Exploring the Need for Region-Specific Calibration Factors
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© 2017 National Academy of Sciences. States, or even large cities, may experience different crash rates in different regions or parts of the city. This variation can be attributed to differences in terrain, population, weather, and other unobserved characteristics. Hence, the variation can affect the calibration procedure and consequently the calibration factor when different crash rates are used for a large area. This study first investigated whether region-specific calibration factors were required and justified for large states, such as Texas and Michigan. Next, a procedure was proposed to determine if a specific region needed a different calibration factor from the one developed for the whole state. If it is determined that a separate factor is needed, the agency should derive a region-specific calibration factor with data that are collected within the region. The proposed procedure is based on the general characteristics of data at the network level: the total number of crashes, the mean value of traffic flow (average daily traffic or annual average daily traffic), and the total segment length (or the number of intersections). The procedure was validated for the two states mentioned and showed that the calculated calibration factor matched the factor proposed in the recommended decision-making procedure.
author list (cited authors)
Geedipally, S. R., Shirazi, M., & Lord, D.