A method for analysing the traffic process in a safety perspective

Sammanfattning: The aim of this study is to extend the traffic safety assessment concept to also include normal road user behaviour, thus not only exceptional behaviours such as accidents and serious conflicts. The goal is to provide a framework for a more thorough description and analysis of safety related road user behaviour in order to better understand the traffic safety processes. The traffic safety process can be seen as a continuum of events with different severity. The aim has been to construct a severity hierarchy for traffic events so that for each event a severity can be estimated. The severity hierarchy can be used for analyses of the traffic safety process, i.e. describing the relationship between accident related events. Relevant events in the traffic safety process are called interactions and are characterised by a collision course. The severity of the process is described by the Time-to-Accident and Speed values. The severity is based on the probability of a serious injury accident and refers to the severity of the event an infinitesimal unit of time before the evasive action. The outcome then depends on when the evasive action is taken and how successful it is. The shape of the hierarchy is affected by different factors, such as type of road user and type of manoeuvre involved, speed of the road users involved, traffic flow, intersectional design, etc. Road user behaviour is studied at two signalised intersections and at one non-signalised intersection with right hand rule. Interactional data, conflict data and accident data are collected for the three different sites with regard to the manoeuvre and the road user taking evasive action. The results suggest a border in the severity hierarchy above which a high occurrence rate of interactions is a sign of unsafety and beneath which a high occurrence rate of interactions is a sign of safety. It is, however, to be noted that a location with a high interaction frequency at low severity levels seems to produce the conditions for occasional events with high injury accident potential. The convexity of these interactions with less severity was in this study widely spread over several severity levels. The opposite pattern, a narrow convexity at reasonably high severities, seems to be the insurance for preventing the most severe types of events from occurring. This is probably due to the learning process, i.e. the increased awareness of the road users brought about by involvement in interactions with reasonably high severity. It is, therefore, from a safety perspective not only interesting to analyse the part of the hierarchy with the most severe events, but also to take the whole shape of the distribution into consideration.