Roundabouts in Signalized Corridors

Typically, roundabouts in the United States are installed at isolated intersections to address location-specific safety or operational needs. The use of roundabouts in signalized corridors, however, has not been well evaluated. It is commonly believed that roundabouts can improve traffic flow and travel speeds along an urban corridor by reducing delay caused by idling at intersections. Concern exists, however, that a roundabout in a coordinated signalized corridor will disrupt continuous traffic flow: downstream signals can more efficiently process vehicles in a platoon, and roundabouts disperse rather than form platoons. Roundabouts also can discharge vehicles more efficiently when traffic arrives randomly. Thus unnecessary queuing may result when roundabouts are downstream of signalized intersections. Since little research was available to compare the traffic flow impacts of roundabouts within a signalized corridor, two case studies were evaluated with the microscopic traffic simulation package, VISSIM. A roundabout and two signalized alternatives, as well as a roundabout and a four-way, stop-controlled alternative, were compared at intersections along signalized corridors in Ames, Iowa, and Woodbury, Minnesota, respectively. The traffic data and corridor geometry were coded into VISSIM, and traditional intersection traffic control within the corridors was compared with a scenario that had a two-lane roundabout. With the microsimulation software, average travel time, stopped delay, and average delay for the entire corridor were compared.