Analyzing the Speed Dispersion Influence on Traffic Safety

Fangqiang Lu School of Transportation

Southeast University Nanjing, China

e-mail: lufangqiang@seu.edu.cn

Xuewu Chen School of Transportation

Southeast University Nanjing, China

e-mail: chenxuewu@seu.edu.cn

Abstract—The aim of this paper is to analyze the influence of speed dispersion on traffic safety. The average speed difference (ASD for short) of two neighboring vehicles was used to define the speed dispersion. Based on the traffic data obtained from Airport freeway in China and A20 freeway in the Netherlands, we carried out a comparative study on speed dispersion characteristics. We found that the speed dispersion of traffic flow on Airport freeway were larger than that on A20 freeway. The comparing results showed that A20 freeway was safer than Airport freeway. Some possible reasons were provided in this paper. This indicates that the ratio of large vehicle, traffic flow and over-speed are the three main factors of speed dispersion influence on traffic safety.

Keywords- speed dispersion; average speed difference; traffic safety; traffic conflict technology

I. INTRODUCTION Speed dispersion has been long recognized as a

contributing factor for freeway crashes. It influences the possibility and the seriousness of the traffic accident. In fact, the 1984 TRB report indirectly supported this point of view by concluding that “if the average speed of the traffic stream could be increased without increasing the variance of the speed, then the adverse effects on safety might be comparatively small”.

Published papers have indicated that the speed dispersion and crash rate have a positive correlation (Fildes [1], Liu [2], Buruga [3], Pei [4]).Prior theories and empirical validations have established speed dispersion-safety relationships, such as the difference of 85th percentile speed and 15th percentile speed and the rate of casualty (Liu [2]), the speed dispersion and accident rate per 100 million vehicle kilometers (Pei [4]). To the author’s knowledge, prior suggestions on structural models, with its potential to provide an improved understanding of the relationships between speed dispersion and traffic accident, was short of the research about the causes of speed dispersion influence on traffic safety, and little information is available in literatures about the distribution of the speed dispersion.

The attempt of this research is to investigate the fundamental characteristics of speed dispersion and discuss the three main factors (the ratio of large vehicle, traffic flow and over-speed) of speed dispersion influencing on traffic safety.

This paper compares the speed dispersion characteristic between Airport freeway in the east of China and A20 freeway in the Netherlands. Then, we analyze the three main factors of speed dispersion influencing on traffic safety. In the end, conclusions and further study are provided.

II. DATE COLLECTION The typical sections of the freeway:

Figure 1. Schematic configuration of the chosen sections of freeways: (a)

Airport freeway in China; (b) A20 freeway in Netherlands.

Airport freeway in the east of China: this section is about 5 km, with 2 lanes in one direction. The speed limit on this road is 120km/h. Traffic data utilized for the present study were obtained by video-tapping, all the videos were tapped during the morning rush hours (7:30 to 10: 30) of 4 weekdays for each detector. Not only does traffic demand for arriving at the airport mainly utilize the airport freeway, but also part of through traffic depends on this freeway.

A20 freeway in the Netherlands: this section is about 1.1 km, with 3 lanes in one direction. All the videos were tapped during the day 22nd June 2005.

In order to ensure a broad range of speed dispersion, 1 minute was used as statistical interval for each lane. A program was applied to calculate the arrival count, time mean speed and ASD based on the “event” data of AUTOSCOPE.

2009 International Conference on Measuring Technology and Mechatronics Automation

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III. COMPARATIVE STUDIES ON SPEED DISPERSION CHARACTERISTICS

A. Definition of speed dispersion In Wang’s recent research (Wang [5]), a new definition

of speed dispersion was proposed: the average speed difference of two neighboring vehicles (ASD for short). He found that the description of average speed difference is better than standard deviation when describing the phenomena of speed dispersion according to the empirical data from urban freeways. ASD can not only be used generally in statistical analysis for sample, but also take consideration of car-following behavior. Therefore, we used it here to describe speed dispersion.

Suppose there are n vehicles passing the given point in a specified time interval, the speed dispersion described by the definition has the form:

1

1

11

−

−=

−

=+

n

vvASD

n

iii

(1)

Where iv is the speed of the ith vehicle passing the given point.

B. Data analysis To get the comparable data, 3 hours data is selected from

the obtained data on the Airport freeway and A20 freeway respectively. The rush hour on Airport freeway is 7:30~8:30am, while it is 7:00~8:00am on A20 freeway. So the data utilized in this study was as followed: (a) 7:30~10:30am, 6th July 2006 on Airport freeway; (b) 7:00~10:00am, 22nd June 2005 on A20 freeway. 1 minute was used as statistical interval for each lane, so there are 180 data points on each lane.

Figure 2. Speed dispersion characteristics of Airport freeway.

As we can see from Figure 2and Figure 3, speed on Airport freeway is more discrete than A20 freeway. Fig. 3 shows that most of the data points distribution range from 2km/h to 6km/h on A20 freeway, there are 75% points (135points) in this range on lane0, 83.8% points (149points) on lane2. While there are 137 points ranged from 8 km/h to

20 km/h on Airport freeway lane0, account for 76.1%, and most of the data points distribute from 9 km/h to 20km/h on Airport freeway lane1, about 128 points that account for 71.1%.

Figure 3. Speed dispersion characteristics of A20 freeway.

C. Comparisons on traffic safety We used traffic conflict technology to study the influence

of speed dispersion on traffic safety. The traffic conflict approach assumes that crash probability is related to potential for conflict among vehicles in the traffic stream (more information about traffic conflict technology see Hyden [6], Zhou [7]). This implies that the crash probability for a driver (to be involved in a multiple-vehicle crash) is a function of the deviation of the individual driver's speed from the speed of other drivers, such as Hauer [8] suggested that drivers with speeds much faster or slower than the median traffic speed are likely to encounter more conflict. We established speed dispersion-serious traffic conflict relationships that based on traffic conflict technology and empirical validations in the prior study. Figure 4 gives the relationships curve between ASD and serious traffic conflict (STC for short) on Airport freeway. The survey methods of serious traffic conflicts see Luo [9]

In the case of this traffic state, the relationships between ASD and STC are not a positive correlation, along with the increasing of ASD; the amount of STC is rising, and then falling. When the ASD ranged from 8km/h to 11km/h, the amount of the STC reached the top (about 13). The reason of rising phase is that speed is discrepant in the state of fluid traffic, and there is fewer disturbances among vehicles, therefore, the amount of STC are few. Then, the speed dispersion is large with the difference increasing among individual vehicles, and the traffic flow is unstable, so the STC is increasing. The reason of fall phase is that when ASD reaches 14km/h and higher, the space of individual vehicles are always large, so the probability of STC is small, but there exists the situation of large ASD and small space headway (sometimes this situation is unsafe), therefore, the amount of STC will not reduce to zero. While this result is not in contradiction to the prior study, the amount of STC can only represent for the crash probability, not reflect the crash serious. The number of STC is small, when the ASD larger

0

10

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30

40

7:30 8:00 8:30 9:00 9:30 10:00 10:30

Time

AD

S (k

m/h

)

Lane0Lane1

0

10

20

30

40

7:00 7:30 8:00 8:30 9:00 9:30 10:00Time

ASD

(km

/h)

Lane0Lane2

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than 15km/h compared to ASD ranged from 9km/h to 11km/h, but the weight of crash severe is bigger when the value of ASD is higher (need further study in the future work).

Figure 4. The relationships curve between ASD and serious traffic

conflict on Airport freeway.

Here we classify safety level as 3 groups: a. both of the possibility and the seriousness of the traffic accident is low; b. the possibility of the traffic accident is high, while the seriousness is medium; c. the seriousness is high, while the possibility is medium. Based on the relationships between ASD and STC, Table gives the detail information about the classification.

TABLE I. TABLE TYPE STYLES

Safety level a b c ASD (km/h) <9 9~11 >11

As we can see from Table , there are noticeable

differences in traffic safety between Airport freeway and A20 freeway. It is apparent that percentages of data in level a of Airport freeway is less than A20 freeway. However, percentages in level b and level c of Airport freeway are much higher than A20 freeway. Therefore, A20 freeway in Netherlands is safer than Airport freeway in China.

TABLE II. NUMBER OF THE ASD DATA POINTS OF AIRPORT FREEWAY AND A20 FREEWAY

Freeway Lane Level a Level b Level c

Airport freeway

lane0 27 (15%)

26 (14.4%)

127 (70.6%)

lane1 26 (14.5%)

22 (12.2)

132 (73.3%)

A20 freeway

lane0 176 (97.7%)

3 (1.7%)

1 (0.6%)

lane2 175 (97.2%)

2 (1.1%)

3 (1.7%)

IV. REASONS One of the reasons is that ratio of large vehicle is higher

on Airport freeway than A20 freeway. The ratio of large vehicle is 14.95% on Airport freeway-lane0 and 29.44% on

lane1, while it is only 0.06% on A20 freeway lane0 and 15.12% on lane2. The ratio of large vehicle would increase the degree of speed dispersion on freeways. Large vehicle is inferior to small car in speed and smart, resulting in the speed difference between large vehicle and small car, which influence on speed dispersion. As discussed by Chen [10], speed dispersion problem is worse in developing countries because of huge demand for freight transportation. Overloading is a common problem on highways. Heavily loaded trucks could not reach the minimum speed of freeway and caused other vehicles’ frequent lane change, which consequently increased crash risks. He suggested that managing speed gap between large and small vehicles be the most crucial step in combating highway crash problem. Besides, vehicle type is diversity in China: the performance is different among large vehicles, small cars, homemade vehicles and incoming cars, huge difference in vehicles’ mechanic capabilities and driving behaviors prohibit the cars and trucks to operate smoothly in harmony. Based on the data obtained from the investigate section, small car is the primary type in the Netherlands, ratio of large vehicle on A20 freeway inner lane is less than 3%, and is only about 15% on outer lane. However, it is about 15% on Airport freeway inner lane, and reaches 30% on outer lane.

The traffic flow under peak hour is larger on A20 freeway (2016 veh/h-lane0, 1320 veh/h-lane2) than on Airport freeway (1516 veh/h-lane0, 1014 veh/h-lane1), speed under peak hour is also larger on A20 freeway (76.78 km/h-lane0, 76.03 km/h-lane2) than on Airport freeway (68.82 km/h-lane0, 68.14 km/h-lane1). However, the traffic state on A20 freeway is steady, and degree of speed dispersion is low, while the flow is low on Airport freeway, and the speed dispersion is serious. In the state of fluid traffic, the drivers have no restricted, the degree of speed dispersion is low. Before the traffic flow reach to saturation, the disturbance is serious among vehicles, and the speed dispersion rises with the flow increasing. In the state of congested traffic, the vehicle freedom is limited, and the speed dispersion begins to fall.

The probability of traffic conflicts increases as the speed difference of adjacent vehicles increasing, along with the frequently acceleration, deceleration or change of lanes to keep up with the front vehicle. Compared with Airport freeway, the traffic flow on A20 freeway is more stable. Under the situation of stable traffic flow with high density, it is necessary to overtake several vehicles or even more than ten to get to destination, which leads to many waving movements and greatly increase the possibility of traffic conflicts. So among the traffic with homogeneous speed, the behavior of overtaking of one vehicle will cause the speed dispersion to increase.

V. CONCLUSIONS AND FURTHER STUDY This paper studies the comparisons of speed dispersion

characteristic between Airport freeway in the east of China and A20 freeway in the Netherlands, and analyzes the influence of Speed Dispersion on Traffic Safety. It is concluded that A20 freeway is safer, based on the index of speed dispersion (ASD). Some of the explanations are given

0

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ASD(km/h)

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above. It is suggested that effective traffic management is necessary to reduce the speed dispersion. In this paper, we just present the method of discussing the relationships between ASD and STC to analyze the influence of speed dispersion on Traffic Safety. We have not obtained the STC data from A20 freeway in the Netherlands, besides the STC is diverse in different traffic state. In order to make an in-depth study, the STC characteristic in different freeway and traffic state should be studied.

ACKNOWLEDGMENT This research has been carried out as a part of the Sino-

Dutch cooperation on Intelligent Transport Systems, and been supported by the project of 2008 Suzhou Technical exclusive program-Railway transportation program. (No.ZXJ0804) This cooperation took place under the Memorandum of Understanding signed by the Dutch Ministry of Transport, Public Works and Water Management and the Ministry of Communications of the People's Republic of China.

The comments and suggestions of Chen Xuewu and Wang Hao are gratefully acknowledged.

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[4] Pei Y.L, Cheng G, “Research on the relationship between discrete character of speed and traffic accident and speed management of freeway”, China Journal of Highway and Transport, Harbin, 2004, pp 74 – 78.

[5] Wang H, and Wang W, “Experimental Features and Characteristics of Speed Dispersion in Urban Freeway Traffic”, Proceeding, 86th Annual Conference of Transportation Research Board, Washington, USA, 2007.

[6] Hyden C, “The development of the Method for traffic safety evaluation: the Swedish traffic conflict technology”, 1987.

[7] Zhou W., Luo S.G, “Traffic conflict identification of road accident black spots”, China Journal of Highway and transport, 2000, Vol.13 No.1 pp 81 – 86.

[8] Hauer E, “Accidents, Overtaking, and Speed Control, Accident Analysis and Prevention”, 1971, Vol. 3, pp. 1 – 13.

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