Right-turn bypass lanes

In general, right-turn bypass lanes (or right-turn slip lanes) should be avoided, especially in urban areas with bicycle and pedestrian activity. The entries and exits of bypass lanes can increase conflicts with bicyclists. The generally higher speeds of bypass lanes and the lower expectation of drivers to stop increases the risk of collisions with pedestrians. However, in locations with minimal pedestrian and bicycle activity, right-turn bypass lanes can be used to improve capacity where there is heavy right turning traffic.

The provision of a right-turn bypass lane allows right-turning traffic to bypass the roundabout, providing additional capacity for the through and left-turn movements at the approach. They are most beneficial when the demand of an approach exceeds its capacity and a significant proportion of the traffic is turning right. However, it is important to consider the reversal of traffic patterns during the opposite peak time period. In some cases, the use of a right-turn bypass lane can avoid the need to build an additional entry lane and thus a larger roundabout. To determine if a right-turn bypass lane should be used, the capacity and delay calculations in Chapter 4 should be performed. Right-turn bypass lanes can also be used in locations where the geometry for right turns is too tight to allow trucks to turn within
the roundabout.

Exhibit 6-41 shows an example of a right-turn bypass lane.

There are two design options for right-turn bypass lanes. The first option, shown in Exhibit 6-42, is to carry the bypass lane parallel to the adjacent exit roadway, and then merge it into the main exit lane. Under this option, the bypass lane should be carried alongside the main roadway for a sufficient distance to allow vehicles in the bypass lane and vehicles exiting the roundabout to accelerate to comparable speeds. The bypass lane is then merged at a taper rate according to AASHTO guidelines for the appropriate design speed. The second design option for a right-turn bypass lane, shown in Exhibit 6-43, is to provide a yield-controlled entrance onto the adjacent exit roadway. The first option provides better operational performance than the second does. However, the second option generally requires less construction and right-of-way than the first.

The option of providing yield control on a bypass lane is generally better for both bicyclists and pedestrians and is recommended as the preferred option in urban areas where pedestrians and bicyclists are prevalent. Acceleration lanes can be problematic for bicyclists because they end up being to the left of accelerating motor vehicles. In addition, yield control at the end of a bypass lane tends to slow motorists down, whereas an acceleration lane at the end of a bypass lane tends to promote higher speeds.

The radius of the right-turn bypass lane should not be significantly larger than the radius of the fastest entry path provided at the roundabout. This will ensure vehicle speeds on the bypass lane are similar to speeds through the roundabout, resulting in safe merging of the two roadways. Providing a small radius also provides greater safety for pedestrians who must cross the right-turn slip lane.