The effect of new technologies on speed distributions.

A multitude of literature confirms the negative effect of speed on road safety. To complement this there are numerous studies that have investigated the effects that various ‘traditional’ safety improvements have had on vehicle speeds and consequently accidents. New technologies, in particular Intelligent Speed Adaptation (ISA), now offer the possibility of traffic calming through the vehicle. Studies on the safety impact of ISA have typically either considered the effect of speed control on particular accident types or have estimated accident reductions based upon an overall reduction in mean speed. Considering only accidents that are clearly speed related ignores the fact that reduced speed allows the driver more reaction and evasion time. This reduces the likelihood of an accident situation arising, increases the chances of an avoidance action being successful and reduces the severity of impact should the evasive action be unsuccessful. Reducing speeds will therefore impact upon a wider range of accident types than just those accidents which are clearly related to excessive speed. On the other hand, considering only the relationship between accidents and speed does not account for the ability of ISA to impact more severely on accidents that occur at higher speeds. Neither method can be considered satisfactory since both include some implicit assumptions about the shape of the speed distribution overall. The advanced technologies currently being considered have the ability to change the speed distributions so dramatically that these methodologies and the associated assumptions are invalidated. In particular they create a ‘spike’ of persons travelling at or just below the speed limit set by the system (Tate, 1997). In order to assess the potential benefits of such new technologies it is important to clearly establish: the link between the accident liability of individuals and their location within the speed distribution, and the likely pattern of relocation when drivers who would typically travel at higher speeds are limited in their speed. (Author/publisher) For the covering abstract see ITRD E116881.