Left Turn Lane Design Factors

Left Turn Lane-01Left turn lanes are exclusive turning lanes that allow for a left turn movement to occur outside of the through lane. They are arguably the best strategy a traffic engineer can employ because of the large safety and capacity benefits they provide versus the relatively low cost to build them.

Both a road’s capacity and safety are improved when a left turn lane is provided. One report suggests that adding a left turn lane increases approach capacity by approximately 25%[1].  The Federal Highway Administration has found crashes can be expected to decrease 10% to 44% after installing a left turn lane, depending on the circumstances[2].

However, left turn lanes may negatively impact pedestrians and bicyclists.  Pedestrians crossing the road will have an extended crossing distance and be exposed to traffic for a longer period of time. Pedestrians and bicyclists may be further impacted if the addition of the left turn lane results in revised traffic signal timing. For instance, exclusive left turn phasing for a signal generally increases the overall timing and will add delay to those looking to cross the road.

Installation Factors

  • Functional Classification: high order roads, such as arterials, are often planned with left turn lanes to provide the desired mobility and access.
  • Vehicle Speeds: left turn lanes help eliminate the conflict between high speed through traffic (45 mph or higher) and vehicles slowing to turn.
  • Capacity Needs: left turn lanes increase the capacity of an intersection approach.
  • Percent of Left Turns: when left turning vehicles account for a high percentage of the approach’s traffic, a left turn lane recognizes that movement as the primary travel route.
  • Site Conditions: sight distance limitations, corridor design consistency, potential roadway obstructions, or other characteristics of an intersection can drive the need for a left turn lane.
  • Crash History: a history of right angle crashes can indicate a left turn lane will improve safety.

Some agencies and regions have developed volume guidelines, called warrants, for when to install a left turn lane. The Highway Capacity Manual (HCM2000) provides a simple volume threshold, suggesting more than 100 left turning vehicles in the peak hour warrants an exclusive left turn lane and 300 left turning vehicles in the peak hour warrants an exclusive dual left turn lane. The National Cooperative Highway Report Program (NCHRP) Report 745 has guidelines based on the peak hour volumes and the intersection configuration, as shown in Table 1.

NCHRP 745 Left Turn Lane Warrants

Design Guidelines

  • Lane Width is generally 11 to 12 feet, but can be as wide as 14 to 16 feet wide if there is a large volume of heavy vehicles using the turn lane.
  • Taper Length provides the shift from the through lane to the left turn lane. The taper rate is typically a 15:1 ratio between taper length and turn lane width on higher speed roads (45 mph or greater). Smaller taper ratios, as low as 5:1, have been used on lower speed roads in confined situations.
  • Deceleration Length is the space needed for a turning vehicle to enter the turn lane and slow to a stop before the intersection. Most agencies assume the deceleration starts at the beginning of the taper.  Assuming a deceleration rate of 6 feet per second2, deceleration lengths of 170 feet (30 mph) to 485 feet (55 mph) are needed.
  • Storage Length is the space needed for cars to stack while waiting to complete their turn. If an analysis of the intersection has been completed, the 95th percentile queue length, the distance at which 95% of the queues will be at or below, can be used for the necessary storage length. Storage length can also be based on the average number of vehicles expected during a two minute period within the peak hour. Without any volume information, at least 50 feet of storage length, about two cars, should be provided.
  • Signing and Striping should be provided per your jurisdiction’s engineering standards (i.e. Manual on Uniform Traffic Control Devices). In general, the guidelines call for a solid white line dividing the through lane from the left turn lane, solid white left turn arrows at each end of the turn lane, and a mandatory movement lane control sign.

Resources

[1]S/K Transportation Consultants, Inc., 2000, National Highway Institute Course Number 133078: Access Management, Location, and Design.

[2]Federal Highway Administration, Effectiveness of Intersection Left- and Right-Turn Lanes

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6 thoughts on “Left Turn Lane Design Factors

  1. Would you kindly explain the purpose of the road markings at a shadowed left-turn lane. I understand the purpose of the shadowed left-turn lane; I don’t understand why the space to the left of the lane on the approach taper and bay taper is marked to exclude traffic. What would be the consequence of just allowing the traffic to split into two streams as the through-lane widens and shifts right to create room for a turning lane?

    Thanks,
    Bill

    • Bill – I’m not sure what you mean by “shadowed,” but the cross hatching and taper are largely there to provide an area and physical clue for slowing down traffic. Mike

    • If all traffic wasn’t shifted to the right and left-turning traffic forced to shift into the left turn lane, you would have many drivers making a mistake by driving straight into the left turn lane at speed. The approach taper gives some protection, or “shadowing” for the left turn lane.

      Unfortunately, that also means the approach taper guides traffic off the through lane to the right as it passes the bay taper because the guidance of the approach taper goes away. If the driver continues on that trajectory, he will end up out of the lane.

      The “partially-shadowed” left turn lane has the approach taper measurement end at the beginning of the full turn lane, which usually has a lane line dividing the turn lane and thru lane. That way, the approach taper guides the driver right to the lane line, which then provides guidance downstream.

      • Thanks very much. Your explanation make sense and is consistent with an explanation given to me by a former police officer.

  2. Is it common to have a non-conforming turn lane designe. We have a highway with 3 lanes north and 3 lanes south traffic with a center turn lane. On the northbound lane at the intersection was a tapered left turn lane. Recently a second turn lane was added without a taper. The lane is marked at the intersection with an arrow in the street and a signe with 2 arrows overhead.
    This lane continues straight on the other side of the intersection, leaving two lanes for through traffic. This is the only instance of this configuration I have ever seen. By my observation and personal expirence this is very confusing to many drivers since it is unusual in the way it limits through traffic. Is there a standard that is used in the designe of these features in Texas.

    • I believe the Texas DOT has standards, but they can be adjusted by the engineer to accommodate extenuating circumstances. Mike