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4 Introduction The field of highway safety has seen many advances in recent years. Estimates of counter- measure effectiveness, once based on anecdotal experience, are now available in the form of research-based crash modification factors and functions. Crash modification factors (CMFs) provide transportation professionals with the kind of quantitative information they need to make decisions on where best to invest limited safety funds. Resources such as the American Association of State Highway and Transportation Officials (AASHTO) Highway Safety Manual (HSM) and the Federal Highway Administration (FHWA) CMF Clearinghouse provide more than 5,000 CMFs for hundreds of countermeasures. Although there is still a strong focus on developing quality CMFs, other questions have arisen concerning how CMFs should be used by practitioners. Two of the most pressing issues deal with applying CMFs in practice. First is the question of the transferability of CMFs. In research studies, CMFs are developed using data from a sample of sites, often within one state. Additionally, a CMF may be available for a countermeasure but is focused only on one road type. Many practitioners want to know if a CMF developed under one set of conditions (e.g., state, roadway type, geography, and agency design practices) can be applied to a site with another set of conditions. Second is the question of estimating the effectiveness of applying multiple countermeasures to the same location. It is often the case that a transportation agency will apply several treat- ments to a location, such as installing advance warning signs, brighter chevrons, and reflective markers to a horizontal curve. While CMFs may exist for each individual treatment, the practi- tioner needs to know how these CMFs can be combined to accurately estimate the effect of the combined treatment. As CMFs are used more widely, the issues raised here become increasingly important. These questions must be addressed for CMFs to continue to be a useful tool in estimating the benefits of safety treatments. C H A P T E R Â 1 Background
