The uses of exposure and risk in road safety studies

This report explores the theoretical possibilities of defining exposure and risk, discusses the problems associated with the use of exposure and risk and gives examples of various safety studies in which use is made of exposure and risk indicators. The report sets out with a definition of the three central terms used: accidents, exposure and risk. Each of the three terms is defined and definition problems and problems concerning accuracy of measurement and reporting are described. The conclusion reached is that there are general definitions of exposure and of risk, as used in the field of health prevention and risk analysis, but that 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 impossible on account of the unavailability of the required information, or the necessity to collect it at great cost. Generally, the more aggregate the exposure measure, the more indirect the variables that are introduced, which casts shadows over the resulting risk calculations. In the case of transport, the most widely used measure of exposure is the number of kilometres travelled for each travel mode. In some cases, useful additional insight is provided by taking into account the speed of travel, in which case exposure is expressed as the amount of time spent in the traffic system. One of the developments in recent years has been the installation of electronic and telecommunication equipment inside vehicles and along roads. Another development is the increasingly widespread use of mobile telephones. As a result it is becoming easier to collect up-to-date and reliable information on a variety of parameters that could be of importance in the calculation of vehicle exposure and risk. Additional information on the distribution of speeds, types of vehicles and following distances also seems to be a future possibility. One of the contexts in which the term risk is used, is in 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 have various activities exposed to equal risks, so as to establish a fair distribution of risks. After discussing the various problems and pitfalls in using this approach, it is concluded that the desire for equal risks in various segments of the transportation system 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.