A Dynamic Left Turn intersection (DLTi) varies the number of left turn lanes available for a given approach at a signalized intersection – and the associated left turn signal phasing for that approach and/or the opposing approach – by time-of-day, in order to reduce delay for left turns and the intersection as a whole

By implementing a variable number of left turn lanes (VNLTL) treatment for a given approach, all left turns are made from the left-most left turn lane during normal, off-peak operation, which improves sight distance for those turning vehicles. Closing the adjacent, right-most left turn lane also improves sight distance for the opposing left turns. This can enable “permitted” left turns and reduce overall intersection delay. Since left turn lane utilization and signal phasing varies by time of day, NCDOT and RTA refers to this operation as a Dynamic Left Turn intersection.

Benefits of DLTi treatment

  • Mitigates or eliminates the “peak capacity vs. off-peak delay” tradeoff
  • Eliminates requirement to show the restrictive protected-only phase when such a phase is not needed
  • Lowers off-peak delay, by reducing both the number and duration of required signal phases
  • Potentially reduces peak delay, by mitigating the tradeoff of building a second left turn lane
  • Increases operational flexibility, on a time-of-day, approach-by-approach basis

 

Design and operational information

 Signs, signals, and markings

  • Static left turn only (R3-5L) signs are posted above the left-most left turn lane
  • LED lane use control signs are posted above the right-most left turn lane
  • A single, four section head is installed above the lane line between the two left turn lanes
  • Left turn lane pavement markings are consistent with typical dual left turn lane installations

Normal, off-peak operational practice

  • Only the left-most left turn lane is available to turn left
  • The right-most left turn lane is closed, with a red “X” displayed in the LED lane use control signs
  • The signal includes flashing yellow arrow to provide protected-permitted left turn operation

Peak period operational practice

  • The right-most left turn lane also becomes available to serve left turns
  • A left turn arrow and “ONLY” is displayed in the LED lane use control signs
  • The signal suppresses the flashing yellow arrow to provide protected-only left turn operation

Design and operational notes

  • The single left turn signal serves and controls both left turn lanes
  • One set of left turn lane signs are installed at the signal itself, directly over the center of the lanes
  • A companion set of left turn lane signs are installed in advance, serving the entry to the turn lanes
  • Left turn phasing can vary by approach as well as by time-of-day, depending on each cross product

 Click here for this overview, including a graphic of the installation treatment

Click here for a list of FAQs, including a graphic of the installation treatment

 

For questions about the innovative DLTi treatment, please contact:

Joe Milazzo II, PE
Executive Director, Regional Transportation Alliance
919.664.7065
joe@letsgetmoving.org

RTA is the voice of the regional business community on transportation in North Carolina’s Research Triangle region.