Urban motorway safety and traffic state

The metropolitan Melbourne motorway network carries 40 per cent of the urban arterial road travel and this trend is increasing. Although it has a low crash risk in terms of crashes per vehicle kilometres travel compared to other urban road types, casualty crash numbers have been increasing on the metropolitan Melbourne motorway network despite a decreasing trend on other urban roads. Infrastructure deficiencies are rarely mentioned as being causes of urban motorway crashes, rather mention is made about traffic conditions and vehicle-to-vehicle interactions. Analysis was undertaken to test the hypothesis that the dynamics of traffic flow, which cause congestion and require complex driver responses, are a very significant contributor to the casualty crash problem. A statistically significant association between traffic state and crashes was observed. Casualty crashes are more likely to occur when flow breakdown has occurred or where flow breakdown is likely or certain to occur. Besides absolute crash numbers, this was based on the exposure, that is on the percentage of traffic processed, for those traffic states. With increasing traffic, there is a growing risk that a motorway will operate in a critical traffic state most hours of the day resulting in more crashes if nothing is done to address this. The results of the analysis can be used to improve safety on urban motorways through the development of Intelligent Transport System (ITS) strategies to keep the motorway operating at conditions that minimise flow breakdown.