Single- and double-lane roundabouts

The resulting maximum service volumes are presented in Exhibit 3-1 for a range of left turns from 0 to 40 percent of the total volume. This range exceeds the normal expectation for left turn proportions. This procedure is offered as a simple, conservative method for estimating roundabout lane requirements. If the 24-hour volumes fall below the volumes indicated in Exhibit 3-1, a roundabout should have no operational problems at any time of the day. It is suggested that a reasonable approximation of lane requirements for a three-leg roundabout may be obtained using 75 percent of the service volumes shown on Exhibit 3-1.

If the volumes exceed the threshold suggested in Exhibit 3-1, a single-lane or double-lane roundabout may still function quite well, but a closer look at the actual turning movement volumes during the design hour is required. The procedures for such analysis are presented in Chapter 4.