Research On Level Of Service Of Safety For Freeway Diverging Area Based On Speed Consistency

Freeway system, which is fast, efficient and comfortable, develops rapidly in the last two decades in China. However, the safety situation is still serious, although the number of crashes and crash rate is in the trend of decrease. Diverging areas are the safety bottle necks on freeways. More than 30% of freeway crashes occurred on diverging areas and their influencing areas. Among these crashes, nearly 75% were related with speeding, which is one of the major factors contributing to freeway crashes. Speeding not ony increases the number of crashes, but also leads to severer accidents. From this point of view, conducting research on speed related safety issues will result in significant social and economic benifits. This paper focuses on the evaluation of level of service of safety on freeway diverging area, based on speed consistency.Firstly, the crash types and their characteristics on freeays and freeway diverging areas are introduced based on historical crash data. The major factors contributing to the crashes occurred on diverging areas are explored from the prospective of driver-vehicle-road-environment system. And the relationships between the factors explored are analized. The inherent relation between crash and speed and speed difference are revealed from the micro analysis to the major crash types. It is concluded that speed related parameters are the most significant and direct factors influencing the crash occurrence on freeway diverging areas.The measure, , named the 85th percentile individual speed difference, is proposed as one of the alternative speed difference measures. is capable of eliminating the the pitfalls possessed by the conventional aggregate measures (the representative is ), which are (1) ecological fallacy; and (2) over-estimation of safety. The relationship that is always equal or greater than is deduced in a mathematic way. Based on the data surveyed on the 17 diverging areas located on freeway G42, the operational speeds, operational speed differences, and the 85th percentile individual speed differences on each section are forecasted. To extract the measure related with speed statistics, mean speed, speed vatiance, skewness, and kurtosis are selected as candidate speed deviation measure. The correlation between safety and the candidates from both speed difference measures and speed deviation measures are analized in SPSS 17. Based the correlation, the safety level of the 17 diverging areas are determined qualititatively.To resolve the problem that the historical crash data related with diverging areas are not available, we find a surrogate solution. The solution is importing the vehicle trajectory files (.trj), which are extracted from the simulation output in VISSIM, to the software SSAM. SSAM is able to analize the trajectory files and obtain the traffic conflict data we need for modeling. Base on the correlation table between traffic conflict rate and speed difference measures, speed deviation measures and their transformations, the most significant measure is selected as the final speed consistency measure. To scale the safety level of target diverging areas, the concept, level of service of safety, is introduced. The criterion of the classification of speed consistency measures is proposed, and the level of service of safety is classified into five grades, from level S1 to S5. With the scale criterion established, the levels of service of safety of the 17 diverging areas are determined quantitatively.At the end of this paper, the countermeasures for improving the safety performance of freeway diverging areas are put forward with respect to reducing the speed difference and speed deviation. Speed zone method is chosen as the main solution to increase speed consistency and thus improve the level of service of safety.