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Benefits of Metering Signals at Roundabouts with Unbalanced Flow: Patterns in Spain

Unbalanced flow patterns may be a problem in roundabouts, even at medium demand levels. One single approach can cause queues on the downstream approaches; the average delay can increase greatly, and the roundabout can fail. This operational problem can be lessened by regulating traffic with metering signals, one of the most cost-effective measures used in Australia, the United Kingdom, and the United States. Even though some Spanish roundabouts exceed capacity, the potential benefit to local conditions has not been studied. Traffic microsimulation was used to analyze capacity improvements and operational performance on roundabouts with metering signals. A field study characterized gap acceptance behavior on conditions close to capacity. Vissim was calibrated and validated, and a signal control logic was implemented in its VisVAP module. The capacity from traffic microsimulation represents the traffic demands observed in the field more accurately because of the variable follow-up headways. In contrast, the U.S. Highway Capacity Manual 2010 underestimates capacity for almost all observed conditions. Almost 400 combinations of design and control parameters for metering signals were required to obtain the optimal (location and timing) model with the lowest average delay. Then, traffic demand scenarios were varied with the optimal metering system. The percentage benefit, calculated as overall average delay, could be up to 60% depending on the combination of controlling traffic demand and conflicting traffic flow. Results allow users to determine the need for metering and quantify the potential benefit from its application to one-lane roundabouts.