The crash prediction models (CPMs) in Part C of the Highway Safety Manual (HSM) are often used to infer the safety effect of a change in facility type and control mode. However, several researchers have observed that the HSM CPM for signal-controlled intersections predicts a larger average crash frequency than the HSM CPM for stop-controlled intersections (for the same volume level). Most recently, some of the CPMs for roundabouts developed in National Cooperative Highway Research Program (NCHRP) Project 17-70 for the second edition of HSM were observed to predict a larger average crash frequency than the HSM CPMs for stop- and signal-controlled intersections. This paper discusses why comparing two HSM CPMs to infer that the safety effect is problematic when the associated characteristic is endogenous to the comparison. The characteristics “convert to signal” and “convert to roundabout” are shown to be endogenous to the HSM CPMs being compared. The correct interpretation of the results from this comparison are described and possible reasons are offered to explain why these results should not be expected to agree with a CMF (for the same change in character). It is recommended that the HSM should offer guidance on the correct interpretation of the results obtained when comparing two HSM CPMs to infer safety effect of a change in character. The roundabout SPFs that were changed for inclusion in HSM2 no longer appear to give consistent or convincing results when comparing roundabouts with three- versus four-legs or with one versus two circulating lanes.
James A. Bonneson (Sun,) studied this question.