Modeling Driver Selected Speeds at Single Lane Roundabouts

The objective of this paper is to investigate the relationship between American roundabout geometric design elements and driver selected speeds. From May to August of 2003, an extensive data collection effort was undertaken as part of the National Cooperative Highway Research Program (NCHRP) Project 3-65 in order to obtain spot speeds and roundabout geometric design information for 32 roundabouts located in 11 states. This study utilizes a portion of this dataset by testing 10 single lane roundabouts which include a total of 37 roundabout approaches. These roundabouts were chosen for their placement in high to low speed environments. Extensive data analysis was performed to estimate the impact of area and geometric design on operating speeds of vehicles at approach, entry, circle, and exiting points. The analysis showed higher approach speeds in rural areas as compared to suburban and urban areas. Geometric parameters of roundabouts found to be having a significant positive or negative impact on speeds were approach width, flare length, inscribed circle diameter, central island diameter, circulatory roadway width, departure width, and split width. These factors were used to developed prediction models for 85th percentile speeds at four locations of a roundabout. Data were further investigated for speed differentials at several key locations and the geometric parameters influencing the smooth transition of speeds were determined. The models developed in this study may be used to gain insight into how roundabout design may affect the driver selected speeds which in turn influence safety and operational performance.