Delay Analysis of Single-Lane Roundabout with a Slip Lane under Varying Exit Types, Experimental Balanced Traffic Volumes, and Pedestrians, Using Microsimulation

A slip lane facilitates right-turning traffic flow, reduces approach delay, and reduces conflict points within a roundabout. In this paper the delay performance of a single-lane roundabout with an adjacent slip lane is modeled with the VISSIM microsimulation tool for three slip lane exit types (free-flow, yield, and stop) and the results are compared with a roundabout having no slip lane. The VISSIM assessment considers four experimental traffic percentage turning volume distributions as balanced flow scenarios (total traffic flow into and out of every roundabout approach is the same). Simulated slip lane right-turning traffic volumes range from 50 to 500 vehicles/h, and the four pedestrian volume levels range from 0 to 100 pedestrians/h. VISSIM results confirm that average delays in a roundabout with a slip lane are a function of circulating conflict volumes and are related exponentially to slip lane volumes regardless of the slip lane exit type. Results also indicate that a free-flow slip lane exit type best reduces total average delay in the roundabout and in the slip lane itself. Yield and stop slip lane exit types also reduce the roundabout total average delay but to a lesser degree. Finally, at a higher traffic volume, a free-flow slip lane exit type can increase roundabout delay from 6.6 to 34.7 s/vehicle if drivers must yield the right-of-way to high pedestrian traffic (100 pedestrians/h) crossing a free-flow slip lane (priority rule).