Design method to avoid path overlap

Achieving a reasonably low design speed at a double-lane roundabout while avoiding vehicle path overlap can be difficult because of conflicting interaction between the various geometric parameters. Providing small entry radii can produce low entry speeds, but often leads to path overlap on the entry, as vehicles will cut across lanes to avoid running into the central island. Likewise, providing small exit radii can aid in keeping circulating speeds low, but may result in path overlap at the exits.

Entry curves

At double-lane entries, the designer needs to balance the need to control entry speed with the need to minimize path overlap. This can be done a variety of ways that will vary significantly depending on site-specific conditions, and it is thus inappropriate to specify a single method for designing double-lane roundabouts. Regardless of the specific design method employed, the designer should maintain the overall design principles of speed control and speed consistency presented in Section 6.2.

One method to avoid path overlap on entry is to start with an inner entry curve that is curvilinearly tangential to the central island and then draw parallel alignments to determine the position of the outside edge of each entry lane. These curves can range from 30 to 60 m (100 to 200 ft) in urban environments and 40 to 80 m (130 to 260 ft) in rural environments. These curves should extend approximately 30 m (100 ft) to provide clear indication of the curvature to the driver. The designer should check the critical vehicle paths to ensure that speeds are sufficiently low and consistent between vehicle streams. The designer should also ensure that the portion of the splitter island in front of the crosswalk meets AASHTO recommendations for minimum size. Exhibit 6-46 demonstrates this method of design.

Another method to reduce entry speeds and avoid path overlap is to use a smallradius (generally 15 to 30 m [50 to 100 ft]) curve approximately 10 to 15 m (30 to 50 ft) upstream of the yield line. A second, larger-radius curve (or even a tangent) is then fitted between the first curve and the edge of the circulatory roadway. In this way, vehicles will still be slowed by the small-radius approach curve, and they will be directed along a path that is tangential to the central island at the time they reach the yield line. Exhibit 6-47 demonstrates this alternate method of design.

Exit curves

To avoid path overlap on the exit, it is important that the exit radius at a double-lane roundabout not be too small. At single-lane roundabouts, it is acceptable to use a minimal exit radius in order to control exit speeds and maximize pedestrian safety. However, the same is not necessarily true at double-lane roundabouts. If the exit radius is too small, traffic on the inside of the circulatory roadway will tend to exit into the outside exit lane on a more comfortable turning radius.

At double-lane roundabouts in urban environments, the principle for maximizing pedestrian safety is to reduce vehicle speeds prior to the yield and maintain similar (or slightly lower) speeds within the circulatory roadway. At the exit points, traffic will still be traveling slowly, as there is insufficient distance to accelerate significantly. If the entry and circulating path radii (R1 and R2 , as shown on Exhibit 6-12) are each 50 m (165 ft), exit speeds will generally be below 40 km/h (25 mph) regardless of the exit radius.

To achieve exit speeds slower than 40 km/h (25 mph), as is often desirable in environments with significant pedestrian activity, it may be necessary to tighten the exit radius. This may improve safety for pedestrians at the possible expense of increased vehicle-vehicle collisions.