When combinations of countermeasures are proposed for implementation, the value of the CMF for the combination is the product of the CMFs of the individual countermeasures, meaning when CMFs overlap, they are combined multiplicatively. CMFs are considered overlapping when they are applied to the same segment of roadway or intersection and the selected treatments address the same crashes. If the selected treatments do not address any of the same crashes, the CMFs are not considered overlapping.
The formulas below summarize how the overlapping CMFs, and overlapping Annual Crashes Reduced are calculated.
CMF1 = CMF for first/previous treatment
CMF2 = CMF for second/following treatment
N = Number of crashes
Calculating Overlapping CMFs
The value for the overlapping CMF can be calculated by calculating the combined CMF and then subtracting the original CMF:
And then reducing it to the following:
Calculating Overlapping Crashes Reduced
The value for the overlapping number of crashes reduced can be calculated by calculating the combined number of crashes reduced and then subtracting the original number of crashes reduced:
In the case of multiple overlaps, the above calculation is applied independently for each of the overlaps, as outlined in the illustration below. For the third and subsequent overlaps, the value for A become the “combined CMF” (i.e. the multiplied value from A and B in the first/previous overlap calculation) and B is just the CMF from the third/following treatment with that concept continuing for as many overlapping treatments as needed without having to consider impacts to previous calculations or making any adjustments to them.