Improved Right Turn Slip-Lane Design

Description

A channelized turn lane with a pork chop island in Boulder, Colorado

Right-turn slip lanes can be a detrimental to pedestrian safety when they allow motorists to maintain high speeds through the turn, do not optimize sight lines to the crosswalk, and do not reduce the crossing distance for pedestrians. 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 also able to cross the right-turn lane and wait on the crossing island for their walk signal. This design should be considered when curb radius reduction is not an option.

Well-designed right-turn slip lanes include several key features, including:

  1. The island (sometimes referred to as the “pork chop�) that forms the channelized right-turn lane is raised and large enough to accommodate waiting pedestrians and accessibility features, such as curb ramps and cut-throughs.
  2. As they enter the right-turn lane, drivers can easily see pedestrians crossing or about to cross the right-turn lane, and have enough space to stop completely once a pedestrian is spotted.
  3. The right-turn lane is as narrow as possible while still enabling the design vehicle to make the turn. Edge lines with cross-hatching can be used to narrow the perceived width of the lane while still accommodating larger vehicles.
  4. The crosswalk is oriented at a 90 degree angle to the right-turn lane to optimize sight lines, and is positioned one car length back from the intersecting roadway to allow drivers to move forward and wait for a gap in oncoming traffic after clearing the crosswalk.
  5. The visibility of the crosswalk to drivers is further enhanced through the use of high-visibility crosswalk striping, flashing beacons, and/or signage. Raised crosswalks may also be used to force motorists to slow down.
  6. The angle at which the right-turn lane intersects the cross street is relatively low (e.g., closer to 110 percent, rather than 140 percent). This feature lowers motor vehicle speeds and makes it easier for drivers to see oncoming traffic.
  7. Acceleration lanes are not provided where the right-turn lane intersects the cross street. Acceleration lanes enable drivers to navigate the channelized right-turn lane at higher speeds than would be possible if drivers had to yield to cross street traffic.
  8. The needs of visually impaired pedestrians are considered as part of the design. For example, rumble strips placed in the right-turn lane can help visually impaired pedestrians judge whether drivers are yielding as they approach the crosswalk.
  9. Active warning beacons may be desirable in locations where there are high traffic volumes and vehicle speeds.

Purpose

Right-turn slip lanes reduce the complexity of an intersection by breaking it into manageable parts. They slow turning vehicles, allow drivers and pedestrians to easily see each other, and reduce pedestrian exposure in the roadway. Drivers are better able to see oncoming traffic as they merge into the receiving roadway.

Considerations

  1. Evaluate first whether a slip lane is necessary.
  2. Right-turn slip lanes are most appropriate at signalized intersections with higher volumes of right-turning vehicles or with geometrics (e.g., skewed) that make right turns infeasible for the design vehicle without substantially increasing pedestrian crossing distances.
  3. In some states, the slip lane must be stop-controlled.

Cost

Costs may vary substantially, depending on a variety of factors.