76 Transportation Research Record: Journal of the Transportation Research Board, No. 2312, Transportation Research Board of the National Academies, Washington, D.C., 2012, pp. 76–85. DOI: 10.3141/2312-08 M. Al-Ghandour, Program Development Branch, North Carolina Department of Transportation, 1534 Mail Service Center, Raleigh, NC 27699-1534. B. Schroeder, Highway Systems Group, Institute for Transportation Research and Educa- tion, North Carolina State University, Centennial Campus Box 8601, Raleigh, NC 27695-8601. W. Rasdorf and B. Williams, Department of Civil, Construc- tion, and Environmental Engineering, North Carolina State University, Campus Box 7908, Raleigh, NC 27606-7908. Corresponding author: M. Al-Ghandour, malghandour@ncdot.gov. roundabout exit leg and forming a new acceleration lane adjacent to exiting traffic, and a yield slip lane, terminating at a sharp angle with the roundabout exit approach so that its right-turning traffic yields to the traffic it is merging with (1). Measured as capacity, a roundabout’s operational performance typically is based on one of three capacity methods: gap acceptance, empirical regression, or a hybrid of gap and empirical methods. FHWA (2) and NCHRP Report 572 (3) describe roundabout capac- ity models as a function of the circulating flow in the roundabout, follow-up headway, and critical gap. Numerous studies have used VISSIM microsimulation to explore roundabout performance. Bared and Edara (4) used VISSIM, a micro- simulation modeling tool from Germany (5), to model roundabouts for various ranges of circulating and entry traffic volumes. They found that simulation results from VISSIM (of roundabout capacity in vehicles per hour) were significantly lower than from the SIDRA analytical and RODEL empirical models. They also verified that VISSIM capacity results were similar to field measured data (traffic volumes and geometry, speed, and video data) that were collected for the development of NCHRP 572 in the United States (3). Bared and Afshar used VISSIM to predict new planning capacity models by lane for two- and three-lane roundabouts (6). They intro- duced capacity models as a function of separate circulatory-lane traffic volume. Trueblood and Dale described key VISSIM features for effective simulation of roundabouts, including link and connector, routing decisions, reduced speed zone, and priority rules (7). These features in VISSIM allow users to code and model roundabouts accurately. Several studies have also used VISSIM to evaluate roundabout geometric and behavior features. For example, Vaiana et al. simu- lated several experimental scenarios to compare some roundabouts with others in regard to their geometric elements, vehicular flow, and behavioral parameters (8). They determined three features that are critical to obtaining a correct simulation: (a) speed (approach speed, reduced speed zones, and circulatory speed), (b) priority rules, and (c) traffic assignment. No research was found that evaluated roundabout operational per- formance for a single-lane roundabout with and without a slip lane. Thus, there is a need to determine the value of slip lanes and to quan- tify their contribution to overall roundabout operational performance. For that purpose, this paper has two specific objectives: 1. To evaluate operational performance—vehicle delays—of single-lane roundabouts with a slip lane under various exit control Delay Analysis of Single-Lane Roundabout with a Slip Lane Under Varying Exit Types, Experimental Balanced Traffic Volumes, and Pedestrians, Using Microsimulation Majed Al-Ghandour, Bastian Schroeder, William Rasdorf, and Billy Williams 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 bal- anced 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 aver- age delays in a roundabout with a slip lane are a function of circulat- ing 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 round- about 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). A slip lane, an optional separate (exclusive) lane that lies adjacent to a roundabout, facilitates right-turning traffic flow and reduces the approach delay by allowing right-turning movements to bypass the roundabout, thereby reducing traffic volumes and vehicle conflicts in the roundabout. Although roundabouts are an increasingly common form of intersection control in the United States, research has yet to quantify slip lane contributions to their operation and safety. NCHRP Report 672 defines two types of slip lanes: a non-yield slip lane (referred to as a free-flow exit type), merging with the