Exit curves

Exit curves usually have larger radii than entry curves to minimize the likelihood of congestion at the exits. This, however, is balanced by the need to maintain low speeds at the pedestrian crossing on exit. The exit curve should produce an exit path radius (R3 in Exhibit 6-12) no smaller than the circulating path radius (R2). If the exit path radius is smaller than the circulating path radius, vehicles will be traveling too fast to negotiate the exit geometry and may crash into the splitter island or into oncoming traffic in the adjacent approach lane. Likewise, the exit path radius should not be significantly greater than the circulating path radius to ensure low speeds at the downstream pedestrian crossing.

The exit curve is designed to be curvilinearly tangential to the outside edge of the circulatory roadway. Likewise, the projection of the inside (left) edge of the exit roadway should be curvilinearly tangential to the central island. Exhibit 6-25 shows a typical exit layout for a single-lane roundabout.

Exit curves at single-lane roundabouts

At single-lane roundabouts in urban environments, exits should be designed to enforce a curved exit path with a design speed below 40 km/h (25 mph) in order to maximize safety for pedestrians crossing the exiting traffic stream. Generally, exit radii should be no less than 15 m (50 ft). However, at locations with pedestrian activity and no large semi-trailer traffic, exit radii may be as low as 10 to 12 m (33 to 39 ft). This produces a very slow design speed to maximize safety and comfort for pedestrians. Such low exit radii should only be used in conjunction with similar or smaller entry radii on urban compact roundabouts with inscribed circle diameters below 35 m (115 ft).

In rural locations where there are few pedestrians, exit curvature may be designed with large radii, allowing vehicles to exit quickly and accelerate back to traveling speed. This, however, should not result in a straight path  tangential to the central island because many locations that are rural today become urban in the future. Therefore, it is recommended that pedestrian activity be considered at all exits except where separate pedestrian facilities (paths, etc.) or other restrictions eliminate the likelihood of pedestrian activity in the foreseeable future.

Exit curves at double-lane roundabouts

As with the entries, the design of the exit curvature at double-lane roundabouts is more complicated than at single-lane roundabouts. Techniques and guidelines for avoiding conflicts between adjacent exit lanes at double-lane roundabouts are provided in Section 6.4.