A new road safety indicator based on vehicle trajectory analysis

In 2014, in France, accidents on rural roads accounted for roughly 63% of road mortality (ONISR, 2014). Yet, the locations of accidents are increasingly spread, and it is becoming difficult for road managers to set priorities for interventions on their road network. They can no longer base their strategy on accident analysis. Road managers need to quickly measure the effectiveness of a modified facility, improvement of facilities or modification of the environment. On this type of site accident analysis requires a period of 3 to 5 years to determine the effectiveness of a change. This period of time does not meet the requirements of road managers or road users. To provide a high safety level, managers are implementing innovative developments: wall effect chicane at crossroads, dynamic warning signs, rural roundabouts, etc. Managers want to quickly assess these innovations. In addition, they have less and less money, so it is necessary to give them tools to help prioritize their intervention. It is also preferable for road managers to take preventive action rather than having to wait for accidents or serious injuries to occur, in order to carry out diagnosis and propose countermeasures. In this article, we propose a new safety indicator based on trajectory analysis. By trajectory, we mean a time-function that describes vehicle movement including not only its path on the road but also speeds and accelerations in both horizontal directions (longitudinal and transverse). These trajectories describe vehicle/infrastructure/driver interactions and reveal inadequacies that can lead to accidents. These trajectories are obtained using two roadside observation systems that analyse vehicle trajectories on two specific locations: curves and intersections on rural roads. The first tool is a local trajectory measurement system. It records trajectories of all vehicles passing through a specific zone. The goal is to observe abnormal paths that reveal dysfunctions in the vehicle/Infrastructure/driver system. The main applications-related contribution of this work focuses on developing a comprehensive solution for accurately estimating vehicle trajectories based on a combination of video images and range finder (telemeter) measurements. The purpose of the second tool is to improve understanding of dysfunctions at a crossroads and to evaluate the effectiveness of facilities. A precise study of patterns encountered on accident-prone intersections on rural roads showed that some situations are much more dangerous and frequent than others. In particular, most accidents at intersections are caused by vehicles crossing main roads. The second system presented in this paper detects and records conflicts between users on the secondary road who are crossing the main road, and users driving straight ahead on the main road. This kind of conflict represents the main type of accidents called crossing at grade accidents. The proposed risk indicator is equal to the number of conflicts (near-misses), weighted by the severity of the conflicts recorded in a given period. We chose to take the number of conflicts per hour. The purpose of these tools is to understand anomalies in thevehicle/infrastructure/driver system, and to evaluate the effectiveness of countermeasures on safety. Associated with risk indicators they make it possible to objectively assess driver behaviours with respect to the road infrastructure. (C) 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.