Potential for Metering to Help Roundabouts Manage Peak Period Demands in the United States

Roundabouts generally provide safety and other advantages. During peak hours, however, even moderate demands on an upstream approach can result in long delays and driver frustration over downstream movements. A metering signal is one way to ensure that all demands at a roundabout are adequately served. A roundabout metering signal regulates flow into the circle from one approach and thereby creates larger gaps in the circle for downstream entrants. Although metering signals have been used successfully around the world, little guidance is available for U.S. designers. The goal of this research was to provide U.S. designers with some guidance on the use of metering signals. The authors developed a simple macroscopic model based on the Highway Capacity Manual, validated it by using a simulation model, and performed exercises with it on a number of demand combinations to see where a meter might help. The macroscopic model should be helpful as a quick screening tool. The results from application of the model provide evidence that a meter may reduce delays compared with those seen in an unmetered roundabout with some demands. Although signalized intersections produced lower delays than did metered or unmetered roundabouts in most cases tested, roundabouts were sometimes better. Analysts should consider more than just peak period delays in deciding on the optimum traffic control at an intersection. If, with the aid of a metering signal for a few peak hours, a roundabout produces slightly higher delay levels during those peak hours than a standard traffic signal, with consideration of its many other benefits, a roundabout could well be the optimum design.