Estimation of Safety Effectiveness of Changes in Shoulder Width with Case Control and Cohort Methods

The use of observational data to estimate safety effectiveness for changes in shoulder width is explored. Common statistical models are applied to estimate safety effectiveness of shoulder width; however, two unique approaches, the case control and the cohort methods, are used in the design stage. Case control and cohort designs are common methods in epidemiological studies; they are used to develop relationships between risk factors and disease. In highway safety, case control and cohort designs show potential for relating geometric and traffic characteristics to crashes. Case control and cohort methods produce estimates of the odds ratio and relative risk, respectively, which indicate the expected percentage change in the probability of an outcome, given a unit change in the risk factor. In the highway safety application, the odds ratio and relative risk may provide a reasonable estimate of the crash modification factor (CMF) for the risk factor in question. Geometric, traffic, and crash data were obtained for two-lane, rural, undivided highway segments in Pennsylvania. Conditional logistic regression models were developed to estimate the odds ratio for shoulder width, and Cox proportional hazard models were developed to estimate relative risk. Confounding variables were identified in the preliminary analysis and adjustments were made for confounders during the design and final analysis stages. Estimates from the two methods indicate that crashes decrease as shoulder width increases. Results are compared with the CMF presented in the Highway Safety Manual, and confidence intervals are included to illustrate the certainty of the estimates.