Two-Way Left-Turn Lane Design Guide

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Photo courtesy of Project for Public Spaces (www.pps.org)

A two-way left-turn lane (TWLTL) is a configuration that provides a center lane exclusively for left turning vehicles from either direction.  This provides for more flexible use of the left turn lane compared to separate left turn lanes that have fixed tapers and pocket storage lengths.  TWLTLs are best on roads with closely spaced driveways or roadways with a heavy commuter pattern.

In most places, TWLTLs are typically found on urban streets that have a single through lane in each direction (three-lane TWLTL).  However, it’s not uncommon in some jurisdictions to find TWLTLs on roads with two lanes in each direction (five-lane TWLTL) or even three lanes in each direction (seven-lane TWLTL).

A common application of TWLTLs is to convert a four lane undivided road (two lanes in each direction, no turn lanes, and no medians) to a three lane road (one lane in each direction with a TWLTL down the middle).  This is usually called a “Road Diet.”

Road Diets have been well studied.  Before/after data typically shows the three lane road have comparable capacity and a lower crash rate (meaning safer) than the four lane road with no center treatment at all.  In addition, vehicle speeds have been found to be reduced with a three-lane facility, making it safer for non-motorized users.  An added benefit of a Road Diet is the pavement width from removing a lane can be used for bicycle lanes, on-street parking, or shoulders.  The road diet transition also benefits pedestrians by reducing the crossing width where they are exposed to conflicting traffic.

Benefits

  • Allows opposing left turning movements to occupy the same lane, reducing the roadway width.
  • Removes left turning vehicles from the through lane.
  • Reduces total crashes by approximately 30 percent or more compared to facilities without turn lanes.
  • Reduces operating speeds by one to two mph compared with multiple through lane facilities.
  • Reduces overall corridor travel times compared to facilities without turn lanes.
  • Converts with pavement marking changes only (four-lane undivided conversion to three-lane TWLTL).
  • Allows space for bicycle lanes without expanding the roadway (four-lane undivided conversion to three-lane TWLTL).
  • Improves pedestrian safety by reducing the vehicle exposure (four-lane undivided conversion to three-lane TWLTL).

Limitations

  • Impractical on roadways that carry about 17,500 or more vehicles per day (three-lane TWLTL) or 28,000 or more vehicles per day (five-lane TWLTL).
  • High volume roadways (20,000 or more vehicles per day) will be safer for vehicles and pedestrians with raised medians and exclusive turn lanes.
  • Seven-lane TWLTL (three lanes of through traffic in both directions) are not generally recommended due to high crash rates.
  • The TWLTL can be mis-used for passing on very long corridors.
  • Not generally recommended for high speed corridors (45 or more mph).
  • May not be appropriate for transit corridors if buses will regularly block the through lanes.
  • Functionality is reduced when driving spacing exceeds 12 per mile (440-foot spacing) as opposing left turning vehicles will overlap.

Design Guidelines

  • The center TWLTL width can be as small as 11 feet, although 14 is preferred. Some agencies will provide a 16- or 18-foot lane to allow for a future raised median if needed.
  • According to the Manual on Uniform Traffic Control Devices (MUTCD), opposing white left turn arrows should be spaced 8-16 feet apart.
  • The opposing left turn arrows are to be placed at the beginning of and/or downstream of the TWLTL and repeated approximately every 300 feet.
  • Pavement markings of a TWLTL shall be a normal broken yellow line on the inside and a normal solid yellow line on the outside adjacent to the through lane.
  • A “R3-9b” sign should be used in conjunction with the required pavement markings and is highly recommended for at least the first set of the opposing left turn arrows from each direction. The sign is often repeated regularly to reinforce the pavement markings and is helpful where this operation is new and/or environmental conditions frequently obscure the pavement markings.
  • Consider pedestrian and bicyclist crossing movements, with the potential need for refuge areas along a corridor in-place of the TWLTL.
  • Pavement textures, coloring and other features can be added with a TWLTL to increase their visibility and potentially their functionality.

Resources

Photo Credit:  City of Charlotte, NC

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