Safety Performance Functions for Diamond Interchange Ramp Terminals

The objectives of this research were to identify factors contributing to ramp terminal crashes and develop safety performance functions for four-leg diamond interchange ramp terminals. Four-leg diamond interchange ramp terminals in Wisconsin were sampled to construct two datasets. The first dataset was comprised of 60 pairs of interchange ramp terminals and was used to do general analysis and identify the effects of geometric factors. The other dataset was comprised of only signalized 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. For signalized interchange ramp terminals, yellow and all-red clearance intervals were also considered as factors. The presence of cross road median is negatively correlated with average crash frequency. For signalized interchange ramp terminals, the crash frequency increases with deficient 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 by 84 percent. While decreasing the deficient all-red clearance interval by 0.5 sec was found to reduce crashes by 39 percent. These findings indicate that providing adequate clearance times at signalized ramp terminals can significantly improve their safety performance. Validation of these SPFs for signalized ramp terminals with an expanded dataset is recommended.