Influence Of Road Feature Variables On Accident Rate

Licentiatavhandling, 2008

The main objective of this traffic safety investigation was to find critical road parameters affecting accident rate (AR). The secondary objective was to analyze any identified risk factors from a vehicle dynamics perspective. A better understanding of how road infrastructure affects safety can be used to predict future safety issues as well as provide guidance in selecting countermeasures when problems are identified. The study was based on accident and road maintenance data in Western Sweden. A total of 2912 accidents reported from 2000 to 2005 on 810 km median-separated roads were collected and combined with the road characteristics: reference speed, carriageway width, vertical and horizontal curves, super-elevation and road surface conditions. Factors that influence accidents have been studied through analysis of vehicle dynamics using the simulation program PC-Crash. The statistical analysis showed that the approach was sufficient for defining critical road parameters. Results showed variations in AR when road geometrical parameters changed and indicate that road characteristics have a significant effect on AR. In particular, a marked increase in AR was observed for decreasing curve radii and higher ARs were observed in right curves. Moreover, results of investigating the influence of various geometric features of curves on specific crash types showed that overtaking accidents are more frequent on right-turn curves than on left curves. Computer simulations with identical lane changing conditions showed both larger accelerations and yaw velocities in left curves than right curves. This reduces the likelihood that vehicle dynamics behaviour alone is behind the difference observed for overtaking accidents. The study also showed adverse effects of poor road surface parameters, such as wheel rut depth and road roughness (IRI), on AR. In addition, roads designed for high speed limit exhibited lower AR, but higher injury severity rates. The investigation approach and results reported in this thesis are useful input for road design guidelines and active safety systems such as ABS and ESC that are sensitive to the road characteristics. More importantly, there is a need increase the dialog between road engineers and vehicle designers in terms of traffic safety. In particular, gradual changes in vehicle characteristics over time do not seem to be reflected in changes in road design standards. For instance, improved road handling can lead to higher operating speeds than intended by road engineers. Therefore, an understanding of the higher operating speeds on specific road safety issues is useful. Similar examples apply for other vehicle-infrastructure interactions.