Evaluation of Roundabouts Versus Signalized and Unsignalized Intersections in Delaware

This study evaluates and compares a single-lane roundabout with an unsignalized (twoway stop controlled) intersection and a signalized (pre-timed) intersection and recommends conditions under which the construction of a roundabout may be more appropriate for an intersection. The measures of effectiveness used for the comparisons were effective intersection capacity, major and minor road entry lane capacity, major and minor road average delay, major and minor road queue length, and emission rates (CO, NOX, HC, and CO2). These measures are provided by the aaSIDRA software package which could then be used to establish “thresholds” in the major road one-way volume. These volumes indicate the threshold values for which the roundabout performs better than the unsignalized and signalized intersections. Two single-lane roundabouts with medium to high traffic volumes were studied in order to obtain local driver gap acceptance characteristics for use in the SIDRA analyses. The roundabouts selected for study are located in Maryland, near its border with Delaware, and so the data collected is assumed to be applicable to Delaware drivers as well. Observations of these sites found critical gaps of 3.85 and 3.91 s and follow-up times of 1.9 and 2.1 s, both lower than those values recommended by the HCM. Analyses using the SIDRA software with a critical gap of 3.9 s and a follow-up time of 2.0 s were then conducted.