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Intersection Safety Case Study

Permissive/Protected Left Turn Phasing

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Intersection Safety Case Study

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Federal Highway Administration
Office of Safety


This case study is one in a series documenting successful intersection safety treatments and the crash reductions that were experienced. Traffic engineers and other transportation professionals can use the information contained in the case study to answer the following questions:


Left-turning movements are generally acknowledged to be the highest-risk movements at intersections. An estimated 27 percent of all intersection-related crashes in the United States are associated with left turns, with over two-thirds occurring at signalized intersections[1]. Vehicles making this type of movement encounter potential conflicts from several sources including opposing through traffic, through traffic in the same direction, and crossing vehicular and pedestrian traffic.

Several intersection treatments have been developed to reduce these risks, including converting from a permissive left-turn mode to permissive/protected phasing. In a "permissive" mode, a green signal permits vehicles to turn left as traffic allows (see left image in Figure 1). In a "permissive/protected" mode, the permissive left-turn phase is immediately followed by an exclusive, protected left-turn phase, initiated by a green arrow signal indication (see right image in Figure 1). The Manual on Uniform Traffic Control Devices (MUTCD) (Section 4D.06) provides more information on the standards and applications of the permissive/protected mode.

Figure 1: Permissive/Protected Mode
Two side-by-side photos of permissive-protected signal heads, with the photo on the left showing the permissive phase (circular green) and the photo on the right showing the protected phase (left-turn arrow).
Photo courtesy of Michigan Department of Transportation (used with permission).

In 1996, American Automobile Association (AAA) Michigan, in partnership with the cities of Detroit and Grand Rapids, Michigan, initiated the Road Improvement Demonstration Program (RIDP) to improve intersection safety. The RIDP's main purpose was to identify high-crash locations and to develop and implement safety measures at those locations. While various measures were taken, the AAA Foundation for Traffic Safety reported that the most successful crash reductions were often accomplished with low-cost projects.

"You don't have to spend a million bucks. The biggest savings is really from a societal perspective, from the reduced injuries[2]."

AAA Foundation for Traffic Safety,
Progress Report. Vol. 6., No 6., December 1999.


The following case study showcases a successful and effective low-cost countermeasure that measurably improved safety at three signalized intersections in Michigan. The treatment included converting the permissive left-turn mode to permissive/protected phasing.

Treatment Summary

All intersection examples used in this report are from Detroit/Grand Rapids, MI. Existing intersection treatments met minimum MUTCD standards. This case study examines the effects of the addition of a permissive/protected left-turn mode at three signalized intersections.

Evaluation Methodology

This case study examines three signalized intersections with a high incidence of injury crashes (many due to left-turn head-on crashes). In this study, left-turn head-on crashes were classified as those in which the impacted vehicles were traveling in opposite directions[8].

Crash reduction results were based on a review of "before and after" data from these intersections during a minimum of 2.5 years, between 1998-2002.1

(The "before" and "after" observation periods ranged from 6 months to 29 months at the treated intersections).


Problem: These intersections were experiencing high incidences of crashes (many with injuries) because of conflicts occurring in the course of the left-turn movement during the permissive left-turn phase.

Solution: The cities upgraded the permissive left-turn mode to a permissive/protected mode to minimize left-turn head-on crashes. They retimed signals as appropriate to accommodate the permissive/protected left-turn phasing.

Westbound Burton Street at Kalamazoo Avenue

Figure 2: "Before" condition
Photo of the the Westbound Burton Street and Kalamazoo Avenue intersection with a permissive signal is installed.
Photo courtesy of Michigan Department of Transportation (used with permission).

Figure 3: "After" condition
Photo of the Westbound Burton Street and Kalamazoo Avenue intersection after a protected-permissive signal was installed.
Photo courtesy of Michigan Department of Transportation (used with permission).

Table 1 summarizes the percent reductions for total, injury and left-turn head-on crash types for each of the three intersections.

Table 1: Summary of crash reductions after installation of the permissive/protected left-turn modes
Location Implementation Date Before After Reduction in Crashes per Year
Months Total Crashes Injury Crashes Left-turn/ head-on Crashes Months Total Crashes Injury Crashes Left-turn/ head-on Crashes Total Crashes Injury Crashes Left-turn/ head-on Crashes
Wyoming Road and
Seven Mile Road,
Mar 02 24 36 8.7 3.3 6 26 4.0 0.0 27.8% 54.0% 100%
Eastern Avenue
and Alger Street, Grand Rapids
Jan 01 24 14.2 1.5 3.8 20 12.7 0.0 0.0 10.6% 100% 100%
Burton Street and
Kalamazoo Avenue,
Grand Rapids
Apr 00 24 36.0 9.5 6.0 29 19.9 4.1 2.1 44.7% 56.8% 65.0%
Total empty 72 86.2 19.7 13.1 55 58.6 8.1 2.1 32.0% 58.9% 84.0%
* Total crashes include injury and left-turn head-on crashes, and have been converted to reflect annual averages.


Implementation Issues

The cities experienced no implementation issues with this countermeasure.


The costs to implement this countermeasure, including the equipment (i.e., the controller), and technician costs (i.e., signal retiming), were estimated at $25,000 per intersection.

Time Frame

The signal mode improvements at each intersection were implemented within one week.


This countermeasure was effective in reducing injury crashes, including left-turn head-on crashes, as well as total crashes.

Summary of Results

The "before" conditions at the three treated intersections met minimum MUTCD standards. The safety enhancements discussed in this case study were added to reduce left-turn head-on crashes. Installation of the permissive/protected left-turn mode cumulatively reduced total targeted left-turn head-on crashes at these intersections by 84 percent, injury crashes by 58.9 percent, and total crashes overall by 32 percent reduction per year.

The average reductions in crashes achieved by this treatment consistent with the overall crash reduction factor of 16 percent for left-turn crashes when converting from permissive to permissive/protected phasing mentioned in the Desktop Reference for Crash Reduction Factors by the Department of Transportation (USDOT). The Highway Safety Manual reports an Accident Modification Factor (AMF) of 0.99 (equivalent to a CRF of 0.01, or 1 percent reduction) for all crashes for this treatment, which is based on rigorous statistical studies and analyses. The results from the three intersections highlighted in this case study are based on simple "before and after" studies and not necessarily statistically significant. However they demonstrate that implementing permissive/protected phasing does reduce crashes, which is consistent overall with the Highway Safety Manual (HSM).


1) O'Connor, T., "Intersection Collision Avoidance Systems Web Page," California Center for Innovative Transportation, August 2004. (http://www.calccit.org/itsdecision/serv_and_tech/Collision_avoidance/intersection.html).

2) AAA Foundation for Traffic Safety, Progress Report. Vol. 6., No 6., December 1999.

3) Federal Highway Administration. Desktop Reference for Crash Reduction Factors, FHWA-SA-07-015 (Washington, DC: September 2007).

4) "NCHRP Report 500, Volume 12: A Guide for Reducing Collisions at Signalized Intersections," Strategy 17.2 A1: Employ Multimode Signal Operation, v-11, ISSN 0077-5614, Transportation Research Board, The National Academies, Washington DC, 2004.

5) Agent, K.R., "Guidelines for the use of protected/permissive left-turn phasing," ITE Journal 57, no. 7, pp. 37-43, 1987.

6) Clark, J.E. and Daniel, J.T., Quantification of the impacts of providing protected left-turns at signalized intersections, 1994.

7) Box, P.C. and Basha, P.A., "A study of accidents with lead versus lag left-turn phasing," ITE Journal 73, no. 5, pp. 24-28, 2003.

8) Datta, K.Tapan and Schattler, L. Kerrie. "Evaluation Studies for the AAA Road Improvement Demonstration Program in Michigan," Wayne State University, 2003.

9) Neuman, T.R. Intersection Channelization Design Guide, NCHRP Report 279, Transportation Research Board, The National Academies, Washington, DC, 1985.

1 Note that crash reduction averages in this report reflect the percent reduction per year based on the difference between the total number of "before" and "after" crashes.

2 The "after" study period closed at the completion of the RIDP.

3 Signal backplates and secondary post-mounted signal heads to improve visibility were also installed at this intersection.

4 Other interventions at this intersection included installing backplates and relocating signal heads over the travel lanes to improve visibility. Note that the signal configuration was changed from diagonal span in the "before" condition to box span in the "after" condition.

For More Information

Ed Rice
Intersection Safety Team Leader,
FHWA Office of Safety


Tapan K. Datta, Ph.D., P.E.
Professor, Civil and Environmental Engineering,
Wayne State University-Transportation Research Group


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U.S. Department of Transportation
Federal Highway Administration

December 2009

Page last modified on September 4, 2014.
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