Developing an accident modification function for speed enforcement

This paper presents an accident modification function for speed enforcement. The function describes the percentage effect of enforcement on the number of injury accidents as a function of the relative change in the amount of enforcement. Thirteen studies that have evaluated the effects of different levels of enforcement, each containing between four and eight data points, were used as data source. A hypothesis regarding the shape of an accident modification function was formulated to help assess the plausibility of the estimated relationships. In the first stage of analysis, functions were fitted to the data points of each study by means of weighted regression analysis. Many of these functions were highly implausible and likely to be influenced by outlying data points and random variation. In the second stage of analysis, data from all studies except two were pooled and data points referring to identical or almost identical levels of enforcement were aggregated to reduce the influence of random variation. The number of data points was thus reduced from 63 to 15 and an accident modification function was fitted to the data points. The function was consistent with the hypothesis guiding the study.

► Thirteen studies of speed enforcement were reviewed.
► An accident modification function was fitted to each study.
► Many of these functions were implausible.
► Data from eleven studies were pooled.
► An accident modification function was fitted to the pooled data.