Exploration of the effects of operational and physical characteristics on operating speeds at modern roundabouts

The modern roundabout has been found to be a safe and effective intersection configuration in the United States. The design of modern roundabouts and their ability to be safe and efficient depends on their low and consistent operating speeds.; This research provides an initial exploration into the relationships of thirteen operational and physical characteristics or the modern roundabout and their effect on operating speeds. These characteristics included two central island diameters, circulating lane width, two turn deflections superelevation, approach width, grade and speed, entry and exit radii, turn angle and measured radius. These thirteen characteristics were used to develop an operating speed prediction model.; Operating speed data from fifty-nine approach movements at twelve modern roundabouts was collected and used in model development. The twelve modern roundabouts studied were located in California, Kansas, Maryland, Mississippi, Nevada and Washington. All data was collected during the summer and fall of 2000.; The recent Federal Highway Administration (FHWA) Roundabout Informational Guide recommends speed consistency for roundabout movements. The Federal Highway Administration equation provided to calculate the operating speeds relies on hand drawn vehicle paths to determine the radius of the fastest vehicle travel path. The FHWA equation also relies on the side friction factor for predicting operating speeds. Values for side friction factors are as yet unknown within the modern roundabout.; The FHWA equation was used to predict operating speeds at the study sites. The FHWA equation was found to be unusable unless the roundabout already existed, or the predictions were found to be significantly different than the observed operating speeds.; Operating speed prediction equations were developed through the multiple regression process. This process was done in an iterative manner so that the final model could be used to predict operating speeds. The variables found to influence operating speed in the final model were circulating lane width, through deflection, approach speed, entry radius, central island diameter two (the part that would effect the second part of a left turn), and turn angle. This speed prediction model provides designers insight into the factors that will effect the operating speed of the modern roundabout.