Analysis of roundabout interchanges

The traffic performance evaluation of the roundabout interchange is the same as for a single conventional roundabout. The maximum entry capacity is dependent on the circulatory flow and the geometry of the roundabouts. The evaluation process is included in Chapter 4.

The benefits and costs associated with this type of interchange also follow those for a single roundabout. A potential benefit of roundabout interchanges is that the queue length on the off-ramps may be less than at a signalized intersection. In almost all cases, if the roundabout would operate below capacity, the performance of the on-ramp is likely to be better than if the interchange is signalized. The headway between vehicles leaving the roundabout along the on-ramp is more random than when signalized intersections are used. This more random ramp traffic allows for smoother merging behavior on the freeway and a slightly higher performance at the freeway merge area compared with platooned ramp traffic from a signalized intersection.

The traffic at any entry is the same for both configurations. The entry capacity is the same and the circulating flow is the same for the large single roundabout (Exhibit 8-6) and for the second configuration of the two teardrop roundabout system (Exhibit 8-8). Note that the raindrop form may be considered and analyzed as a single large roundabout as in the circular roundabout interchange, but with a “pinched” waistline across or under one bridge rather than two. The relative performance of these systems will only be affected by the geometry of the roundabouts and islands. The system with the two circular roundabouts will have a slightly different performance depending upon the number of U-turns.