Conflict Models for Single-Lane Roundabout Slip Lanes from Microsimulation

Abstract

Conflict patterns at single-lane roundabouts with and without slip lanes were evaluated and compared through VISSIM, the microscopic simulation program for multimodal traffic flow modeling, and surrogate safety assessment model (SSAM) analysis. From a sensitivity analysis of several volume distribution scenarios of the percentage of turning traffic, five zone-based conflict prediction models were developed through the use of Poisson regression. The models captured simulated conflict differences that resulted from the addition of a right-turn slip lane. The models were evaluated under three exit control scenarios (yield, stop, and free-flow merge). With SSAM analysis, the models predicted the occurrence of conflicts for roundabout zones with different R-squared values, which ranged from .69 to .97. The models were compared with national and inter-national crash prediction models for single-lane roundabouts and were further validated by actual crash data from 10 single-lane roundabouts in the city of Carmel, Indiana. The number of conflicts for a single-lane roundabout was predicted as a function of approach entry, circulation, and slip lane traffic flows and was determined to be sensitive to the slip lane exit type. Results confirmed that conflicts in the merge area were more frequent than in the roundabout approach area and that the installation of a free-flow slip lane exit type reduced overall conflict occurrence. The results demonstrated the usefulness of SSAM analysis for evaluating roundabout safety and developing an empirical relationship between simulated conflicts and field-observed crashes.