Case–control and cross-sectional methods for estimating crash modification factors: Comparisons from roadway lighting and lane and shoulder width safety effect studies

ProblemWhile observational before–after studies are considered the industry standard for developing crash modification factors (CMFs), there are practical limitations that may preclude their use in highway safety analysis. There is a need to explore alternative methods for estimating CMFs.MethodThis paper employs case–control and cross-sectional analyses to estimate CMFs for fixed roadway lighting and the allocation of lane and shoulder widths.ResultsBased on the case–control method, the CMF for intersection lighting is 0.886, while the cross-sectional study indicates a CMF of 0.881. The CMFs developed for lane and shoulder widths are also similar when comparing the two methods.ConclusionsThis paper suggests that case–control and cross-sectional studies produce consistent results if care is taken in the study design and model development.Impact on industryCase–control and cross-sectional studies may provide a viable alternative to estimate CMFs when a before–after study is impractical due to data restrictions.

Research highlights
► Observational before–after studies are the industry standard for developing CMFs.
► Practical limitations sometimes preclude the use of before–after studies.
► Cross-sectional and case–control studies are alternatives for estimating CMFs.
► Case–control studies account for many sources of variation in cross-sectional data.
► Cross-sectional analyses can produce results similar to the case–control method.