Positive offset left-turn lanes are one solution to improving left-turning motorists’ visibility of opposing through and right-turning traffic.
MnDOT is revising its Road Design Manual and seeks to incorporate more information, policies and design guidance regarding positive offset left-turn lanes.
Researchers from the University of Wisconsin’s Traffic Operations and Safety Laboratory reviewed safety performance data from research that examined left-turn offsets. They also consulted 23 state DOT road design guides to understand the extent of available guidance.
What Was the Need?
In Minnesota, 42 percent of all severe crashes from 2008 to 2012 occurred at intersections. Four-way intersections require a high level of driver decision-making, and motorists turning left account for a substantial percentage of intersection crashes. Blocked sight lines at left-turn lanes can increase crash risk. Motorists turning left may not see vehicles approaching in the opposing through lanes and misjudge the available gap to cross.
When left-turn lanes of opposing traffic are directly opposite each other—called zero offset—drivers’ sight lines of the opposing through-traffic lanes are somewhat obscured. When those opposing turn lanes are positioned in a closer overlapping pattern—known as a negative offset—sight lines of oncoming through-lane traffic can be further obstructed, reducing drivers’ ability to view oncoming traffic and judge gaps sufficiently for turning.
In addition, drivers may take longer to complete a turn, slowing traffic flow. Modifications of left-turn lane design can provide motorists making left turns with clearer sight lines, increasing safety and improving traffic flow.
“Positive left-turn lane offsets allow drivers to see beyond the opposing left-turning vehicle, thus improving the motorists’ ability to view oncoming traffic. This design improves visibility for left turns, reducing crash risk and promoting efficient traffic flow,” said Kristi Sebastian, traffic engineer, Dakota County.
MnDOT and the Local Road Research Board sought to learn from previous research that has examined left-turn lane variations and their safety performance results. They also wanted to know of relevant road design guidance from other state departments of transportation (DOTs) concerning left-turn lanes. Researchers at the University of Wisconsin’s Traffic Operations and Safety (TOPS) Laboratory investigated the left-turn lane offset issue and developed recommendations for revising the MnDOT Road Design Manual.
What Was Our Goal?
The primary objective of this project was to develop Minnesota-specific guidelines to improve sight lines at intersections, particularly those of left-turn lanes. These new guidelines would be developed through close examination of research and road design manuals from other DOTs in states similar to Minnesota. This guidance would then be incorporated in an upcoming revision of the MnDOT Road Design Manual.
What Did We Do?
Researchers’ work encompassed three broad areas. First, they examined previous research that had been conducted to investigate the effects of various left-turn lane offset designs and their safety results. Second, researchers reviewed design and guidance manuals of state DOTs to identify existing policies and guidance regarding offset turn lanes at intersections. Finally, researchers developed general policies and guidance for designing offset left-turn lanes for the new edition of the Road Design Manual. They also recommended changes to MnDOT’s Traffic Engineering Manual and the Traffic Control Signal Design Manual to improve consistency among manuals addressing intersection design and operation.
What Did We Learn?
Researchers learned that obstructed left-turn sight lines had been a concern as early as the 1970s. Some guidelines were developed in the 1990s to address the problem; however, only a small number of research studies have evaluated the safety performance of offset left-turn lane treatments.
A 2009 Federal Highway Administration presentation showed the effects of left-turn lane offset improvements applied in Florida, Nebraska and Wisconsin. For positive offset left-turn lane implementation, as was done in Wisconsin, 1994 to 2006 crash data showed a 33.8 percent reduction in total crashes. Changes in lane offsets other than positive offsets showed insignificant reductions in crashes and in some cases, increased crashes.
Other studies showed that 72.5 percent of left-turn crashes involve the through-lane traffic. Increased sight lines could be beneficial in reducing these crashes. Research data showed a robust safety and performance benefit associated with positive offset left-turn lanes.
“Left-turning motorists’ visibility and sight distance can be improved by providing positive left-turn lane offsets that eliminate obstructed views of opposing traffic. Providing positive offsets will undoubtedly improve an intersection’s operational efficiency and safety,” said David Noyce, director, University of Wisconsin–Madison TOPS Lab.
Researchers’ examination of design manuals from 23 state DOTs showed that the majority have no or only minimal discussion of sight distance or left-turn safety design. Most rely on the American Association of State Highway and Transportation Officials (AASHTO) Geometric Design of Highways and Streets 2011 Edition. Only Florida DOT provides comprehensive policies and guidance on design standards for assuring unobstructed sight lines at left-turn lanes.
Researchers developed revisions for the Road Design Manual that address offset left-turn lanes. Recommended standards, policies and guidance are provided for new, reconstruction and preservation projects. Topics include a wide range of design factors, suggested designs for urban and rural multilane roadways and expressways, accommodating U-turns, considerations for pedestrians and bicyclists, and winter maintenance.
The recommendations of this project will be incorporated into the revised version of MnDOT’s Road Design Manual and other manuals associated with intersection design and safety.
This post pertains to Report 2019-32, “Design Standards for Unobstructed Sight Lines at Left-Turn Lanes,” published August 2019.