This study was commissioned and funded by the Department for Transport (DfT) formerly the Department for the Environment, Transport and the Regions (DETR). The objective of the study was to improve understanding of the extent to which accident risk increases in congested traffic and the changes in road user behaviours which give rise to this increase. Despite this presumption in the initial study objectives that accident risk may increase in congested conditions, the reality of the situation was that, with the exception of motorway sites, accident rates during periods of recurrent congestion are lower than those in uncongested conditions. This may in part be due to the fact that there will be more regular road users familiar with site conditions during periods of congestion. However, it is also likely to reflect the substantially lower speed of vehicles. The fact that for two-wheeled motor vehicles (twmv’s), cyclists and pedestrians the proportion of fatal or serious accidents remained the same in congested conditions may be a reflection of the vulnerability to injury of these particular road user groups or that the impact speeds are not changed in congested conditions. In practice, the study set out to address three key issues: a) do accident rates change in congestion; b) does driver behaviour change in congestion; and c) are there causal links between changes in behaviour and changes in accident risk. At an early stage in the study, definitions of congestion were developed that were easily identifiable and measurable. A motorway or peri-urban link was defined as being congested when the point average speed taken over 3 minutes was below 50 per cent of the speed limit. An urban link (with a signalised exit) was defined as congested when traffic could not exit the link within a time equivalent to one signal cycle while an urban link (with an unsignalised exit) was defined as congested when traffic could not exit the link within a time equivalent to one signal cycle. Two types of congestion were identified; ‘recurrent’ congestion which occurs at regular times at a site and ‘non-recurrent’ congestion, which is unexpected and usually the result of an incident such as an accident or vehicle breakdown. Because of the difficulty of collecting historic data relating to non-recurrent congestion, only sites suffering from recurrent congestion were considered in this study. The sites covered motorway, peri-urban and urban situations. (Author/publisher)
Abstract