Scientific paper ID 2546 : 2024/3

The number of people killed or injured in road accidents depends basically on the three factors: exposure, accident rate and injury severity. Exposure denotes the amount of activity in which accidents may occur. Any human activity is exposed to the risk of accident, but as far as road traffic is concerned, the amount of activity usually refers to the amount of travel, that is number of person kilometers of travel performed. There are various ways in which one can travel by road: as a pedestrian, by cycling, by driving a car, by taking the bus, etc. Not all of these ways involve the same level of accident risk. The type of exposure chosen is therefore an aspect of exposure that influences the number of people killed or injured in road accidents. Furthermore, the risk to which one is exposed as a road user is probably not independent of the combination of various means of transport in traffic. The risk of accident to each pedestrian, for example, is usually lower, the higher the proportion of pedestrians in traffic. Hence, the relative proportions of the various means of transport in a traffic stream may influence the overall level of risk represented by traffic stream. Accident rate is the risk of accident per unit of exposure and is an indicator of the probability of accident occurrence. Although an accident rate is not identical to an estimate of probability, it is a useful indicator as the probability of accident occurrence, in the theoretical sense, can be assumed to be proportional to the accident rate. The higher the accident rate, the higher the probability of an accident on a given trip of a given length. Sometimes the terms risk of accident, level of risk, or accident risk will be used synonymously with accident rate. Although the accident rate is defined per unit of exposure, it is not independent of exposure. Ideally, an exposure measure ought to be defined in a way that accident rate and exposure are independent.