Platooned arrivals on roundabout approaches

The performance of a roundabout is affected by its proximity to signalized intersections. If a signalized intersection is very close to the roundabout, it causes vehicles to enter the roundabout in closely spaced platoons; more importantly, it results in regular periods when no vehicles enter. These latter periods provide an excellent opportunity for traffic on the next downstream entry to enter. Since the critical gap is larger than the follow-up time, a roundabout becomes more efficient when the vehicles are handled as packets of vehicles rather than as isolated vehicles.

When the signalized intersection is some distance from the roundabout, then the vehicles’ arrival patterns have fewer closely spaced platoons. Platoons tend to disperse as they move down the road. The performance of a roundabout will be reduced under these circumstances when compared with a close upstream signal. If arrival speeds are moderate, then few longer gaps allow more drivers to enter a roundabout than a larger number of shorter gaps. If arrival speeds are low, then there are more opportunities for priority-sharing (where entering and circulating vehicles alternate) and priority-reversals (where the circulating vehicles tend to yield to entering vehicles) between entering and circulating traffic streams, and the influence of platoon dispersal is not as marked.