Accident Study Terminology
Using accident data to determine the cause(s) of crashes and finding ways of preventing them or reducing their severity is a primary objective of SHA. In order to understand how highway safety professionals accomplish these goals, one needs to first understand a few commonly used terms.
Accident Frequency
Accident frequency is the number of accidents that occur at an intersection or on a section of highway over a period of time. Frequency is often used to assess trends of accidents at a single location. These trends are studied for three, five, or ten year periods, so that the average annual experience can be assessed, smoothing out the high and low fluctuations that occur yearly.
Accident frequency alone is not sufficient to compare the relative hazard of one location to that of another; the traffic volume exposure and the highway section length must be taken into consideration to have a valid comparison. For instance, if two intersections have the same accident frequency, the location having less traffic exposure, and hence less opportunity for accidents, is considered to be more hazardous.
Traffic Volume Exposure
Traffic volume exposure is the amount of traffic traveling along a section of road or entering an intersection. This is commonly expressed as average daily traffic (ADT).
Accident Rates and Percentages
For a section of highway, the rate is calculated as accidents per 100 million vehicle miles of travel (Acc/100MVM). For an intersection, the rate is calculated as accidents per million vehicles entering (Acc/MVE).
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