Safe Transportation Research & Education Center

Safety interventions (e.g. median barriers, photo enforcement) and road features (e.g. median type and width) can influence crash severities, crash frequencies, or both. Both dimensions—crash frequency and crash severity—are needed to obtain a full accounting of road safety. Extensive literature and common sense both dictate that all crashes are not ‘created’ equal—with fatalities costing society more than 1000 times the cost of property damage only crashes. Despite this glowing disparity, the profession has not unanimously embraced or successfully defended a non-arbitrary severity weighting approach for analyzing safety data and conducting safety analyses.

This paper argues that both the crash frequency and severity dimensions are made available by intelligently and reliably weighting crash frequencies and converting all crashes to property damage only crash equivalents (PDOEs) using comprehensive societal unit crash injury costs. This approach is analogous to calculating axle load equivalents when predicting pavement damage—for instance a 40,000 lb truck causes 4025 times more stress than does a 4000 lb car and so simply counting axles is not sufficient. Calculating PDOEs using unit crash costs is the most defensible and non-arbitrary weighting scheme, allows for the simple incorporation of severity and frequency, and leads to crash models that are sensitive to factors that affect crash severity. Moreover, we show that using PDOEs diminishes the errors introduced by under-reporting of less severe crashes—an added benefit of the PDO equivalent analysis approach. The method is illustrated using rural road segment data from South Korea (which in practice should apply Korean crash cost data). Finally, the results and remaining issues with PDOEs and their adoption in safety analysis are discussed.