Roundabout Slip Lanes: Performance and Safety Analysis

A slip lane, an optional separate (exclusive) right-turn lane that lies adjacent to a roundabout, allows right-turning movements to bypass the roundabout itself. Research reveals a need to assess the value of slip lanes. Therefore, the goal of this research is to articulate a more thorough understanding of a single-lane roundabout with a slip lane, quantifying its operational (traffic volumes and delays) and safety performance (conflict frequencies and pedestrian traffic).;Performance of a single-lane roundabout is evaluated under three slip lane exit types (free-flow, yield, and stop) and the results are compared to a roundabout having no slip lane (base). Ten experimental traffic percentage distribution matrices are considered for gap acceptance-based performance assessment (SIDRA) and simulation assessment (VISSIM). Conflict patterns at single-lane roundabouts with and without slip lanes are evaluated and compared through a Surrogate Safety Assessment Model (SSAM) analysis. Five zone-based conflict prediction models are developed using Poisson regression (GENMOD).;SIDRA and VISSIM results confirm that average delay and circulating conflict volumes---operational efficiency---in a roundabout with a slip lane are related exponentially to slip lane volumes and are sensitive to changing gap acceptance parameter (SIDRA) or priority rules (VISSIM). Using SIDRA analysis, the models predict the relationship between slip lane average delay and entering volumes off the northbound approach (assumed slip lane approach) with an R-squared value up to 0.82. In contrast to the estimated VISSIM 95% confidence interval between -26% and -20%, SIDRA results showed a 95% confidence interval of reduction in roundabout delays for a free-flow slip lane, estimated between -17% and -10%, before the roundabout is oversaturated for unbalanced flow scenarios. A free-flow slip lane exit type helps to reduce total average delay in the roundabout and the slip lane approach.;Both yield and stop slip lane exit types also reduce roundabout total average delay but to a lesser degree than a free-flow slip lane. Also, the performance of the next approach of a slip lane (westbound approach) is not always significantly improved, because of variations in circulation flow and entry flow.;At a higher traffic volume, however, a free-flow slip lane exit type can increase roundabout delay from 6.6 to 28.1 seconds per vehicle if drivers must yield the right-of-way to high pedestrian traffic (100 pedestrians per hour) crossing a free-flow slip lane (priority rule). Finally, results suggest that theoretical capacity threshold values for slip lane volumes are estimated to range from 150 to 350 vehicles per hour (balanced) and from 400 to 500 vehicles per hour (unbalanced) for traffic percentage distribution volume scenarios before roundabout oversaturated conditions.;Using a SSAM (Surrogate Safety Assessment Model) analysis and Poisson regression to evaluate safety, the conflict prediction models predict the occurrence of conflicts for different roundabout zones, with R-squared values ranging from 0.69 to 0.97. The models are compared to national and international crash prediction models for single-lane roundabouts and are further validated using actual crash data from ten single-lane roundabouts. The numbers of conflicts for a single-lane roundabout were predicted as a function of approach entry, circulating, and slip lane traffic flows and were determined to be sensitive to the slip lane exit type. Results confirm that conflicts in the merge area are more frequent than in the roundabout approach area, and that the installation of a free-flow slip lane exit type reduces overall conflict occurrence. Finally, the results demonstrate the usefulness of SSAM analysis for evaluating roundabout safety and developing an empirical relationship between simulated conflicts and field-observed crashes.