The relationship between speed and the risk of serious injury in light vehicle crashes

Abstract

The current set of guiding principles for road safety, the Safe System, has the ultimate aim of eliminating serious and fatal injury from road crashes. Safe speeds are considered one of the key elements of the Safe System. Suggestions have been made regarding what these ‘safe speeds’ may be, but they have not been derived empirically. In order to define these ‘safe speeds’, the relationship between speed and the risk of serious and fatal injury must be determined. The aim of this thesis was to examine the relationship between speed and the risk of serious injury in road crashes. Speed was further categorised into three speed indices that represent the speed of a vehicle specific to different phases of the crash sequence: travel speed, impact speed, and change in velocity (delta-v). A further aim was to use a relatively new and highly accurate source of speed data, event data recorders (EDRs). The relationship between speed and injury risk for crashes involving light vehicles was examined across four studies. The first study examined the relationship between the speed limit and the risk of a crash being fatal for different crash types. This was done using data from police reports on all crashes in two Australian states. Two subsequent studies examined the risk of serious injury relative to travel speed and impact speed, by impact type. Both of these studies used data extracted from an American database that includes EDR speed data and AIScoded injury data. The final paper, utilising the same database, examined the relationship between travel speed, impact speed and delta-v. Speed limit, travel speed, and impact speed were all found to be positively related to the risk of serious and/or fatal injury. The risk was found to differ by crash or impact type. Head-on crashes and head-on impacts generally carried the highest risk for a given impact speed, travel speed or speed limit. The exception to this was that, for travel speeds above 92 km/h, side impacts carried a higher risk of serious or fatal injury. Hit-fixed-object crashes were also found to have a similarly high risk at a given speed limit to head-on crashes. The relationships between the speed indices showed that part of the reason that head-on crashes and hit-fixed object crashes have a higher risk of serious or fatal injury at a given speed is that a higher proportion of the travel speed is translated to delta-v. The findings highlight the role of the speed indices in the injury outcome of road crashes and provide the risk curves necessary to define ‘safe speeds’ once the question of acceptable risk has been settled. The high risk at a given speed found for head-on and hit-fixed-object crashes suggests that measures to prevent these types of crashes should be prioritised. Measures that reduce travel and/or impact speed will be effective in improving road safety and moving towards achieving the ultimate goals of the Safe System.