A safe road transport system : Factors influencing injury outcome for car occupants

Abstract: Given that over a million people are fatally injured in road accidents each year, the need for a systematic proactive approach is undeniable. The Swedish Road Administration (SRA) has developed a model for a safe road transport system based on the Vision Zero philosophy, to identify and prevent deviations from a safe system approach with regard to crashes. The overall objective of this thesis was: to study road crashes using this system approach, to identify whether the SRA model could be used to classify fatal crashes, and to identify system weaknesses as well as important factors that need to be addressed to further develop a safe road transport system. The thesis comprises four studies based on real-world crashes in Sweden. Three kinds of data were used in the studies: in-depth fatal crash data, in-depth car crashes involving cars fitted with an on-board crash pulse recorder, and observational data. In two studies, the aim was to investigate the interactions between a few safety performance indicators (SPIs), and how these indicators could be used to identify the most important factors in road crashes. The other two studies focused on SPIs of the vehicle and the road, to evaluate whether the SPIs used reflect the most important factors in the system. The aims of these two studies were also to present average crash severity, depending on collision partner as well as road safety standard. Most road traffic injuries are related to an interaction between the three components: the road, the vehicle and the road user. Therefore a system approach is needed to analyse crashes and to find preventive interventions. The SRA model was found to be useful for classifying in-depth fatal crashes. However, to identify weaknesses in the road traffic system, a more sophisticated model is needed. Based on crashes involving cars fitted with an on-board crash pulse recorder, crash severity was found to differ depending on collision partner. Frontal two-vehicle crashes and single-vehicle crashes with rigid roadside objects were shown to generate the highest crash severity. The least harmful crash type was single-vehicle crashes into deformable objects. Furthermore, crash severity was lower in crashes occurring on roads with a good safety rating than in those that occurred on roads with a poor safety rating. While it was found that a higher speed limit resulted in higher crash severity on roads with a poor safety rating, the opposite was found on roads with a good safety rating. The main reason for this was that lanes for traffic travelling in opposite directions were more often separated at higher speeds on roads with a good safety rating. On divided roads, no crashes resulted in a crash severity above the level corresponding to a 10% risk of sustaining serious or fatal injury. Simultaneous 100% fulfilment of a set of SPIs (sober driver, non-excessive speed, seat belt use and divided roads) also supports this finding, since only 5% of all fatalities on rural roads in Sweden occurred under these circumstances. Divided roads are therefore one of the most important SPIs for car occupants. The overall the results of the thesis indicates that it is necessary to establish a system approach, where the road infrastructure is based on the capabilities and limitations of human beings through good road and vehicle design.