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					                          Effect of Heavy Vehicles on Safe Operation on Freeway
                  ZHANG Jie1, CHEN Yongsheng1, SUN Xiaoduan1, 2, LIU Xiaoming1 & ZHONG Liande1
                       (1 Beijing Key Lab of Traffic Engineering, Beijing University of Technology, Beijing 100022, China;
                              2 Civil Engineering Department, University of Louisiana at Lafayette, LA 70504, USA)

Abstract: With the rapid development of transportation, the proportion of large-scale and heavy-loading vehicles is increasing notably. Because of limited
performance of heavy vehicles, the negative effect of heavy vehicles on safe operation for the freeways is remarkable. Analyzing the impact of heavy vehicles,
speed on traffic accidents would significantly enhance the efforts of traffic engineers to determine suitable countermeasures to reduce the occurrence and
severity of crashes, and also improve efficiency of operation for freeways. Statistics package of SPSS 10.0 version was utilized for model fittings and
calibrations. Via statistical analysis, findings have been revealed that crash rate and speed have strong correlation with the percentage of heavy vehicles. The
relationship was indicated a “bell shaped” curve between crash rate and average speed differential between cars and heavy vehicles. Quadratic models could fit
it. This study also showed that the average speed would turn small respectively when percentage of heavy vehicles is pretty high or pretty low. Such research
proved that the heavy vehicle was an important factor that influenced the level of operation and safety of freeway. Consequently, the safety management of
heavy vehicles is the vital contents of improving traffic safety level for the freeways.
Keywords: freeway; traffic safety; crash rate; percentage of heavy vehicles; average speed; speed differential


1    Introduction
      The amount of heavy trucks has been steadily increasing in China with the swift development of national economy. The
transportation of heavy trucks is 67% of overall trucking transportation in USA, which is 55.5% in China [1]. The negative effect of
large-scale and heavy-loading development for trucks on traffic safety is more and more evident, such as increasing operation speed
differential between trucks and other vehicles that affects capacity, freedom and efficiency of transportation. Especially on freeways
the improvement of speed augments difficulties of driver controlling trucks companied by serious over-loading problem, which
threaten the safe operation of freeways.
      Truck crashes are complex events. Several factors interact to contribute to their occurrence: the vehicle, the driver, the
environment, or another vehicle. Most often, evidence indicates that a large-truck crash is caused by driver error or inattention rather
a problem with the highway environment [2]. To our knowledge, on the one hand a few studies have investigated the detailed
relationships among crashes, speed and trucks. On the other hand the variable of percentage of heavy trucks is introduced in many
models of crashes. This study describes and models the relationships among crash rate, speed and percentage of heavy vehicles for a
freeway which is located in the suburb of Beijing with 4 lanes, 2 directions and higher traffic volumes. Speed limit is 110 km/h for
basic segment of the freeway.
2    Purpose and Scope
      The purpose of this research was to identify the relationships among percentage of heavy vehicles, average speed, speed
differential between different vehicles and crash rate for a freeway. Vehicles were divided into 2 types: cars and heavy vehicles. The
type of cars includes passenger cars and small trucks. The type of heavy vehicles includes medium and large sized vehicles (vans,
buses, large-sized trucks), and articulated vehicles. Table 1 gives the detailed sorts of vehicles.
      The specific objectives of the study were to identify the relationships between percentage of heavy vehicles and average speed,
percentage of heavy vehicles and crash rate, percentage of heavy vehicles and average speed differential between different vehicles
for the freeway.

                         Table 1      Types of vehicles in «Technical standard of highway engineering in China» of 2003
                    Types                                                                  Description
                     Cars              Cars with seats not greater than 19 or trucks with loads not greater than 2 T
            Medium-sized cars          Cars with seats greater than 19 or trucks with loads greater than 2 T and not greater than 7 T
             Large-scale cars          Trucks with loads greater than 7 T and not greater than 14 T
                    Tows               Trucks with loads greater than 14 T

3    Data Collection
      The data from the freeway are including the following several respects.
      1) The data of speed and flow. The data were obtained from 1oop detectors including volumes of cars and heavy vehicles
respectively. Time interval is one minute. The traffic flow data were collected at 23 within 2004.
      The first step was to delete abnormal data, such as 0 value or unusually low values because of instrument failures. Distribution
of speed for cars, heavy vehicles and the total sample respectively approaches normality nearly in the frequency analysis. Through
the arrangement of speeds in 24 h for 1 representative day for each direction, the following variables were calculated: hourly average
speed, average speeds for cars and heavy vehicles respectively, average speed differential between heavy vehicles and cars, hourly
traffic volume, percentages of heavy vehicles and cars respectively, etc. Table 2 shows the statistical values of speed for heavy
vehicles, cars and the all. Analyzing average speed differential between heavy vehicles and cars contributes to studying the
interaction among the vehicles and effects on crash rates for the mixed traffic. Table 2 also indicates that average speed, standard
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deviation of speed, 15th percentile speed and 85th percentile speed of cars are all higher than those of heavy vehicles, mainly because
the performance of cars is better than that of heavy vehicles. The standard deviation of speed for the overall sample is higher than
that of 2 single samples, because the more style differs, the more speed is scattered.

                                Table 2      Statistics of speeds of the second segment for up direction
                                Statistics                      Total               Heavy vehicles             Cars
                     Mean /(km/h)                          76.984 3              68.112 6                    86.092 7
                     Std. Deviation /(km/h)                13.995 9              9.064 9                     12.202
                     Skewness                              0.248                 0.187                       -0.346
                     Std. Error of Skewness                0.047                 0.066                       0.067
                     Kurtosis                              -0.242                0.996                       0.958
                     Std. Error of Kurtosis                0.094                 0.131                       0.133
                     Minimum /(km/h)                       29                    29                          39
                     Maximum /(km/h)                       141                   107                         141
                     Percentiles    15 /(km/h)             62                    59                          75
                     Percentiles    85 /(km/h)             92                    77                          97


      2) The data of crashes. The data were obtained from police traffic accident records in 2004 for the freeway. In order to ensure
homogenous traffic type and flow characteristics, the freeway was divided into 3 segments along with up and down 2 directions. The
first segment is from starting point to mileages of K6 +500, the second is from mileage of K6 +500 to mileage of K72 +20, the third
segment is left. The first segment plays the role of connecting urban. The traffic volume and traffic composing have a greater change
for the last segment because of long-distance heavy vehicles going out of the freeway from this segment. Table 3 shows statistics of
crashes in 2004.

                                 Table 3      Statistics of crashes for up and down directions in 2004
                        Property-damage                                                            No type
            Up                                    Injury accidents      Fatal accidents                                 Aggregate
                              accidents                                                        accident records
         The first                 52                     104                      1                     0                   157
        The second                 73                     145                      13                   40                   271
         The third                 7                       14                      5                    39                   65
                        Property-damage                                                            No type
           Down                                   Injury accidents      Fatal accidents                                 Aggregate
                              accidents                                                        accident records
         The first                 74                      92                      3                     0                   169
        The second                 108                    251                      18                    6                   383
         The third                 2                        5                      2                    14                   23

     After crashes of every hour were collected, crash rate was defined as:
                                                                           N  1 000 000
                                                            Crash rate 
                                                                            365  L  T
Where N—the number of crashes including fatal accidents, injury accidents and property-damage accidents occurred within a
specific hour; L—length of the segment, km; T—hourly traffic volume of one way, PCU/h, a heavy vehicle is equivalent to 2.234
cars.
4   Data Analysis
     The freeway was divided into 3 segments and 2 directions in the data analysis, which are 6 bins. On the one hand the traffic flow
characteristic has directionality, on the other hand comparing the relationships among speed, heavy trucks and crash rate for up, down
and 2 directions has contributed to drawing reliable conclusions. Table 4 shows the variables of traffic flow for 3 segments per
direction.
4.1 Percentage of Heavy Vehicles and Crash Rate
     The conventionally dominated knowledge is that the crash rate reduces with increasing percentage of heavy vehicles, because
the increase of heavy vehicles has strengthened the alertness of the drivers around, the driver behaviors are safer. The freeway’s data
introduced another characteristic. Fig.1 shows that when the percentage of heavy vehicles is lower than 50%, the relationship
between the crash rate and percentage of heavy vehicles is remarkable, that is, the crash rate decreases as the percentage of heavy
vehicles increases. When the percentage of heavy vehicles is higher than 50% and lower than 85%, the drivers’ behaviors of driving
are different from each other, so crash rate is stochastic and the relationship is not clear. When the percentage of heavy vehicles is
higher than 85%, the value of crash rates is very low. As the percentage of heavy vehicles is lower than 10% on the freeway, the
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broadcast and variable message broads should be used to arouse the awareness of drivers to stay in safer conditions.

              Table 4    Statistics of speeds and traffic volumes in 24 h for 3 segments from up and down directions
                           Mean /                                                                    85% /                                           SD /         Volumes of           Volumes of         Percentage of
                Up
                           (km/h)                                                                    (km/h)                                         (km/h)       heavy vehicles           cars          heavy vehicles /%
             The first      73.74                                                                                           86                      11.92             5 113              32 062              13.75
            The second      81.84                                                                                           98                      15.14            4 988               13 712              26.67
             The third      83.34                                                                                103                                18.05            4 738               5 845               44.77
                           Mean /                                                                    85% /                                           SD /         Volumes of           Volumes of         Percentage of
               Down
                           (km/h)                                                                    (km/h)                                         (km/h)       heavy vehicles           cars          heavy vehicles /%
             The first      69.36                                                                                           80                      10.70            6 570               38 586              14.55
            The second      76.98                                                                                           92                      14.00            6 604               15 747              29.55
             The third      80.12                                                                                103                                19.70            4 682               5 872               44.36
                                  Hourly number of crashes per




                                                                                                                                         2.5
                                                                                               million vehicle kilometers




                                                                                                                                          2

                                                                                                                                  1.5

                                                                                                                                          1

                                                                                                                                  0.5

                                                                                                                                          0
                                                                                                                                                0           20       40        60         80      100
                                                                                                                                                            Percentage of heavy vehicles /%
                                Fig.1                                                                                  Percentage of heavy vehicles and crash rate for 2 directions

4.2   Average Speed Differential Between Cars and Heavy Vehicles and Crash Rate
     The average speed differential is from 3.22 km/h to 39.01 km/h between cars and heavy vehicles. Intuitive impression believed
that crashes wouldn’t tend to happen if different vehicles travel at the same speed. The freeway’s analysis didn’t support such
recognition. Fig.2 shows a relationship with “bell shape” for the 2 variables, that is, when the average speed differential of 2 vehicle
types falls in the range between 3 km/h and 33 km/h, the crash rate decreases with increasing average speed differential. While that
differential is higher than 33 km/h and lower than 39 km/h, the crash rate would increase with increasing average speed differential
between cars and heavy vehicles. When the average speed differential is 10 km/h, the crash rate is high. When the differential is from
20 km/h to 35 km/h, the crash rate is low. Actually, the range of low speed differential represents the condition of congested traffic
flow, at that time vehicles are difficult to keep the adequate safe headway, so the rear crashes are apt to happen. On the other hand,
when the speed differential is very high, slow heavy vehicles will impede the free running of the high-speed cars, cars urge to
overtake the trucks so that speeding is frequent and risk of crashes rises, thereupon the crash rate is high. Table 5 returns to results of
quadratic model analysis for the 2 variables. It is emphasized that the results may be inconsistent with these when the analytic
conditions of space and time are different.
                                                        Hourly number of crashes per million




                                                                                                                                          2.5

                                                                                                                                          2.0
                                                                                                                    vehicle kilometers




                                                                                                                                          1.5

                                                                                                                                          1.0

                                                                                                                                         1.5

                                                                                                                                          0.0

                                                                                                                                         -0.5
                                                                                                            0         10         20        30        40        50
                                                                                                     Average speed differential between cars and heavy vehicles /(km/h)
                                                                                                                            Fig.2                   Average speed differential between cars and

                                                                                                                                                                     1982
                                              heavy vehicles and crash rate for 2 directions
                  Table 5    Regression by quadratic model between the average speed differential of cars and
                             heavy vehicles and crash rate for 2 directions
              Dependent variable: Crash rate                                                 Method: Quadratic
              Multiple R           0.726 73
              R Square              0.528 14
              Adjusted R Square    0.521 04
              Standard Error       0.404 78
                                                                           Analysis of Variance:
                                                      DF       Sum of Squares         Mean Square
              Regression                               2          24.390 287           12.195 143
              Residuals                               133         21.791 219            0.163 844
              F =74.431 54                               Signif F = 0.000 0
                                                                            Variables in the Equation
              Variable                                               B             SE B          Beta          T       Sig T
              DIFFERENCE                                         -0.119 641      0.018 059     -1.668 393   -6.625   0.000 0
              DIFFERENCE**2                                      0.001 804      0.000 449 1.010 788          4.014   0.000 1
              (Constant)                                          2.111 986     0.167 238                   12.629   0.000 0


4.3   Percentage of Heavy Vehicles and Average Speed
     Fig.3 illustrates that the percentage of heavy vehicles significantly influences the average speed. When the percentage of heavy
vehicles is higher than 20%, the average speed has a negative correlation with the percentage of heavy vehicles. The average speed
increases with an increase in the percentage of heavy vehicles for 2 directions when the percentage of heavy vehicles is lower than
20%. Maybe on the one hand a small amount of heavy vehicles means regularly congested traffic flow on the first segment because
of lots of cars exiting, the average speed is low, on the other hand a few heavy vehicles on the second and third segments have
encourage the car drivers to overtake frequently, and then the increase of speed of cars raise the average speed of the whole traffic
flow. When a large number of heavy vehicles exist on the freeway, dynamic performance of heavy vehicles is limited and the car
drivers are willing to adopt low speed due to the pressure of heavy vehicles, so the average speed of the whole traffic flow decreases.
4.4 Percentage of Heavy Vehicles and Speed Differential Between Cars and Heavy Vehicles
     Fig.4 shows that when the percentage of heavy vehicles is lower than 50%, the average speed differential between cars and
heavy vehicles increases as the percentage of heavy vehicles increases. When the percentage of heavy vehicles is higher than 50%,
the average speed differential between cars and heavy vehicles shows stochastic distribution. Car drivers would tend to speeding
when the percentage of heavy vehicles is lower, which would augments the average speed differential between cars and heavy
vehicles. As the percentage of heavy vehicles is much great, different driver behaviors lead to sophisticated features of speed choice.
Some conservative drivers travel at low speeds to ensure safety, some aggressive drivers overtake the slow heavy vehicles very
frequently, and thus the average speed differential could not appear specific distributions.


                                                      100
                              Average speed /(km/h)




                                                        80

                                                        60


                                                        40

                                                       20


                                                        0
                                                             0           20          40          60         80       100
                                                                          Percentage of heavy vehicles /%

                             Fig.3                      Percentage of heavy vehicles and average speed for 2 directions




                                                                                     1983
                                   Average speed differential between
                                                                                                          45




                                                                        cars and heavy vehicles /(km/h)
                                                                                                          40
                                                                                                          35
                                                                                                          30
                                                                                                          25
                                                                                                          20
                                                                                                          15
                                                                                                          10
                                                                                                          5
                                                                                                          0
                                                                                                               0        20          40          60             80        100

                                                                                                                             Percentage of heavy vehicles /%

                                                                        Fig.4                              Percentage of heavy vehicles and average speed differential
                                                                                                            between cars and heavy vehicles for 2 directions

5     Conclusions
     The relationships among percentage of heavy vehicles, average speed differential between cars and heavy vehicles and crash
rate are strong for the freeway in this study. The results of the study show that crash rate is greater when average speed differential
between cars and heavy vehicles is very high or very low.
     The results of this research are unable to represent the whole freeways in China. However, its methodology contributes to
studying the relationships among speed, traffic flow components and crash rates for any individual roads. The research has proved
that heavy vehicle is a dominant factor to influence the operation efficiency, especially the safety of freeways. Consequently, the
safety management on heavy vehicles is the vital contents of improving traffic safety level for the freeways.

Acknowledgements
     The authors gratefully acknowledge the support of the company that manages the freeway and department that records accidents
of the freeway. Our thanks also go to other members of the research team of traffic safety of our lab, who played a great part in the
effort to assemble the required data for the study.

References:
[1]   Gao Jianping, Liu Benmin, Guo Zhongyin. Influence of heavy vehicles on traffic safety of freeways. Journal of Tongji University (Natural Science),
      2005, 33(6): 763-767(in Chinese)
[2]   Nicholas J Garber, Kirsten A Black. Advanced Technologies for Improving Large- truck Safety on Two-lane Secondary Roads. VTRC 95-R17. Virginia:
      Virginia Transportation Research Council, 1995
[3]   Transportation Research Board. Twin Trailer Trucks: Effects on Highways and Highway Safety. Special Report No. 21 1. Washington, DC, 1986
[4]   Hedlund J. The Severity of Large-Truck Crashes. DOT HS 802-332. NHTSA Technical Note. Washington D.C.: U.S. Department of Transportation, 1977
[5]   Garber J J, Gadiraju R. Factors Affecting Speed Variance and Its Influence on Accidents. Transportation Research Record No. 1213, 1989. 64-71
[6]   Nicholas J Garber, Angela A Ehrhart. The Effect of Speed, Flow, and Geometric Characteristics on Crash Rates for Different Types of Virginia Highways.
      VTRC 00-R15, 2000
[7]   Kara M Kockelman, Jianming Ma. Freeway speed and variations preceding crashes, within and across lanes. 2004 Annual Meeting of the Transportation
      Research Board, 2004
[8]   Rune Elvik, Peter Christensen, Astrid Amundsen. Speed and Road Accidents. TOI Report, 2004
[9]   Garber N J, Ravi Gadiraju. Factors Affecting Speed Variance and Its Influence on Accidents. AAA Foundation for Traffic Safety. Falls Church, VA, 1988
[10] West L B Jr, Dunn J W. Accidents, speed deviation and speed limits. Traffic Engineering, 1971, 41(10): 52-55




                                                                                                                                1984

				
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