Accident Analysis

Reduction in accidents, and in particular injury and fatal accidents, is a primary focus of many ITS deployments. Integral to these is an understanding of how the existing traffic situation (driver behaviour, vehicle dynamics and road environment) relate to safety. Also key is understanding that an ITS scheme which is not in itself targeted specifically at road safety – may nevertheless result in changes in the level of safety as an unintended side effect.

Deployment of ITS can alter the balance of accident types. It is not uncommon with traffic schemes for one type of incident to be substantially reduced and another type to increase (perhaps of lower severity). In general, ITS deployments that reduce congestion and smooth traffic flows will reduce accidents. High variability in speeds — of the vehicle (rapid deceleration or acceleration) and between vehicles — is more likely to cause disturbances and incidents than steady vehicle speeds. In general higher overall speeds will increase both accident risk and severity.

Accident analysis is a major tool in obtaining an understanding of the existing situation and how it could be improved by ITS. It helps to provide an understanding of the most effective solutions and is essential for monitoring and evaluating the safety of the road network. It should be undertaken, following deployment of a system, to:

• confirm that there are no unexpected negative side effects of deployment
• evaluate the safety outcome of system deployment

Monitoring is used to verify that, after deployment, the system has produced the desired effects and there are no unexpected negative side-effects. An example might be the case of a VMS, where incidents could occur as a result of drivers slowing down - in order to read the VMS or to give themselves more time for decision-making after passing the VMS.

Evaluation compares the before and after situation - ideally also comparing it with a control road or location in which there has been no intervention. This provides reassurance that an observed improvement was not simply the result of an overall trend such as a general improvement in safety performance. A rigorous evaluation will require a statistically significant change in the number of accidents to demonstrate that the change observed is not the result of chance. (See Evaluation)

ITS systems themselves can provide data to enhance accident analysis. The systems can notify the emergency services and traffic management centre directly that an accident has taken place – for example via eCall, for which the in-vehicle technology is mandated in new European cars from 2018. (See Driver Support)

More generally, the data that is available from in-vehicle data recorders and roadside systems can be used to enhance accident analysis. Accident data could include information on traffic flow, weather conditions or the status of real-time traffic management systems. For some accidents, relevant information may be captured on roadside video.

ITS has also improved data collection at the accident scene through providing sophisticated mobile hardware which is capable of:

• pinpointing the location of the collision
• capturing digital images of the site
• accessing remote databases on-line for back-up information – such as car registration databases