The Uses of Exposure and Risk in Road Safety Studies
A-S Hakkert, Technion, IL; L Braimaister and I van Schagen, Dutch Road Safety Research Institute (SWOV), NL
This paper explores the theoretical possibilities of the definition of exposure and risk, discusses the problems associated with the use of exposure and risk and brings a variety of examples of safety studies in which use is made of exposure and risk indicators. The paper sets out with a definition of the three central terms used- accidents, exposure and risk. Each of the three terms is defined and problems of definition, accuracy of measurement and reporting are described. The conclusion reached is that, whereas there are general definitions of exposure and of risk used in the health prevention and risk analysis fields, in road safety practice these terms should be defined within the context of the issue studied. For each application, the correct exposure measure should be used. This is sometimes made impossible because the required information is not available, orhas to be collected at great cost. Generally, the more aggregate the exposure measure, more indirect variables are introduced which casts shadows over the resulting risk calculations.
In the case of transport, the most widely used measure of exposure is the amount of travel for each travel mode. In some cases, useful additional insight is provided by taking into account the speed of travel, in which case exposure can be expressed as the amount of time spent in the traffic system. With the developments in recent years in the use of electronics and telecommunications inside vehicles and along the roads, and the increasingly widespread use of mobile telephones, it is becoming easier to collect up-to-date and reliable information on a variety of parameters that could be of importance in thecalculation of vehicle exposure and risk.
One of the contexts in which risk is used, is comparing risks between different parts of the transport system, different transport modes or even different activities outside the field of transport. The desire is to make various activities exposed to equal risks, so as to make them "fair". It is concluded that the desire for equal risks is not practical. It is more useful to search for ways to make each segment of the transport system as safe as possible, keeping cost-effectiveness considerations into account.
The paper presents some of the general issues associated with the use of risk and exposure, over-representation, the setting of national targets and risk- effectiveness. It also discusses a variety of examples of safety studies that use risk definitions and some safety studies that were conducted without the use of risk to evaluate safety effects. The relationships between accidents, exposure and risk as applied in those studies concerned with the safety of road infrastructure, including the relationship between accidents and traffic volumes and the issue of risk as applied to the identification of hazardous locations are discussed in detail.
Association for European Transport