RNS
Browse Projects > Detailed View

Safety Effectiveness of Dual Left-Turn Lanes

Description:

To help meet increased traffic demand, many agencies have installed dual left-turn lanes at signalized intersections as a way of providing additional capacity. While there have been many studies regarding the operational characteristics of dual left-turn lanes, little is known about their safety impacts. This gap in existing knowledge makes it challenging for practitioners to appropriately evaluate the safety of dual left-turn lanes and to compare their safety performance with a single left-turn lane intersection.

Dual left-turn lanes could present safety challenges such as the potential for sideswipe collisions. In addition, there are many different factors that may affect the safety performance of intersections with dual-left-turn lanes such as signal phasing, cycle length, storage length, traffic counts, land use, number of intersection approaches, turning radius, lane usage, driveways, pedestrians, signage, pavement markings, sight distance, horizontal curvature, and intersection skew. There is a need for practitioner guidance to help understand the safety tradeoffs of dual-left-turn lanes with consideration of these factors.

Objective:

The objectives of the research include:

· Determine CMFs for dual left-turn lanes for different types of signalized intersections;

· Explore the relationships between the safety performance of dual left-turn lanes and traffic, geometric, and other characteristics; and

· Provide practitioners with tools and guidance to help improve the safety performance of dual left-turn lanes.

Benefits:

The project will result in several products that can be easily implemented. First, CMFs will be developed for dual left-turn lanes at different types of signalized intersections. These CMFs could be widely distributed through the CMF clearinghouse and possibly included in future editions of the HSM. Second, the project will provide practitioners guidance regarding the factors (e.g. signal phasing, cycle length, storage length, traffic counts, land use, number of intersection approaches, turning radius, lane usage, driveways, pedestrians, signage, pavement markings, sight distance, horizontal curvature, and intersection skew) that influence safety at dual left-turn lane intersections and the relative importance of these factors in assessing safety. The study may include recommendations regarding types of markings and signage that have been the most effective with regard to safety. Another product of the study will be potential short-term and long-term countermeasures that can be implemented at dual left-turn lane intersections to help improve their safety. The guidance and list of countermeasures could be provided as a separate quick reference guide or incorporated into other publications such as the AASHTO Green Book, FHWA Signalized Intersections Informational Guide, AASHTO Guide for the Development of Bicycle Facilities, the Access Management Manual and AASHTO Guide for the Planning, Design, and Operation of Pedestrian Facilities.

Related Research:

A review of literature shows that many of the existing studies pertain to general design guidance, operational impacts of dual left-turn lanes, or development of guidance for other types of facilities. The 2011 Edition of the AASHTO Green Book (1) provides some general design guidance for double and triple left-turn lanes regarding swept path width and offtracking. Brich (2) provided recommendations regarding positive guidance pavement markings at dual left-turn lane locations.

The most recent research regarding operational impacts of dual left-turn lanes was undertaken by Fitzpatrick et al. (3). This study identified a number of factors that affect saturation flow rates such as U-turns, number of vehicles in the queue, and friction on the receiving leg caused by bus stops, channelized right-turn lane exits, and driveways or minor intersections. Although the study was focused on operations, an investigation of driver behaviors found that downstream driveways could cause more lane changing activity. A study of dual left-turn lane capacity was also undertaken by Wortman (4) while other studies have assessed operations of triple left-turn lanes (5) (6).

While some studies have investigated safety impacts of multiple turn lanes, they are out-of-date or focused on other multiple turn lane configurations. Cooner et al. (7) analyzed both operational and safety performance of triple left-turn lanes and dual right-turn lanes. While their study did not find any major safety issues based on analysis of 25 sites in Texas, they did not investigate safety impacts of dual left-turn lanes. Ackeret et al. (8) investigated sideswipe crashes at both dual left-turn lane and triple left-turn lane intersections in Las Vegas. Although they found that the proportion of sideswipe crashes at dual left-turn lane intersections was relatively low, their study was completed in 1999 and focused on one metropolitan area. There is a need for a timelier and more thorough assessment of the safety impacts of dual left-turn lanes for signalized intersections with a wide variety of characteristics. This need is further demonstrated by the lack of any existing crash modification factors (CMFs) for dual left-turn lanes on the CMF clearinghouse (9).

References

  1. American Association of State Highway and Transportation Officials, A Policy on Geometric Design of Highways and Streets, Washington, DC, 2011.

  2. Brich, S. C. (1993). Positive Guidance Pavement Markings for Dual Left-Turn Lanes. Virginia Transportation Research Council.

  3. Fitzpatrick, K., Brewer, M. A., Dorothy, P., & Park, E. S. (2014). Design Guidance for Intersection Auxiliary Lanes (NCHRP Report 780).

  4. Wortman, R. H. (1987). Capacity of Dual Left-Turn Lanes. State of the Art. Final Report (No. FHWA-AZ87-829).

  5. Sando, T., & Moses, R. (2009). Influence of intersection geometrics on the operation of triple left-turn lanes. Journal of transportation engineering, 135(5), 253-259.

  6. Sando, T., & Mussa, R. (2003). Site characteristics affecting operation of triple left-turn lanes. Transportation Research Record: Journal of the Transportation Research Board, (1852), 55-62.

  7. Cooner, S. A., Ranft, S. E., Rathod, Y. K., Qi, Y., Yu, L., Wang, Y., & Chen, S. (2011). Development of guidelines for triple left and dual right-turn lanes: technical report (No. 0-6112-1). Texas Transportation Institute.

  8. Ackeret, K. W., Nambisan, S. S., & Menon, R. (1999). Evaluation of Side Swipe Crashes at Triple and Dual Left Turn Lanes in Las Vegas, Nevada. In Transportation Frontiers for the Next Millennium: 69th Annual Meeting of the Institute of Transportation Engineers (No. Publication No. CD-006). Federal Highway Administration. (2016). “Crash Modification Factors Clearinghouse.” <http://www.cmfclearinghouse.org/> (Aug. 4, 2016).

Implementation:

Research products include:

· CMFs for dual left-turn lanes for different types of signalized intersections;

· List of factors that influence safety at dual left-turn lane intersections and their relative importance;

· Recommendations regarding types of markings and signage to help improve safety at intersections with dual left turn lanes;

· List of potential short-term and long-term safety improvements that can be implemented at dual left-turn lane intersections.

The research products will be developed in a format suitable for incorporation into various resources such as the CMF Clearinghouse, Highway Safety Manual (HSM), AASHTO Green Book, FHWA Signalized Intersections Informational Guide, AASHTO Guide for the Development of Bicycle Facilities, the Access Management Manual, and AASHTO Guide for the Planning, Design, and Operation of Pedestrian Facilities.

The target audience for the research results includes all practitioners who are responsible for the design, planning, and operations of dual left-turn lane intersections. The results can be implemented by both geometric designers and traffic engineers. Champions to help implement the research could include representatives from state DOTs, ITE, APWA, local agencies, consultants, MPOs, and other organizations. Aaron Frits, Road Design Leader at the Kansas Department of Transportation (KDOT), has indicated that he is willing to serve as the AASHTO monitor for the project. The AASHTO Standing Committee on Highway Traffic Safety, Standing Committee on Highways, and Subcommittee on Design could facilitate the implementation of the research results. The research results will be easily implementable although various state DOTs could be selected as early adopters to begin using the results. Candidate DOTs for early implementation of the results include Caltrans, TxDOT, North Carolina DOT, and Michigan DOT. Webinars could be used to help disseminate the research results. There are no known barriers to implementation of the research products.

Relevance:

The most significant benefit of this project is improved safety. Attainment of the project objectives will help to improve the safety of intersections with dual left-turn lanes through implementation of better practices and will facilitate more balanced consideration of safety and mobility impacts when planning and designing these facilities. As the use of dual-left turn lanes becomes more widespread, there is an urgent need to better understand the safety impacts of these facilities.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Research Period:12 - 24 months
Research Priority:Medium
RNS Developer:Henry Brown, Richard C. Coakley, Kay Fitzpatrick, Gilbert Chlewicki, Larry F. Sutherland, Aaron Frits
Source Info:Developed as part of the 2016 mid-year meeting of the TRB Committee on Geometric Design (AFB10), TRB Committee on Operational Effects of Geometrics (AHB65), and AASHTO Technical Committee on Geometric Design.
Date Posted:10/26/2016
Date Modified:11/14/2016
Index Terms:Signalized intersections, Left turn lanes, Highway capacity, Side crashes, Highway safety, Traffic signal phases, Turning radius,
Cosponsoring Committees: 
Subjects    
Highways
Design
Operations and Traffic Management
Safety and Human Factors

Please click here if you wish to share information or are aware of any research underway that addresses issues in this research needs statement. The information may be helpful to the sponsoring committee in keeping the statement up-to-date.