Elementary Units of Exposure

Most road safety studies rely on summary measures of exposure. The term "summary measure" denotes any aggregate indicator of exposure that does not directly identify and count the number of opportunities for accidents to occur. This paper shows how elementary units of exposure can be developed on the basis of known aggregate measures, such as annual average daily traffic (AADT). An elementary unit of exposure refers to any event that generates an opportunity for an accident to occur. Four such events are identified: (a) encounters (i.e., vehicles passing each other in opposite directions of travel); (b) simultaneous arrivals at points of intersection between potentially conflicting directions of travel, in particular vehicles entering intersections at the same time or within a very short time interval; (c) change of travel lane on multilane highways; and (d) braking or stopping. These events describe traffic movements that occur before a wide range of crash types. The only major group of accidents that is not directly related to particular events is running off the road. The number of events expected to occur for each of the four types identified is estimated on the basis of the following assumptions: (a) AADT is known [when estimating the number of events, mean hourly volume (AADT/24) is used]; (b) vehicles or road users arrive at a point of potential conflict according to a Poisson process; and (c) simultaneous arrivals within a very short time interval (such as 1 s) have the potential to generate a conflict. The number of encounters and simultaneous arrivals in intersectioris increases considerably faster than AADT. The number of events that may generate conflicts involving lane changes, braking, or stopping increases more slowly than does AADT.