Mini-Roundabouts
As discussed in Chapter 1, a mini-roundabout is an intersection design alternative that can be used in place of stop control or signalization at physically constrained intersections to help improve safety problems and excessive delays at minor approaches. Mini-roundabouts are not traffic calming devices but rather are a form of roundabout intersection. Exhibit 6-50 presents an example of a mini-roundabout.
Mini-roundabouts should only be considered in areas where all approaching roadways have an 85th-percentile speed of less than 50 km/h (30 mph). In addition, mini-roundabouts are not recommended in locations in which high U-turn traffic is expected, such as at the ends of street segments with access restrictions. Miniroundabouts are not well suited for high volumes of trucks, as trucks will occupy most of the intersection when turning.
The design of the central island of a mini-roundabout is defined primarily by the requirement to achieve speed reduction for passenger cars. As discussed previously in Section 6.2, speed reduction for entering vehicles and speed consistency with circulating vehicles are important. Therefore, the location and size of the central island are dictated by the inside of the swept paths of passenger cars that is needed to achieve a maximum recommended entry speed of 25 km/h (15 mph). The central island of a mini-roundabout is typically a minimum of 4 m (13 ft) in diameter and is fully mountable by large trucks and buses. Composed of asphalt, concrete, or other paving material, the central island should be domed at a height of 25 to 30 mm per 1 m diameter (0.3 to 0.36 in per 1 ft diameter), with a maximum height of 125 mm (5 in) (14). Although fully mountable and relatively small, it is essential that the central island be clear and conspicuous (14, 15). Chapter 7 provides a sample signing and striping planing plan for mini-roundabout.
The outer swept path of passenger cars and large vehicles is typically used to define the location of the yield line and boundary of each splitter island with the circulatory roadway. Given the small size of a mini-roundabout, the outer swept path of large vehicles may not be coincident with the inscribed circle of the roundabout, which is defined by the outer curbs. Therefore, the splitter islands and yield line may extend into the inscribed circle for some approach geometries. On the other hand, for very small mini-roundabouts, such as the one shown in Exhibit 6-50, all turning trucks will pass directly over the central island while not encroaching on the circulating roadway to the left which may have opposing traffic.