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Detection of Safety Deterioration in a Series of Accident Counts

The task is to develop a workable procedure with the object of detecting sites where the mean accident frequency has increased over time to more than what can be attributed to changes in traffic or to general trends. Two kinds of increase are to be detected: a steady but gradual deterioration and a sudden increase in mean accident frequency at an unknown time. The procedure is to be applied to many sites, with the detected sites subsequently examined in detail. Both the gradual and the sudden increase in accident frequency are to be detected by a statistical test of significance for the difference between the means of two Poisson random variables. The software for doing such tests of significance has been developed and is available. An identification procedure is suggested to take account of (a) the number of sites that can be practically subjected to detailed examination and (b) the marginal yield of correctly identified sites.