Which statement best describes a crash modification factor (CMF)?

• A. The ratio of injuries to crashes
• B. The ratio of costs to benefits
• C. The ratio of crashes before to after
• D. The ratio of crashes after to before at the same site and exposure

Answer: (D)

Crash modification factor shows how a safety countermeasure changes crash frequency by comparing what happens after implementation to what happened before, at the same site and with the same level of exposure. That’s why the best description is the ratio of crashes after to crashes before, measured at the same location and with the same exposure. This setup isolates the effect of the countermeasure from other changes like traffic volume. If the CMF is less than 1, crashes drop after the change; if it’s greater than 1, crashes rise; if it’s about 1, there’s little or no effect. CMFs are not about injuries per crash, nor about costs and benefits; they’re specifically about crash counts adjusted for exposure at a given site.