Mini-roundabouts

Mini-roundabouts are distinguished from traditional roundabouts primarily by their smaller size and more compact geometry. They are typically designed for negotiation speeds of 25 km/h (15 mph). Inscribed circle diameters generally vary from 13 m to 25 m (45 ft to 80 ft). Mini-roundabouts are usually implemented with safety in mind, as opposed to capacity. Peak-period capacity is seldom an issue, and most mini-roundabouts operate on residential or collector streets at demand levels well below their capacity. It is important, however, to be able to assess the capacity of any proposed intersection design to ensure that the intersection would function properly if constructed.

At very small roundabouts, it is reasonable to assume that each quadrant of the circulatory roadway can accommodate only one vehicle at a time. In other words, a vehicle may not enter the circulatory roadway unless the quadrant on both sides of the approach is empty. Given a set of demand volumes for each of the 12 standard movements at a four-leg roundabout, it is possible to simulate the roundabout to estimate the maximum service volumes and delay for each approach. By making assumptions about the proportion of left turns and the proportion of cross street traffic, a general estimate of the total entry maximum service volumes of the roundabout can be made, and is provided in Exhibit 3-2. AADT maximum service volumes are represented based on an assumed K value of 0.10. Note that these volumes range from slightly more than 12,000 to slightly less than 16,000 vehicles per day. The maximum throughput is achieved with an equal proportion of vehicles on the major and minor roads, and with low proportions of left turns.