Safety Effectiveness of Dual Left-Turn Lanes
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.
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.
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.
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).
Association of State Highway and Transportation Officials, A Policy on Geometric Design of Highways and Streets, Washington,
S. C. (1993). Positive Guidance Pavement
Markings for Dual Left-Turn Lanes. Virginia Transportation Research
K., Brewer, M. A., Dorothy, P., & Park, E. S. (2014). Design Guidance for Intersection Auxiliary Lanes (NCHRP Report 780).
R. H. (1987). Capacity of Dual Left-Turn
Lanes. State of the Art. Final Report (No. FHWA-AZ87-829).
T., & Moses, R. (2009). Influence of intersection geometrics on the
operation of triple left-turn lanes. Journal
of transportation engineering, 135(5), 253-259.
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.
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.
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).
· CMFs for dual left-turn lanes for different types of
· 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.
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
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|
|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.|
|Index Terms:||Signalized intersections, Left turn lanes, Highway capacity, Side crashes, Highway safety, Traffic signal phases, Turning radius, |
Operations and Traffic Management
Safety and Human Factors