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VTInotat
Nummer: 08 Datum: 1987-02-12
Titel: The Effect of Overtaking Sight Distance on Journey Speed in Two
' Lane Rural Road
Forfattare: Tang Shumei
Avdelning: Trafikavdelningen
Projektnummer: 7700501-5
Projektnamn: Kinesisk gastforskare
Uppdragsgivare: VTI
Distribution: fri
Statens vag- och trafikinstitut
w6Eff-och Trafih- Pa: 58101 LinkGping. Tel. 013-1152 00. Telex 50125 VTISGI! SIIIStit tet Besok: Olaus Magnus vag 37, Linkoping
ACKNOWLEDGEMENTS
The author would like to thank Mr Arne Carlsson for his valuable
comments and Miss Gunilla Berg for her very substantial assistance in
some figures.
TABLE OF CONTENTS
ABSTRACT
INTRODUCTION
METHODOLOGY
SIMULATION RESULTS AND ANALYSIS
The relationship between sight distance andovertaking concentrationThe relationship between overtaking concentra-tion and journey speedThe effect of sight distance on journey speedThe effect of the percentage of sight distanceabove 300 m and 600 m on journey speedThe effect of traffic volume on overtaking rate
DISCUSSION
CONCLUSIONS
REFERENCES
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The Effect of Overtaking Sight Distance on Journey Speed in Two LaneRural Road
by Tang ShumeiSwedish Road and Traffic Research Institute
5-581 01 LINKOPING Sweden
ABSTRACT
This paper presents some findings of a study of the effect of overtaking
sight distance on journey speed by means of Swedish VTI simulation
model.
The findings reveal that there are strong relationship between sight
distance and overtaking concentration, overtaking concentration and
journey speed.
The implication of the findings is the provision of the level of service
evaluation and the determination of the criteria of controling geometric
design for two lane rural roads.
This paper concludes with a recommendation that the criteria of over
taking sight distance be an essential element in two lane rural road
planning and design. Overtaking lanes, climbing lanes and paved shoulders
are all viable alternatives to new roads.
1 INTRODUCTION
In designing two lane rural roads, overtaking sight distance as well as
stoping sight distance is needed. Overtaking sight distance is normally
stipulated according to the design speeds. These values are based on some
hypotheses of design speed and acceleration. It has been also proposed
that road sections with better sight distance should constitute certain
proportion in the case of heavier traffic and higher speed. But there is no
criteria yet. Since the consideration for different sight distance in
geometric design has a direct impact on the cost of roads, normal design
criteria is the major concern for road designers. However, despite the
accordance with standards, different sight distance condition will result
in different journey speeds and capacities. In the sections with
heterogeneous traffic, if no segregation, the poor sight distance will lead
to platoons and delay thereby a lower level of service.
How sight distance influences journey speeds. Very few literatures dealing
with this are available. It is needed to study the quantitative relationship
(1). This paper studies the effect of sight distance on travel speeds by
using a traffic simulation technique.
2 METHODOLOGY
Two types of method can be applied to study the effect of sight distance
on journey speeds: emperical method and traffic simulation technique.
Since the interactions between vehicles are very complicated, it has been
adviced that traffic simulation technique be a useful aid.
This study was carried out by means of Swedish VTI traffic simulation
model (2) which has shown its potential in long experience. This model is a
kind of microscopic simulation model which describes a single vehicle in
the flow along defined stretches of road forgiven traffic volumes and
traffic compisition. The vehicle are assigned the following characteri-
stics:
a) Vehicle type
b) Basis desired speed in m/s
c) p-value (power/mass ratio) in W/Kg
The stretch of a road considered consists of a sequence of consecutive
road block objects and sight distance function in each direction of travel.
Each object is homogeneous with regard to the follows:
a) Road width
b) Auxiliary lane/Wide shoulder
c) Slope
d) Horizontal curvature
e) Speed limit and
f) Overtaking restrictions
The input data for this study involves road sight distance profile, road
width and traffic data. They are discribed as follows:
a) Sight distance data
The minimum sight distance 200 m
The maximum sight distance 1 000, 800, 600, 400 (m)
The frequency of maximum sight distance 2, l, 0.5/Km respectively
b) Road data
Road width
Road length
c) Traffic data
Volume
Composition
1. cars
2. lorries
3. semi-trailers
4. truck-trailers
9m
8km
1200, 750, 300 Veh/h (both directions)*
88%
4%
4%
4%
Figure l is one of the examples showing the maximum and the minimum
sight distance as well as the frequency of maximum sight distance in each
kilometer.
1000 q
800
600
400
200
Figure 1.
ll SIGHT DISTANCE (m)
A
LENGTH (mlT
1600 2000
Sight distance profile
The main outputs of the simulation studied in this paper are journey
speeds of cars, overtaking concentration (overtakings/km. h) and overe
taking rate (overtakings/ car. hm). .
*Depending of a random distribution for generating traffic, the actual
flow was 1 240, 725 and 300 veh/h at the simulation.
3 SIMULATION RESULTS AND ANALYSIS
3.1 The relationship between sight distance and overtaking con-
centration
Overtaking is defined as such a manoeuvre that a driver moves from a
position behind to one in front of another vehicle travelling in the same
direction. To complete an overtake manoeuvre, the requirement needed
are as follows:
a) The available sight distance should be large enough for overtaking
safely
b) The acceleration ability should be available
If requirement b) is fulfilled, according to the simulation, the better the
sight distance condition, the higher the overtaking concentration. This
implies better sight distance can disperse the traffic queues. Fig. 2 shows
the effect of different sight distance on overtaking concentration under
the condition of different traffic volumes. As can be seen, overtaking
concentration is directly proportional to sight distance. The range of
incremental rate (of overtaking concentration with different sight distan-
ce) is from 0.02 o 0.19 with different traffic and maximum sight distance
frequency, shown in the table of Fig. 2. Again, with large maximum sight
distance frequency, incremental rate of overtaking increases more largely
than small maximum sight distance frequency.
ll OVERTAKINGI kmhr
Traffic volume Q 1 240 725 300
Z 0.18 0.14 0.03
Frequency F l 0.11 0.08 0.03
0.5 0.06 0.05 0.02
150 -
100
50-
o zoo 200 360 1.50 560 600 760 860 960 1600 5.07;)
Figure 2 The relationship between sight distance and overtaking
concentration.
3.2 The relationship between overtaking concentration and
journey speed
It is always the case that faster vehicles and slower vehicles are
travelling on the same lane in rural two lane road. When a car travelling
on the road is catching up a slower one, it is constrained and has to tail
the slower vehicle at a lower speed, which subsequently constrains the
following cars behind it and results in extending the platoon and affects
the capacity.
If the traffic and road conditions are suitable for overtaking, the
constrained vehicles would be released. After overtaking, the faster
vehicles can travel at its normal speed. As can be seen, overtaking is
playing a positive role in increasing the average journey speed and
therefore save time.
The relationship between overtaking concentration and journey speed
from the simulation is shown in figure 3. It is concluded that
a) journey speed is linear with overtaking concentration;
b) The heavier the traffic, the smaller the influence of overtaking
concentration on journey speed, namely, for the traffic volume of 300
veh/h, the increase in journey speed with increased overtaking con-
centraion is faster than that for a traffic volume of l 240 veh/h.
c) With the same traffic flow, large sight distance results in larger
overtaking concentration.
d) Lines of journey speed varying with overtaking concentration form
three lines which can be expressed by three regression equations with
traffic flow Q being 300, 725, 1240 (veh/h) respectively are as
follows:
For Q = 300 veh/h V : 0.233D + 78.5
Q = 725 veh/h V : 0.048D + 74.0
Q = l leO veh/h V : 0.025D + 70.9
Where, V is the journey speed (Km/h);
D is the overtaking concentration (overtaking/Km/hr)
_ 90 _ JOURNEY SPEED {mm
531000
9: 300
S: N
5:1000
80" 0"" s- 1.00
0:750
S = 1000
S 600 Q: 1200
5:600
$ 0
o 10 £035 46 56 so i036 923160110120130150OVERTAKING/ kmhr
Figure 3. The relationship between overtaking concentration andjourney speed of cars.
3.3 The effect of sight distance on journey speed
From the findings of previous sections, it is very clear that journey speed
is directly proportional to sight distance. Figures in Table l are calculated
'from FiO. 2 and the regression equations.
Journey speed and overtaking concentration of cars under
Table 1.different traffic flow and sight condition
_- .. _ , _._-, - __ - a ..- WWW if..- s _3n ago i mo mm m _, WWW)
Q L"r" 0.3" " ' l n 5 ' l _~ 0.5 m ,_ i _________ j J - _L , 6 . l0 [3 m- _, l '6 H r i » T71 '50 4 iv [K ) I m 1
300 236 ' 0 38')
v 79.89 80.18 31:? 7M-. i3 :R °1 ??3_ 3~l§ a. 3'-39 3} 5 R 75 - . °2 3] 9 »7 37'71C 1 }, ""275 "3'2"" " , _G K, (31 3a )1 0r) + a"; 7n 1 m
750 100*; 77: l07a
L V 7 4-72 7"; 36 ,_ 25.7? 7, ' 77-2., 73.73 7.5 3 75:63. 7.6. E L w. h _-1.1_._77 36.-. .7'3.(:¢;_.-.(; , r .. r ' 4 : U ,l 4, ll] (,7 ll;, + (:3 r ) l ,
1200 :33 I ms 9%
v 71.23 71.n5 71.73 72.n2 77 no *v.a3 7|.W? vs 71 #3 *7 >* a :3 7a.n:L........ ., _~ ...-,._._, .. l -,_M.-.. _. .M, .a .i__..- .. -- _ ._ -- - _..l,. ...--
Note:
1) SD - Sight distance (m)
F - Sight distance frequency/Km
C - Overtaking concentration
V - Journey speed of cars
2) Figures with " * " are the time (in sec.) saved for all cars in the traffic
flow comparing sight distance frequency 2 to sight distance frequency
0.5.
Table 1 gives alternatives of sight distance condition with same journey
speed under different terrian condition. For instance, when traffic flow is
300 veh/h, journey speed for two 800 m sight distance in one kilometer
and for one 1 000 m sight distance in one kilometer is the same.
In addition, with the same sight distance condition, the time saved for car
traffic is more when traffic volume is 725 veh/h than 300 veh/h and l 240
veh/ h.
10
3.4 The effect of the percentage of sight distance above 300 m
and 600 m on journey speed
The percentage of sight distance above a certain limit can be calculated
as follows if L is the total length. See Fig. ll.
P(2A):._Z_ £sT _{_3_A_)_x100
SIGHT DlSTANCE (M)ll
T 1.
A
L +LENGTH (M)
_LF L 1
Figure 4. Determination of percentage of sight distance above acertain limit.
The dependance of journey speed to sight distance above 300 m and 600 m
is shown in Fig. 5 a and Fig. 5 b repectively.
Figure 5 a
Figure 5 b
Figure 5
ll
WYSFEEDvsWOFSlG-TTDSTANE
SIGHT MAXIMUMA 5:11:00 :1 5:600
88.mwm x 3:500 o 5:400
86- AA x
3 0:300 veh/hA x D
82-0 ( x DO
80 o O A
x78 A Q=725veh/ ..I
A X D
76 D Ax DO0 ° o A
74 xA
724 5k 6* x D:1° °° <2:va
70 r r T l lO 20 4O 60 80 100
Percentage of sight distance > 300m (2:)
The dependance of journey speed to sight distance above300 m.
JOlREYSFEEDvsFERSEN'MOl- SG-ITDSTANCE
SGHTMAXIMUM
as~ ' A3 (2:3me
84
882 XA
80 A
X78 A 0:725de
A<75 xA
A74"
K Q"'12.40v°h/h72" XA E
70 1 I I T 1
O 20 40 50 80 100
Pereenfageofelgf dietance>600m(%)
The dependance of journey speedito sight distance above600 m.
The effect of the percentage of sight distance on journeyspeed.
12
For SD _>_ 300 m
Q = 300 veh/h V = 5.68Ps + 79.9
Q = 725 veh/h V = 4.50Ps + 74.3
Q =1 240 veh/h V = 2.63Ps + 71.1
For SD _>_ 600 m
Q = 300 veh/h v = 5.37Ps + 81.5
Q = 725 veh/h v -_- 4.37135 + 75.7
Q = 1 240 veh/h v = 2.75Ps + 71.8
where SD, Q and V are defined as before. P5 is the percentage of sight
distance above certain value.
Table 2. Journey speed of cars under different traffic flow and sightcondition.
Sight Distancelm) SD 3 300 m SD _>_ 600 m
percentage (%) 20 40 60 80 100 20 40 60 80 100
300 (veh/h) 81.0 82.2 83.3 84.4 85.6 82.6 83.6 84.7 85.8 86.9
725 " 75.2 76.1 76.7 77.9 78.8 76.6 77.4 78.3 79.2 80.1
1240 " 71.6 72.2 72.7 73.2 73.7 72.4 72.9 73.5 74.0 74.6
Figures in table 2 are calculated according to the regression equations
above. Comparison between Table 2 and Table 1 show that the same speed
may be received from different sight distance condition. For instance, the
journey speeds of the road with one maximum sight distance 1 000 meters
per kilometer and the road with the percentage P5 (3 300 m) being 80%
are the same when traffic flow is l 240 veh/h. This implies that if the
terrain condition is limited, the alignment is controlled by certain sight
distance limits, the determination of the fixed overtaking sight distance
value is not necessary, because any suitable sight distance percentage of
above certain sight distance can be used for this purpose. Therefore, the
most economical sight distance alternative can be chosen.
13
3.5 The effect of traffic volume on overtaking rate
Overtaking rate (overtakings/car/km) is notincreased with the increase-
ment of traffic volume all the time. Fig. 6 gives an example and indicates
that the largest overtaking rate happens when traffic volume is between
#00 veh/h and 700 veh/h under different sight condition. Combining the
analysis made above, in which the larger overtaking concentration results
in higher journey speed, it may be concluded that in the design of a two
lane rural road with traffic volume being 400 veh/h - 700 veh/h the
consideration of the improvement of sight distance will get high potential
benefit. This outcome is in accordance with the one made above: the
travel time saved for the car traffic is more when the traffic volume is
725 veh/h than 300 veh/h and l 2% veh/h.
OVERTAKlNGI [All in
0.20
018- \
(:16 - \
\ \
01m \\ \
\\ \
\ \ m\\
-4 \011 \
\\.
\\\n
0105:
\ _,
\\0.06 / \
\\
, \/ \// \// /-\ \.
00!:
0.02
FLDV (VEHW,
o 550 vsooq
Figure 6. The effect of traffic flow on overtaking rate.
14
4 DISCUSSION
Platoons are caused by slow-moving vehicles constraining faster-moving
vehicles, the latter ones having difficulties in overtaking because of
alignment or high oncoming traffic flow. In effect the very existence of
platoons and the opportunity or lack of opportunity to overtake slow-
moving vehicles may be considered as a measure of the level of service
provided by a highway (3). The study of the effect of sight distance on
journey speed reveals that to improve the road capacity and the level of
service, it is necessary to improve overtaking condition. This can be
achieved through follows:
1) Build roads with better alignment so that vehicles have better sight
distance to carry out overtakings. This is achieved by avoiding sharp
curves both horizontally and vertically.
2) Build auxiliary lanes (climbing lanes or overtaking lanes) or paved
shoulders with enough width, so that slower vehicles can use them for
faster vehicles to pass. According to the estimation made in Sweden,
100% of vehicle drivers use climbing lanes to overtake, 70% of car
drivers and 90% of other vehicle drivers use paved shoulders to
overtake.
In planning a new road or determining an improvement measure, one has
to take terrain, traffic volume, funds etc. into account and take more
suitable way to better sight condition thereby the capacity.
Arne Carlsson studied three road width alternatives of a road of length 20
km to be built by VTI simulation model (4). The three alternatives are:
l) 9 m
2) 9 m + 4 climbing lanes (both directions)
3) 13 m
Simulation shows that the slope of journey speed - traffic flow curve of 9
m + 4 climbing lanes road and 13 m road has slight difference. The cost of
9 m + 3 climbing lanes road is only about 80% of 13 m road.
15
As can be seen, climbing lane is one of the effective and economical way
to improve road capacity.
Another potential way to improve capacity is the change of earth
shoulders to paved shoulders. A study made by India (5) shows that, by
changing earth shoulders to brick shoulders, the journey speed of different
vehicles or road users can increase as follows: car by 9%, bus by 8%, lorry
by 15%, two wheel vehicle by 6%.
In China, two lane rural roads contribute 83% of national road net work, it
will remain the main part in the future. Owing to the heterogeneous
traffic in two lane rural roads, road capacity and safety is more
conspicuous with traffic increasing. How to improve road capacity is
becoming more urgent. From this study, climbing lanes or overtaking
lanes and hard shoulders offer a potential for substantial improvement in
the level of service on rural roads at relative cost. Due to the frequently
current shortage of road funds and uncertainities about travel trend on
some roads, auxiliary lanes and paved shoulders merit particular atten-
tion.
16
CONCLUSIONS
This study of the effect of sight distance on journey speed for different
traffic volume can be concluded as follows:
1)
2)
5)
3)
4)
6)
Overtaking concentration increases with the increment of traffic flow
and sight distance. With better sight distance condition, incremental
rate of overtaking concetration increases more largely than worse
sight distance condition with the increment of traffic.
With the same traffic volume, better sight distance results in higher
journey speed. With the same sight distance condition, the time saved
for car traffic is more when traffic volume is 725 veh/h than 300
veh/h and l 240 veh/h. This is because overtaking rate is larger when
traffic flow is between 400-800 veh/h. This means that with traffic
volume being 400 veh/h - 800 veh/h, the consideration of improvement
of sight distance will obtain greater benefit.
In the case of the limitation of terrain condition, the determining of
fixed overtaking sight distance value is not necessary. Any suitbal
sight percentage of above certain sight distance can be used for this
purpose. Therefore, the most economical alternative can be chosen.
Sight distance condition or the percentage of sight distance above
certain value can be used for measuring the level of service. Since
delay reduction results from better sight distance.
In planning a two lane rural road, the sight percentage can be chosen
based on design speed and level of service for controlling geometric
design.
Auxiliary lanes (climbing lanes and overtaking lanes) and paved shoul-
ders are effective and economical way to improve road capacity. It is
suggested that to some bottle neck sections in existing roads, to build
auxiliary lanes and hard shoulders or paved shoulders show great
benefit.
l7
7) It is necessary to further study the effect of overtaking sight distance
on journey speed under Chinese condition to establish the criteria of
the percentage of above certain sight distance for different traffic
volume, design speed and the level of service.
18
REFERENCES
S LylyHelsinki University of Technology, Finland.
"The Importance of passing sight distance in Highway Design"Proceeding of the symposium on method for determing Geometric Road Design Standard (1976).
Anders Brodin and Arne Carlsson
"The VTI Traffic Simulation model and programme systemSwedish Road and Traffic Institute".Meddelande nr 321 A 1986
John F MorralDepartment of Civil Engineering. The University of Calgary.
A1 WernerAlberta Transportation
"Measurement of level of service for two-lane rural high-ways.
Arne Carlsson
Redovisning av trafiksimulering pa Rv 31.bggestorp-NassjoSwedish Road and Traffic InstituteMemoranda (in Swedish) 1984-02-29
Central Road Research Institute
"Road cost study in India"Final ReportIndia 1982