Introduction to Crash Modification Factors
What is a CMF?
A Crash Modification Factor (CMF) is a multiplicative factor that indicates the proportion of crashes that would be expected after implementing a countermeasure. Examples of countermeasures include installing a traffic signal, increasing the width of edgelines, and installing a median barrier. CMFs with a value less than 1.0 indicate an expected decrease in crashes. CMFs greater than 1.0 indicate an expected increase in crashes.
Example: A particular stop-controlled intersection is expected to experience 5.2 total crashes per year. The city is considering installing a traffic signal and has identified a CMF for installing a traffic signal of 0.56 for total (or “all”) crashes (Harkey et al., 2008). The expected total crashes after installing the signal would be 5.2 x 0.56 = 2.9 total crashes per year.
A Crash Reduction Factor (CRF) is another way of representing the expected effect of a countermeasure in terms of the percentage decrease in crashes. A CRF is equal to 100*(1-CMF). In the example above, the CRF of the countermeasure would be 100*(1-0.56) = 44. The CRF terminology has traditionally been used by departments of transportation around the U.S., but in recent years, the safety field has moved to using CMFs. A major reason for this is the confusion that can arise when a countermeasure is expected to increase crashes. For instance, a newly installed traffic signal would be expected to increase rear end crashes. A CMF for this countermeasure would be a value larger than 1.0 (e.g., 1.58) whereas the CRF would need to be confusingly represented as a negative reduction (e.g., -58).
Another way of representing the safety effect of a countermeasure is through a Crash Modification Function (CMFunction). A CMFunction is an equation used to calculate a CMF based on the characteristics of the site where it will be applied. For example, this CMFunction is used to express the effect of changing lane width on rural frontage roads:
Using this CMFunction, the CMF for converting a 10 foot lane to a 12 foot lane would be equal to e^(-0.188(12-10)) = 0.687, which represents an expected decrease in crashes.
Often a study develops CMFs for other crash types, such as all (or total) crashes, rear end, and left turn. It is important to use a CMF that was developed for the same crash type as the crashes to which it will be applied. For instance, it would be incorrect to use another CMF from the study in the example, such as the CMF of 0.23 for angle crashes. Applying that CMF, which is lower than the 0.56 for total crashes, would overestimate the benefit of installing the traffic signal, and would be incorrect since it should only be applied to angle crashes.
Other examples of applying CMFs can be found in the CMFs in Practice series, the Highway Safety Manual, or the various training links provided on the CMF Clearinghouse.
How are CMFs used?
A CMF provides a quantitative estimate of the effectiveness of a countermeasure. This allows a CMF user to:
- Identify the most cost-effective strategy when considering various countermeasures. CMFs can indicate which countermeasure will have the greatest impact on decreasing crashes and quantify the benefits for each alternative under consideration.
- Identify the most cost-effective locations for using safety funding. CMFs can assist in determining where to deploy countermeasures in order to yield the most crash savings.
- Compare the results of new analyses to existing CMFs. CMFs can assist researchers by giving a context to newly produced CMFs to check for reasonableness in the results.
- Check validity of assumptions in cost-benefit analyses. CMFs provide a basis for conducting cost-benefit analysis to determine if a proposed project is worth undertaking.
