Estimation of the Safety Effectiveness of Lane and Shoulder Width: Case-Control Approach

Abstract

There is a need to better understand and quantify the safety implications of geometric improvements and countermeasures. A commonly accepted measure of safety effectiveness is the crash modification factor (CMF) defined as the expected number of crashes with a countermeasure divided by the number expected without the countermeasure. This paper evaluates the use of a case-control design to directly estimate the safety effectiveness of lane and shoulder width using the odds ratio: the probability of a crash with the countermeasure divided by the probability of a crash without a countermeasure. The case-control design is well established in epidemiology where it is used to relate risk factors within a study population to a particular outcome or disease. In the highway safety context, the “outcome” is defined as a crash, the “risk factor” is a particular geometric feature or countermeasure and the “subjects” are roadway segments. This paper describes the outcome of an experiment in which commonly available crash, roadway and traffic data are used to estimate odds ratios which are compared to CMFs from the literature. The comparison of these slightly different effectiveness measures are intended as a test of the efficacy of the case-control method in safety effectiveness evaluation. Geometric, traffic and crash data were obtained for more than 28,000 rural two-lane undivided highway segments in Pennsylvania for years 1997 to 2001 inclusive. A matched case-control design is developed to estimate the safety effectiveness of different lane and shoulder widths while adjusting for the confounding variables: speed limit, average daily traffic and segment length. Conditional logistic regression models are developed to estimate the odds ratio for lane and shoulder width separately. Lane and shoulder width odds ratios compare favorably to CMFs from the literature and have the additional advantage of including confidence intervals on the estimates. The paper concludes with a discussion of strengths and weaknesses of the method and recommendations for future research.