Developing a Guideline for Selecting Appropriate Treatment Options for Approaches with Two Left Turn Lane

Dual left turn lanes are crucial for increasing capacity at busy intersections during peak hours with high left turn volumes; however, they can lead to an increase in delays for all intersection users during low traffic demand periods. This increased delay is experienced since most intersections with dual left turn lanes in North Carolina utilize protected-only left turn signal phases. The protected mode can result in additional delays during off-peak hours due to (a) not being able to utilize gaps in opposing traffic due to the lack of a permissive phase, (b) longer cycle lengths as a result of the additional protected left-turn phases, and (c) increased lost time from additional signal phases. In North Carolina, lost time per signal phase can average around five to six seconds; therefore, adding phases can affect overall intersection efficiency. Despite the importance of selecting the appropriate treatment options for dual left turn lanes in minimizing delays and travel times, there are notable research gaps in the existing literature, and comprehensive guidelines are lacking to inform the decision-making processes. The primary objective of this research is to develop guidelines and a spreadsheet-based tool to assist North Carolina Department of Transportation (NCDOT) in selecting the most appropriate treatment options for approaches with dual left turn lanes. The research team will evaluate the advantages and disadvantages of various treatments, including protected-only, permissive, and protected-permissive phasing and a “dynamic left-turn intersection” (DLTi) approach. The research team will also consider exploring other innovative time-of-day treatments similar to DLTi. The project team is aware of potential issues of DLTi with accommodating pedestrians and will look into them in coordination with the steering and implementation committee.​

Language

• English

Project

• Status: Active
• Sponsor Organizations:

  North Carolina Department of Transportation

  Research and Development
  1549 Mail Service Center
  Raleigh, NC  United States  27699-1549
• Project Managers:

  Kim, Jay

• Performing Organizations:

  North Carolina State University, Raleigh

  Department of Civil Engineering, Campus Box 7908
  Raleigh, NC  United States  27695-7908
• Principal Investigators:

  Hajbabaie, Ali

• Start Date: 20250716
• Expected Completion Date: 20270715
• Actual Completion Date: 0

Subject/Index Terms

• TRT Terms: Highway design; Left turn lanes; Signalized intersections; Traffic delays
• Geographic Terms: North Carolina
• Subject Areas: Design; Highways; Operations and Traffic Management;

Filing Info

• Accession Number: 01966079
• Record Type: Research project
• Source Agency: North Carolina Department of Transportation
• Files: RIP, STATEDOT
• Created Date: Sep 18 2025 12:53AM