A section of roadway is being considered for two countermeasures at the same time. CMF1 is 0.69 and CMF2 is 0.78. If each targets a different crash type, what is the best estimate of the resultant combined CMF?

• A. 0.69 * 0.78 = 0.54
• B. 0.69 - 0.78 = -0.09
• C. 0.69 + 0.78 = 1.47
• D. sqrt(0.69 * 0.78) ≈ 0.83

Answer: (A)

When two countermeasures affect different crash types, their effects multiply because each CMF reduces the frequency of its own crash type independently. Multiply the two factors: 0.69 × 0.78 = 0.5382, about 0.54. So the best overall CMF is 0.54, meaning roughly a 46% reduction in total crashes across both types. Subtracting, adding, or taking a square root don’t reflect this independent, multiplicative combination.