The effect of overtaking sight distance on journey speed in two lane rural road

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V TInotat

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

w 6Eff-och Trafih-

_{Pa: 58101 LinkGping. Tel. 013-1152 00. Telex 50125 VTISGI! S}

IIIStit tet Besok: Olaus Magnus vag 37, Linkoping

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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.

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TABLE OF CONTENTS

ABSTRACT INTRODUCTION METHODOLOGY

SIMULATION RESULTS AND ANALYSIS The relationship between sight distance and overtaking concentration

The relationship between overtaking

concentra-tion and journey speed

The effect of sight distance on journey speed

The effect of the percentage of sight distance

above 300 m and 600 m on journey speed

The effect of traffic volume on overtaking rate DISCUSSION CONCLUSIONS REFERENCES Page O O O x 10 l3 14 16 18

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The Effect of Overtaking Sight Distance on Journey Speed in Two Lane Rural Road

by Tang Shumei

Swedish 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.

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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

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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)

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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 (ml T

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.

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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.

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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 z50-o50-o 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

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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);

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_ 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 923160110120130150 OVERTAKING/ kmhr

Figure 3. The relationship between overtaking concentration and

journey 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.

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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_C _{1 }, ""275} 31:? 7M-. i3 :R_"3'2"" _{" ,} _{_G} °1 ??3_ 3~l§ a._K, ₍₃₁ 3'-39 3} 5 R 75 - . °2 3] 9 »7 37'71_3a ₎₁ _0r) ₊ _a"; _7n _{1 m} 750 100*; 77: l07a L V_.-.(; , r7 4-72 7"; 36 ,_ 25.7? 7,.. r ' 4 :' 77-2., 73.73 7.5 3U ,l 4, 75:63. 7.6. E L w.ll] (,7 ll;, + h _-1.1_._77 36.-. .7'3.(:¢;(: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.

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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) _L

F

L

1

Figure 4. Determination of percentage of sight distance above a certain limit.

The dependance of journey speed to sight distance above 300 m and 600 m

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Figure 5 a Figure 5 b Figure 5 ll WYSFEEDvsWOFSlG-TTDSTANE SIGHT MAXIMUM A 5:11:00 :1 5:600 88 .mwm x 3:500 o 5:400

86-

A

A x

3 0:300 veh/h

A x D 82- _{0 ( x DO} 80 o O A x 78 _A _{Q=725veh/ ..I} A X D

76 D Ax DO

0 ° o _A 74 _x A

724 _{° °°}

5k 6*

:1

x D

<2:va

70 r r T l l O 20 4O 60 80 100

Percentage of sight distance > 300m (2:)

The dependance of journey speed to sight distance above 300 m. JOlREYSFEEDvsFERSEN'MOl- SG-ITDSTANCE SGHTMAXIMUM

as~

' A

3 (2:3me

84 8 82 XA 80 _A X 78 A 0:725de A< 75 xA A 74" K _{Q"'12.40v°h/h} 72" XA E 70 1 I I T 1 O 20 40 50 80 100 Pereenfageofelgf dietance>600m(%)

The dependance of journey speedito sight distance above 600 m.

The effect of the percentage of sight distance on journey speed.

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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 sight condition.

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.

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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 010 _5: \ _, \\ 0.06 / \ \ \ ,

/

\

//

// /-\

\

\.

00!: 0.02 FLDV (VEHW, o 550 vsooq

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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.

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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.

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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.

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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.

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18

REFERENCES

S Lyly

Helsinki University of Technology, Finland.

"The Importance of passing sight distance in Highway Design" Proceeding of the symposium on method for determing Geo

metric Road Design Standard (1976).

Anders Brodin and Arne Carlsson

"The VTI Traffic Simulation model and programme system

Swedish Road and Traffic Institute". Meddelande nr 321 A 1986

John F Morral

Department of Civil Engineering. The University of Calgary. A1 Werner

Alberta Transportation

"Measurement of level of service for two-lane rural high-ways.

Arne Carlsson

Redovisning av trafiksimulering pa Rv 31.

bggestorp-Nassjo

Swedish Road and Traffic Institute

Memoranda (in Swedish) 1984-02-29

Central Road Research Institute "Road cost study in India"

Final Report

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