Roundabout departure pattern

Traffic leaving a roundabout tends to be more random than if another type of intersection control were used. A roundabout may therefore affect the performance of other unsignalized intersections or driveways more than if the intersection was signalized. However, as this traffic travels further along the road downstream of the roundabout, the faster vehicles catch up to the slower vehicles and the proportion of platooning increases.

In the case of a well-defined platoon from an upstream signalized intersection arriving at a downstream unsignalized intersection just after a well-defined platoon arrives from the other direction, it may be difficult for the minor street drivers at this unsignalized intersection to enter the link. If, on the other hand, one of these signalized intersections were to be replaced by a roundabout, then the effect of the random traffic from the roundabout might be relatively advantageous. Under these conditions, more dispersed platoons (or random) traffic could assist drivers entering along the link at the unsignalized intersection.

If a roundabout is used in a network of coordinated signalized intersections, then it may be difficult to maintain the closely packed platoons required. If a tightly packed platoon approached a roundabout, it could proceed through the roundabout as long as there was no circulating traffic or traffic upstream from the left. Only one circulating vehicle would result in the platoon breaking down. Hence, the use of roundabouts in a coordinated signalized network needs to be evaluated carefully. One possibility for operating roundabouts within a signal network is to signalize the major approaches of the roundabout and coordinate them with adjacent upstream and downstream signalized intersections.

Another circumstance in which a roundabout may be advantageous is as an alternative to signal control at a critical signalized intersection within a coordinated network. Such intersections are the bottlenecks and usually determine the required cycle length, or are placed at a signal system boundary to operate in isolated actuated mode to minimize their effect on the rest of the surrounding system. If a roundabout can be designed to operate within its capacity, it may allow a lowering of the system cycle length with resultant benefits to delays and queues at other intersections.

Because roundabouts accommodate U-turns more easily than do signals, they may also be useful as an access management tool. Left-turn exits from driveways onto an arterial which may currently experience long delays and require two-stage left turn movements could be replaced with a simpler right turn, followed by a U-turn at the next roundabout.