Crash modification factors (CMFs) are a critical component of the highway safety management process and the Highway Safety Manual (HSM) – See Special Note B. CMFs can be used to compare the relative effectiveness of alternative treatments and adjust baseline estimates from safety performance functions (SPFs) – see Special Note C. CMFs have been (and continue to be) developed for a range of crash countermeasures. While considerable effort has been expended to develop CMFs, important questions have arisen from state departments of transportation (DOTs) safety engineers and other practitioners about the accuracy of these CMFs when used for some real-world applications. For example, will a CMF that is developed using data from one region of the country and at sites with specific geometric design and traffic control features be equally applicable to a different region for slightly different design and control features? Or if several treatments are applied to the same site, will their combined effectiveness be found to equal the value obtained by multiplying their respective CMF values? With regard to the first question, there are several instances where independent studies have examined the same treatment but reported different CMF values. The studies are sound but the findings suggest that treatment effectiveness is influenced by unmeasured site characteristics (e.g., geographic location, terrain, traffic demand, geometric design, traffic control features). In other words, there are characteristics associated with a site that may cause a treatment to be more (or less) effective than at the reference site. These factors could be used to define the situations for which the CMF is applicable or to calibrate an equation that can be used to compute the CMF value as a function of characteristics at individual sites. The ability to transfer CMFs will help agencies effectively identify cost-effective treatments for individual sites.With regard to the second question, there has been considerable research to quantify the effectiveness of individual treatments; however, there has been limited research to quantify the effectiveness when combining multiple treatments. This is an important question because transportation agencies frequently implement multiple treatments in a single location as part of their roadway safety management and project development processes. Research is also needed to produce guidelines for developing CMFs that can be used nationwide.
The objectives of this research were to develop:
- Guidelines for calibration of current CMFs to assess treatment effectiveness at sites for which the site characteristics (e.g., geographical location, terrain, traffic demand, geometric design, traffic control features) may be different.
- Guidelines for how existing and future CMFs can be combined in a single location with multiple treatments.
- Recommended procedures for formulating and calibrating future CMFs that identify key influential site characteristics.
These guidelines and procedures will assist transportation agencies in applying existing and future CMFs more accurately throughout the United States.
