A Microsimulation Traffic Model to Estimate Accidents due to Reduced Sight Distance

The sight distance available on existing roads is very often lower than the stopping sight distance needed to avoid the collision with standing obstacles in the travelled way. When this occurs many rear-end crashes can happen because each follower could not see his/her leader. The investments needed to increase the sight distance to the value arising from the actual speed of the vehicles are usually very high. Due to budget constraints road agencies have to select the road segments to improve by estimating their safety performance. This activity is not an easy task. The most useful approach to address this type of problem is to use a microsimulation traffic model, which has to consider roadway configuration, sight distance, roadway surface condition, braking lights, human behaviour, etc. With this type of tool it would be possible to define the number of collisions or loosing control vehicles, their corresponding speed, the number of vehicle at collision “risk” in the queue, etc. In this paper the structure of a microsimulation model tacking into account most of the influencing factors is presented. It has been conceived with the aim to reproduce what happens in situations where the sight distance is reduced. The first part of the paper concerns the description of the modules in which the model is organized. The second part presents an application to a road segment where a standing obstacle exists in a curve and the sight distance is limited by a retaining wall. This situation has been compared with a similar road segment where the sight distance complies with the Italian Standard. The results obtained show the higher is the flow rate the lower are the rear-end crashes and loosing control vehicles. This is due to the fact that for high flow rate the vehicles are slower and a very high interaction exists among them, so that the braking lights of the leaders suggest to the followers to adopt a prudential behaviour.