is defined as one marked for no passing or any road section

where the passing sight distance is 450m or less.

1.9 LOS criteria for two-lane highways

LOS criteria for two lane highways address both mobility and accessibility concerns.

The primary measure of service quality is the percent time delay. Speed and capacity

utilization are secondary measures.

LOS criteria are defined for peak 15 minutes flow periods applicable for sections of

significant length.

LOS-A: The highest quality of traffic service occurs when motorists are able to drive

at their desired speed without strict enforcement. This would result in average speeds

approaching 96 kmph on two lane highways. Passing demand is well below passing

capacity and no platoons of 3 or more vehicles are observed. Drivers are not delayed

by more than 30% of time by slow vehicles. A maximum flow rate of 420pcph total in

both directions may be achieved under ideal conditions.

LOS-B: Traffic flow speeds of about 85 kmph are expected in level terrain. Passing

demand becomes significant and equal to passing capacity. Drivers are delayed upto

45% of time. Service flow rate of 750 pcph total in both directions can be expected.

Number of platoons begins to increase dramatically.

LOS-C: Noticeable increase in the number of platoons formation, platoon size and

frequency of passing impediments are observed. Average speeds still exceed 83 kmph

in level terrain. Traffic flow is stable but is susceptible to congestion due to turning

and slow moving traffic. Percent time delay up to 60%, Service flow rate of 1200

pcph total in both directions can be expected.

LOS-D: Unstable flow is approached. Passing becomes extremely difficult. Passing

demand is high. Passing capacity approaches zero. Platoon size of 5 to 10 vehicles can

be observed. Speed of 80 kmph can still be achieved under ideal conditions. Turning

vehicles and roadside distractions cause major shockwaves in the traffic stream.

Percent time delay approaches 75%. Maximum flow rate of 1800 pcph can beobserved.


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LOS-E: Defines a traffic flow condition on two-lane highways having a percent time

delay of more than 75% under ideal conditions. Speeds will drop to 80 kmph. Passing

is virtually impossible. Platooning becomes intense. Under ideal conditions the

capacity is 2800 pcph in both directions.

LOS-F: represents a heavily congested condition with traffic demand exceeding

capacity. Volumes are lower than capacity, speeds are lower than capacity speed.

1.10 Directional Split of Traffic

Ideal condition for two-lane roads is when the directional split of traffic is 50/50.

When the direction split moves away from the ideal, capacity decreases as shown in

the table below.

Directional split Total capacity Ratio of capacity

to ideal capacity50/50 2800 1.00

60/40 2650 0.94

70/30 2500 0.89

80/20 2300 0.83

90/10 2100 0.75

100/0 2000 0.71

1.11 Determination of Level of Service

LOS of a given facility under existing conditions or projected traffic demand can bedetermined as explained below. The analysis is based on the flow rates for a peak 15

minutes period within the hour of interest, which is usually the peak hour.


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Table 1.3 Level of Service criteria for general two-lane highway segments

v/c ratio ( ratio of flow rate to an ideal capacity of 2800 pcph in both directions

Level terrain Rolling terrain Mountainous terrain

Percent no passing zones Percent no passing zones Percent no passing zonesLOSPercentTime

DelayAvgb

Speed 0 20 40 60 80 100

Avgb

Speed 0 20 40 60 80 100

Avgb

Speed 0 20 40 60 80 100

A 30 !94 0.15 0.12 0.09 0.07 0.05 0.04 !92 0.15 0 .10 0.07 0.05 0.04 0.03 !90 0.14 0.09 0.07 0.04 0.02 0.01

B 45 !89 0.27 0.24 0.21 0.19 0.17 0.16 !87 0.26 0 .23 0.19 0.17 0.15 0.13 !87 0.25 0.20 0.16 0.13 0.12 0.10C 60 !84 0.43 0.39 0.36 0.34 0.33 0.32 !82 0.42 0 .39 0.35 0.32 0.30 0.28 !79 0.39 0.33 0.28 0.23 0.20 0.16

D 75 !81 0.64 0.62 0.60 0.59 0.58 0.57 !79 0.62 0 .57 0.52 0.48 0.46 0.43 !73 0.58 0.50 0.45 0.40 0.37 0.33

E > 75 !73 1.00 1.00 1.00 1.00 1.00 1.00 !65 0.97 0 .94 0.92 0.91 0.90 0.90 !57 0.91 0.87 0.84 0.82 0.80 0.78

F 100 < 45 - - - - - -


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Computer Aided Geometric Design of Highways

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Table 1.4 Adjustment factors for directional distribution (fd)

for two-lane highways

Directional

Distribution100/0 90/10 80/20 70/30 60/40 50/50

Adjustment

factor0.71 0.75 0.83 0.89 0.94 1.0

Source: Table 8-4 Highway Capacity Manual, Transport Research Board, National Research Council, Washington DC, USA

Table 1.5 Adjustment factor for combined effect of Narrow lanes

And restricted shoulder width (fw)3.7 m lanes 3.4 m lanes 3.0 m lanes 2.7 m lanesUsable

shoulder

width

(m)

LOS

A-D

LOS

E

LOS

A-D

LOS

E

LOS

A-D

LOS

E

LOS

A-D

LOS

E!1.8 1.0 1.0 0.93 0.94 0.84 0.87 0.70 0.76

1.2 0.92 0.97 0.85 0.92 0.77 0.85 0.65 0.740.6 0.81 0.93 0.75 0.88 0.68 0.81 0.57 0.700 0.70 0.88 0.65 0.82 0.58 0.75 0.49 0.66

Note: where shoulder width is different on either side of the road use the average width


Table 1.6 Average Passenger Car Equivalent for Trucks and Busses

on two-lane highways over different terrain segments

Type of Terrain

Vehicle Type LOS Level Rolling Mountainous

A 2.0 4.0 7.0

B & C 2.2 5.0 10.0Trucks ETD & E 2.0 5.0 12.0

A 1.8 3.0 5.7

B & C 2.0 3.4 5.7Buses EBD & E 1.6 2.9 6.5



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Table 1.7 Values of v/c ratiosavs Speed, Percent Grade and

Percent no passing zones for specific grades

Percent no passing zonesPercent Grade

AverageUpgrade Speed (Kmph) 0 20 40 60 80 100

89 0.27 0.23 0.19 0.17 0.14 0.12

85 0.42 0.38 0.33 0.31 0.29 0.27

81 0.64 0.59 0.55 0.52 0.49 0.47

73 1.00 0.95 0.91 0.88 0.86 0.84

69 1.00 0.98 0.97 0.96 0.95 0.94

65 1.00 1.00 1.00 1.00 1.00 1.003

89 0.25 0.21 0.18 0.16 0.13 0.11

85 0.40 0.36 0.31 0.29 0.27 0.25

81 0.61 0.56 0.52 0.49 0.47 0.45

73 0.97 0.92 0.88 0.85 0.83 0.81

69 0.99 0.96 0.95 0.94 0.93 0.92

65 1.00 1.00 1.00 1.00 1.00 1.00

4

89 0.21 0.17 0.14 0.12 0.10 0.08

85 0.36 0.31 0.27 0.24 0.22 0.20

81 0.57 0.49 0.45 0.41 0.39 0.37

73 0.93 0.84 0.79 0.75 0.72 0.70

69 0.97 0.90 0.87 0.85 0.83 0.82

65 0.98 0.96 0.95 0.94 0.93 0.92

57 1.00 1.00 1.00 1.00 1.00 1.00

5

89 0.12 0.10 0.08 0.06 0.05 0.04

85 0.27 0.22 0.18 0.16 0.14 0.13

81 0.48 0.40 0.35 0.31 0.28 0.26

73 0.49 0.76 0.68 0.63 0.59 0.55

69 0.93 0.84 0.78 0.74 0.70 0.67

65 0.97 0.91 0.87 0.83 0.81 0.78

57 1.00 0.96 0.95 0.93 0.91 0.9049 1.00 0.99 0.99 0.98 0.98 0.98

6

89 0.00 0.00 0.00 0.00 0.00 0.00

85 0.13 0.10 0.08 0.07 0.05 0.04

81 0.34 0.27 0.22 0.18 0.15 0.12

73 0.77 0.65 0.55 0.46 0.40 0.35

69 0.86 0.75 0.67 0.60 0.54 0.48

65 0.93 0.82 0.75 0.69 0.64 0.59

57 1.00 0.91 0.87 0.82 0.79 0.767

49 1.00 0.95 0.92 0.90 0.88 0.86

aRatio of flow rate to ideal capacity of 2800 pcph, assuming car operation is unaffected by grade

Note: Interpolate for intermediate values of percent no passing zones, round Percent grade to the next higher integer value


CE309-Highway Design


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Table 1.8 Adjustment Factors for Directional Distribution

For specific grades (fd)

Percent of Traffic

on Upgrade

Adjustment factor

(fd)100 0.58

90 0.64

80 0.70

70 0.78

60 0.87

50 1.00

40 1.2030 1.50



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CE309-Highway design

Table 1.9 Passenger Car Equivalent for Specific Grades

on two-lane highways E and E0Average Upgrade Speed (kmph)Grade % Length of grade

Meters 73 64 48

0 All 1.4 1.3 1.3

400 1.7 1.6 1.5

800 2.0 1.8 1.7

1200 2.3 2.0 1.9

1600 2.6 2.3 2.1

2400 3.4 2.9 2.5

3200 4.6 3.7 2.9

4800 7.3 5.6 3.8

3

6400 11.3 7.7 4.9

400 1.8 1.7 1.6

800 2.2 2.0 1.91200 2.7 2.3 2.1

1600 3.2 2.7 2.4

2400 4.7 3.8 3.1

3200 6.9 5.3 3.8

4800 12.5 9.0 5.5

4

6400 22.8 13.8 7.4

400 2.0 1.8 1.7

800 2.5 2.2 2.0

1200 3.1 2.7 2.4

1600 4.0 3.3 2.8

2400 6.3 4.9 3.8

3200 10.2 7.5 4.84800 22.0 14.6 7.8

5

6400 55.0 25.0 11.5

400 2.1 1.9 1.8

800 2.8 2.4 2.2

1200 3.7 3.1 2.7

1600 4.9 4.0 3.3

2400 8.5 6.4 4.7

3200 15.3 10.7 6.3

4800 38.0 23.9 11.3

6

6400 90.0 45.0 18.1

400 2.2 2.0 1.9800 3.2 2.7 2.4

1200 4.3 3.6 3.0

1600 6.1 4.8 3.8

2400 11.5 8.4 5.8

3200 22.8 15.4 8.2

4800 66.0 38.5 16.1

7

6400 A a 28.0

a - Speed not attainable on grade specified. Note: Round percent Grade to next higher integer value



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V

v = PHF

where, v = flow rate for 15 minutes total flow for both directions in vph

V = full hour volume total for both directions in vph

PHF = Peak Hour Factor

Service flow rate for two-lane roads on general terrain segments is given by the equation

SFi = 2800 x (v/c)i x fd x fwx fHV

where, SFi is the total service flow rate in both directions for prevailingroadway and traffic conditions for level of service i in vph

(v/c)i is the ratio of flow rate to ideal capacity for level of service i obtainedfrom table 2.3

fd is the adjustment factor for directional distribution of traffic obtained from

table 2.4

fw is the adjustment factor for narrow lanes and restricted shoulder widths

obtained from table 2.5

fHV is the adjustment factor for the presence of heavy vehicles in the traffic

stream where,fHV = 1/ [1+ PT(ET 1) + PB(EB 1) ]

and, PT = Proportion of trucks in the traffic stream

expressed as a decimal

PB = Proportion of buses in the traffic stream

expressed as a decimal

ET = Passenger Car Equivalent for trucksObtained from table 2.6

EB = Passenger Car Equivalent for buses

Obtained from table 2.6

Note: General terrain segment is a section of road where the gradient is not more than 3% for

any length less than 1600 meters and where the gradient is more than 3%, the length of such

grade is not more than 800 meters.


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The effect of grade on two-lane roads is more severe than on multi-lane roads because of the

need to use the opposing traffic lane for passing. When the traffic volume increases passing

will become more difficult and even when there are no heavy vehicles in the traffic stream, the

effect of the grade is experienced by passenger cars.

Service flow rates on two-lane road segments on specific grades are adjusted for any given

upgrade speed, by applying an adjustment factor fg.

Service flow rate for two lane roads on specific grade segments for any upgrade speed is given

by the equation:

SFi = 2800 x (v/c)i x fd x fwx fg x fHV

and, fg= 1/ [ 1+ ( PpIp) ]

fHV= 1/ [ 1+ PHV( EHV1 )

Ip= 0.02 ( E E0)

EHV = 1+ ( 0.25 + PT/HV)( E 1)

where, SFi = Service flow rate for LOS i or speed i, total vph in both directions for

prevailing roadway and traffic conditions

(v/c)i = volume/capacity ratio for LOS i or speed i obtained from table 2.7

fd = adjustment factor for directional distribution of traffic (table 2.4)

fw = adjustment factor for narrow lanes and restricted shoulder widths

(table 2.5)

fg = adjustment factor for the effects of grades on passenger cars

fHV = adjustment factor for presence of heavy vehicles in the upgradetraffic stream

Pp= proportion of passenger cars in the upgrade traffic stream, expressed

as a decimal

Ip= impedance factor for passenger cars

PHV= total proportion of heavy vehicles in the upgrade traffic stream

EHV= passenger car equivalent for specific mix on heavy vehicles present

in the upgrade traffic stream

E = base passenger car equivalent for a given percent of grade and a

given speed selected from table 2.9

E0 = base passenger car equivalent for zero percent grade and a given


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upgrade speed selected from table 2.9

PT/HV= proportion of trucks among heavy vehicles

Note: Passenger Car Equivalent values in table 3.7 is from the Highway Capacity Manual and

represents an average mix of trucks, recreation vehicles and buses in the traffic stream. The

average mix is 14% trucks, 4% recreation vehicles and no buses. In Sri Lanka, the vehicle mix

is different. There are no recreation vehicles in the traffic stream. There are buses and lesser

trucks than that assumed in the Highway Capacity Manual. No study has yet been done to

ascertain if the values in the tables of the Highway Capacity Manual need to be amended.

The existence of heavy vehicles in the traffic stream on two-lane highways on grade tends toincrease the platoon formation at the same time as passing restrictions increase. Therefore, it is

necessary to provide climbing lanes for the heavy vehicle to negotiate the grade while allowing

the passenger cars to pass them. It is always advisable to provide a climbing lane if the vehicleoperating speed falls to 15 kmph or less.

Computation of Capacity of two lane roads

Example 1.

A two-lane road in rural area has to be designed for the following roadway and traffic

characteristics.

Roadway characteristics- Design speed : 96 kmph

Lane width : 3.7 m

Paved shoulders : 1.25 m

Terrain : level

No passing zones : nil

Length of segment : 8 km

Traffic characteristics- Directional split : 70/30

Percentage of trucks: 10 %Percentage of buses : 2 %

Percentage of passenger cars: 88%

What is the capacity of this roadway segment?

What is the maximum flow rate that can be accommodated at Level of Service C?

Solution:

Calculate the service flow rates for LOS-C and LOS-E (capacity)

SFi= 2800 x (v/c)ix fd x fwx fHV

fHV = 1/ [1+ PT(ET 1) + PB(EB 1) ]


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(v/c)iis obtained from table 2.3, for level terrain, % no passing zones = 0, LOS-C

(v/c)C= 0.43

For level terrain, % no passing zones = 0, LOS-E, (v/c)E= 1.00 (table 2.3)

fd= 0.89 for 70/30 directional split from table 2.4

fw = 0.92 for shoulder width 1.25, lane width 3.7, LOS-C from table 2.5

fw = 0.97 for same conditions but LOS-E from table 2.5

ET= 2.2 from table 2.6 for LOS-C and 2.0 for LOS-E , level terrain

EB= 2.0 LOS-C and 1.6 for LOS-E

PT= 0.10 given 10%

PB= 0.02 given 2%

Then, fHV(LOS-C) = 1/[1+0.1(2.2-1) + 0.02 (2.0-1)] = 0.88

fHV(LOS-E) = 1/[1+ 0.10(2.0-1)+0.02(1.6-1)] = 0.82

SfC= 2800 x 0.43 x 0.89 x 0.92 x 0.88 = 867 vph

SFE= 2800 x 1.0 x 0.89 x 0.97 x 0.82 = 1982 vph

Thus the road segment has an expected capacity of 1982 vph in both directions andcan accommodate 867 vph at LOS-C.

If the usable shoulder width is reduced to 0.6 meters and the lane width is reduced to 3.0

meters the road can accommodate only 640 vph at LOS-C and the capacity (LOS-E) will be

reduced to 1790 vph total for both directions.

Example:2A two lane rural road carries a peak hour volume of 180 vph and has the following

characteristics.

Roadway characteristics Traffic characteristics

Design speed= 96kmph Direction split=60/40

Lane width= 3.4m Percentage of trucks=5%

Shoulder width=0.6m Percentage of buse=5%

Terrain is mountainous Percentage of passenger cars=90

80% no passing zones

length of road segment=16km


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At what level of service will the road operate during peak periods, if the flow rate for the peak

15 minutes total for both directions is 87% of the total flow rate for the peak hour?

Solution:Calculate the service flow rate for each level of service and compare with the actual flow rate.

V

The actual flow rate v =

PHF

Where V = peak hour volume given as 180vph

and PHF= Peak Hour Factor given as 0 .87

Therefore 180=0.87 x v

v=207 vph

SFi= 2800 x (v/c)ix fd x fwx fHV

fHV = 1/ [1+ PT(ET 1) + PB(EB 1) ]

From table 2.3, for mountainous terrain and 80% no passing zones, v/c is obtained forthe LOS -A to E

LOS-A, v/c = 0.02

LOS-B, v/c = 0.12

LOS-C, v/c = 0.20

LOS-D, v/c = 0.37

LOS-E, v/c = 0.80

fd = 0.94 for 60/40 directional split from table 2.4fw= 0.75 for LOS-A, B, C, and D from table 2.5

fw = 0.88 for LOS-E from table 2.5

ET = 7 for LOS-A, mountainous terrain, (table 2.6)

EB = 5.7 for LOS-A, mountainous terrain, (table 2.6)

ET= 10 for LOS-B and LOS-C, mountainous terrain (table 2.6)EB= 6 for LOS-B and LOS-C, mountainous terrain (table 2.6)

ET= 12 for LOS-D and LOS-E, mountainous terrain (table 2.6)

EB= 6.5 for LOS-D and LOS-E, mountainous terrain (table 2.6)

PT= 0.05 givenPB= 0.05 given

Then, fHV(LOS-A)= 1/[1+0.05(7-1)+0.05(5.7-1) =0.65

fHV(LOS-B and C)= 1/[1+0.05(10-1)+0.05(6-1) =0.59

fHV(LOS-D and E)= 1/[1+0.05(12-1)+0.05(6.5 -1) =0.55

and, SF(LOS-A) = 2800 x 0.02 x 0.94 x 0.75 x 0.65 = 26 vph

SF(LOS-B) = 2800 x 0.12 x 0.94 x 0.75 x 0.59 = 140 vph

SF(LOS-C) = 2800 x 0.20 x 0.94 x 0.75 x 0.59 = 233 vph

SF(LOS-D) = 2800 x 0.37 x 0.94 x 0.75 x 0.55 = 401 vph


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SF(LOS-E) = 2800 x 0.80 x 0.94 x 0.88 x 0.55 = 1019 vph

If the actual flow rate is compared with these values, it is seen that the actual flow rate

207 vph, which is the peak 15 minute flow is higher than the service flow rate for LOS-B but lower than service flow rate for LOS-C. Therefore, the road segment operates at

LOS-C.

Example 3

A segment of Mavanella- Hemmathagama road 3.25 km length has a 6% grade. The

terrain is considered mountainous. The road has two lanes 3.7m wide and the shoulders

are 2.4m wide from the edge of carriageway clear from obstructions. 60% of the road

segment has passing prohibition. The directional split of traffic is 70/30 with 12%trucks, 3% buses and 85% passenger cars in the traffic stream. The effects of other

vehicles may be ignored.

What is the maximum volume that can be accommodated on the grade at a speed of65kmph (LOS-D) if the Peak Hour Factor is 0.85.

Solution

Compute the service flow rate using the equation

SFi= 2800 x (v/c)ix fd x fwx fgx fHV

where, fg= 1/[1+PpIp] and Ip= 0.02 (E-E0)

fHV = 1/[1+PHV(EHV1)]

EHV

= 1+(0.25+PT/HV

)(E - 1)

From table 2.7, find (v/c)Dfor 65kmph, 6% grade, 60% no passing zones)(v/c)D= 0.83

fd = 0.78 (table 2.8)

fw = 1.0 (table 2.5)

E = 10.7 (table 2.9)

E0 = 1.3 (table 2.9)PHV = PT+ PB=0 .12 +0 .03 = 0.15

PT/HV = PT/ PHV= 0.12/0.15 =0.80

Then compute fgand fHVIp= 0.02 (E-E0) = 0.02(10.7 1.3) =0.188fg= 1/[1+PpIp] = 1/[1 + (.85 x .188)] = 0.86

EHV= 1+(0.25+PT/HV)(E - 1) = 1 + (.25 + .80)(10.7 - 1) =11.185

fHV = 1/[1+PHV(EHV1)] = 1/[1 + .15(11.185-1 )] = 0.395

Now, compute the service flow rate for the peak 15 minutes for LOS-D

SFD= 2800 x (v/c)Dx fd x fwx fgx fHV= 2800 x .83 x .78 x 1.0 x .86 x .395

= 615 vph

Therefore the maximum volume that can be accommodated at 65 kmph or LOS-D on the gradeis V = 615 x PHF = 615 x .85 = 523 vph

chapter 1 - traffic volume

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