Closely Spaced Roundabouts

It is sometimes desirable to consider the operation of two or more roundabouts in close proximity to each other. In these cases, the expected queue lengths at each roundabout become important. Exhibit 8-3 presents an example of closely spaced T-intersections. The designer should compute the 95th-percentile queues for each approach to check that sufficient queuing space is provided for vehicles between the roundabouts. If there is insufficient space, then drivers will occasionally queue into the upstream roundabout and may cause it to lock.

Closely spaced roundabouts may improve safety by “calming” the traffic on the major road. Drivers may be reluctant to accelerate to the expected speed on the arterial if they are also required to slow again for the next close roundabout. This may benefit nearby residents.

When roundabouts are used at offset T-intersections, there is an opportunity to bypass one through lane direction on the major road at each roundabout. Exhibit 8-4 presents sketches of through bypass lanes for the two basic types of offset T-intersection configurations. In both cases, through traffic in each direction needs to negotiate only one roundabout, and capacity is therefore typically improved. The weaving section should be analyzed both for capacity and for safety through an evaluation of the relative speeds of the weaving vehicles.

Of the two arrangements shown in Exhibit 8-4, Option A (roundabout precedes bypass) is preferred. The roundabout offers a visual cue to drivers to slow in Arrangement A and encourages slower (and therefore safer) driving through the two roundabouts. If Option B (bypass precedes roundabout) is used, the merges and diverges could occur at higher speeds. It may be appropriate in this case to omit the bypass lane and pass all through traffic through both roundabouts. Another advantage of Option A is that there would be less queuing of traffic on the road space between the roundabouts.

Note that when conventional T-intersections are used, Option A is less preferable than Option B due to the need to provide interior storage space for left turns in Option A. Therefore, roundabouts may be a satisfactory solution for cases like Option A.