The objectives of this research were to identify factors contributing to ramp terminal crashes anddevelop safety performance functions for four-leg diamond interchange ramp terminals. Four-legdiamond interchange ramp terminals in Wisconsin were sampled to construct two datasets. Thefirst dataset was comprised of 60 pairs of interchange ramp terminals and was used to do generalanalysis and identify the effects of geometric factors. The other dataset was comprised of onlysignalized diamond interchange ramp terminals and was used to study the effect of signal timing.The major factors considered included traffic volume, traffic control, and geometric design. Forsignalized interchange ramp terminals, yellow and all-red clearance intervals were alsoconsidered as factors. The presence of cross road median is negatively correlated with averagecrash frequency. For signalized interchange ramp terminals, the crash frequency increases withdeficient yellow or all-red intervals and decreases if there is an exclusive right turn phase.Decreasing the deficient yellow clearance interval by one sec was found to reduce the crashes by84 percent. While decreasing the deficient all-red clearance interval by 0.5 sec was found toreduce crashes by 39 percent. These findings indicate that providing adequate clearance times atsignalized ramp terminals can significantly improve their safety performance. Validation ofthese SPFs for signalized ramp terminals with an expanded dataset is recommended.
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