Counting Roundabouts: A Model to Obtain Detailed Roundabout Turning Movements

Abstract

To determine whether a roundabout or an intersection would work best at a particular location, it is important to know the number of vehicles making each turning movement. Methodologies have been developed for analyzing roundabout levels of service without knowing the detailed turning movements. To analyze whether a roundabout or signalized intersection would work better in a specific location, however, the detailed turning movements are needed. A methodology to determine existing turning movements at roundabouts with minimal labor is presented. Using only four persons, a typical four-leg roundabout can be counted, and all turning movements (including U turns) can be accurately estimated. The study to update the Master Transportation Plan of Abu Dhabi—the Capital of the United Arab Emirates—provided the opportunity to develop and test this approach. The roundabout counting methodology starts with procedures to count traffic volumes, including identification of specific movements to be counted. The counts are then entered into a model, which initially estimates all turns using percentages of vehicles exiting at various locations. The model then iteratively adjusts the turns until all entry and approach values match the counted values. Adjustment coefficients have been added into the process and calibrated, and the model has been successfully applied to obtain reliable turning movement estimates for three-, four-, and five-leg roundabouts. The model has been tested using data where all turns were known in advance and has proven to be quite reliable.