What does a crash modification factor quantify in safety analysis?

• A. The ratio of crashes after the countermeasure to crashes before
• B. The expected total number of crashes over the study period
• C. The ratio of total cost to crashes avoided
• D. The probability of a crash occurring in any given year

Answer: (A)

The crash modification factor is a multiplier that shows how a countermeasure changes crash frequency. It is the ratio of crashes expected after implementing the countermeasure to crashes that would have occurred without it. A CMF less than 1 indicates a crash reduction, greater than 1 indicates more crashes, and exactly 1 means no detectable effect. To estimate post-implementation crashes, you multiply the baseline crash count (or rate) by the CMF. For example, if there are 100 baseline crashes and the CMF is 0.7, you’d expect about 70 crashes after installing the countermeasure, assuming exposure stays the same. It’s not the total number of crashes over the study period, not a cost figure, and not a yearly probability of a crash; it’s the relative change due to the countermeasure.