Improved Right-Turn Slip-Lane Design

Intersections should be designed to accommodate safe pedestrian crossings using tight curb radii, shorter crossing distances, and other tools as described in this application. While right-turn slip lanes are generally a negative facility from the pedestrian perspective—due to the emphasis on easy and fast motor vehicle travel—they can be designed to be less problematic. At many arterial street intersections, pedestrians have difficulty crossing due to right-turn movements and wide crossing distances. Well-designed right-turn slip lanes provide pedestrian crossing islands within the intersection and a right-turn lane that is designed to optimize the right-turning motorist's view of the pedestrian and of vehicles to his or her left. Pedestrians are able to cross the right-turn lane and wait on the crossing island for their walk signal.

The problem for pedestrians is that many slip lanes are designed for unimpeded vehicular movement. The design of corner islands, lane width, and curb radii of right-turn slip lanes should discourage high-speed turns, while accommodating large trucks and buses. The triangular "porkchop" corner island that results should have the "tail" pointing to approaching traffic. Since the traffic signal is timed based on a shorter crossing, the pedestrian crossing time has a much smaller influence on the timing of the signal. This design has an additional advantage for the pedestrian; the crosswalk is located in an area where the driver is still looking ahead. Older designs place the crosswalk too far down, where the driver is already looking left for a break in the traffic.

One way to make a slip lane work well for pedestrians is to create a raised crosswalk for pedestrians between the sidewalk and the island. This treatment has been used extensively and successfully in Boulder, Colorado, where it is now the standard design for slip lanes.

Channelized right turn-lanes remain a challenge for visually-impaired pedestrians. First, there are difficulties associated with knowing where the crosswalk is located or knowing where to cross. Second, it is difficult for a pedestrian who is visually-impaired to know when a vehicle has yielded right-of way. While accessible pedestrian signals can help with these issues, more research is currently underway through the National Cooperative Highway Research Program (NCHRP) to further explore the problem and develop potential solutions. Refer to NCHRP Project 3-78, Crossing Solutions at Roundabouts and Channelized Turn Lanes for Pedestrians with Vision Disabilities for the latest status report.

Purpose

  • Separate right-turning traffic
  • Slow turning-vehicle speeds and improve safety
  • Allow drivers to see approaching cross-street traffic more clearly
  • Reduce the crossing distance for pedestrians

Considerations

  • Evaluate first whether a slip lane is really necessary.
  • Consider adding a raised crosswalk for the pedestrian crossing.
  • Modify old slip lane designs to the preferred, safer design.

Estimated cost

Approximately $50,000 to $200,000 to reconfigure roadway, add striping, and construct an island and a raised crosswalk, assuming additional right-of-way is not required. If lane lines are moved, be sure to include costs of moving or replacing inlaid signal wires.