Methodology to Compute Travel Time of a Roundabout Corridor

Urban and suburban arterials with roundabouts in series are becoming more prevalent in North America. While the Highway Capacity Manual (HCM) provides a methodology for computing segment travel time of urban streets with various forms of intersection control, the manual does not provide a similar procedure for corridors with interdependent roundabouts. Such a methodology is necessary to evaluate performance of roundabout corridors and will allow the practitioner to compare both roundabout and signalized treatments for the same series of intersections. This paper presents a series of models intended to predict arterial travel time for a corridor with roundabouts, including a model for free-flow speed (FFS), a model to predict the length of the roundabout influence area (RIA, the area where geometric delay has incurred), a model for geometric delay, and models for impeded delay and average travel speed. These models were calibrated with data from seven roundabout corridors. The resulting models suggest that while FFS is a function of segment length, posted speed limit, and central island diameter, RIA length and geometric delay are functions of the FFS itself, as well as other geometric elements. The impeded delay and average travel speed are functions of traffic congestion and FFS. After validating the models with travel time data from two additional roundabout corridors not used in model development, the authors present a framework for incorporating the procedure for travel time prediction into the HCM analysis of urban streets.