Signal Control of Roundabouts

During the last decade many traffic signal controlled intersections have been replaced by roundabouts. There has been a trend towards the establishment of roundabouts where pedestrians and cyclists need to be considered specifically, sometimes by including one or more signal controlled crosswalks. Knowledge on adequate design, control and effects of different solutions, particularly with accessibility and traffic safety in mind, has been limited in Sweden. This lack of knowledge was the basis for a project in 2007 financed by the Swedish National Road Administration and which was divided into two parts. In the first part an inventory of available knowledge on signal controlled roundabouts both in Sweden and abroad was carried out. The following alternative forms of signal control were determined for the projects aspects: A1: Signal controlled crosswalks at the approach and in the direct vicinity of the gyratory (off signal). A2: Signal control of crosswalks at the approach up-and downstream. B1: Complete signal control of an approach. The crosswalks on both directions are passed in a single step. B2: Complete signal control of an approach. The crosswalks are passed in two steps (with intermediate stops). C: Coordinated, fully signal control of the roundabout. In the second part of the project accessibility analysis for the different alternatives was carried out with the help of field studies and calculations using the traffic modeling tool TRANSYT for signal optimization and VISSIM for evaluation of the effects. The traffic safety aspects were analyzed with the aid of specific interviews with the regulatory organizations, accident statistics from STRADA (Swedish Traffic Accident Data Acquisition) and field studies of traffic behavior at two roundabout locations. The project concluded following recommendations regarding accessibility and safety: Alternative (A1) should be avoided from both a capacity and a traffic safety aspect. Use B. Alternative (A2) should be placed a minimum of 22 m from the roundabout due to both capacity and a traffic safety aspects. Alternative (B2), which has higher capacity than (B1) can be applied if there is a need for signalized crosswalks. Alternative (C) can be considered due to capacity constraints at high pedestrians' flow of several approaches (C) 2011 Published by Elsevier Ltd.