This report describes an effort in estimating crash modification factors (CMFs) associated with different left-turn phasing schemes, at intersections where the major approach speed limit exceeds 40 mph. For installation of signals at previously thru/stop-controlled intersections, rear-end crashes increased while right-angle crashes decreased. Installation of the signal had no effect on either major or minor approach left turn crashes as long as the protected-only left turn phasing was used on the major approaches. At one intersection where a signal was originally installed with permitted/protected phasing on the major approaches, we found evidence for an increase in major approach left-turn crashes, which vanished when the major approach left-turn treatment was changed to protected-only. For several other phasing changes it was not possible to construct an after-treatment data set of sufficient size to permit reliable estimation of an effect.

This report also describes a simple simulation model for left-turn cross-path crashes, where a probabilistic gap acceptance model for the turning driver is combined with a standard braking model for the opposing driver. The model characterizes left-turn crashes as resulting when the turning driver accepts a minimal gap and takes an atypically long time complete his/her turn, while the opposing driver takes an atypically long time to react before braking. R reconstruction of an actual fatal crash however was more consistent with the opposing driver reacting normally, but with the turning driver selecting an atypically short gap. Characterizing the rate at which such selection errors occur would then be necessary to accurately predict left-turn crash frequencies.