FHWA-SA-09-015

Intersection Safety Case Study

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:

•	 Whataresomeinexpensivetreatmentoptions toconsider toreduce

crashes (many with injuriesand manyduetoleft-turn head-on crashes)

at signalizedintersections?

•	 How manycrashesdidthetreatments reduce?

•	 Arethereimplementation issuesassociated with this treatmentand,

if so, how can theybeovercome?

Permissive/Protected

Left-Turn Phasing

Photo by April Armstrong (used with permission).
2 FHWA | Permissive/Protected Left-Turn Phasing

Introduction

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. 

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 highcrash 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.

Figure 1: Permissive/Protected mode

Photo courtesy of Michigan Department of Transportation (used with permission).Photo courtesy of Michigan Department of Transportation (used with permission).
FHWA | Permissive/Protected Left-Turn Phasing 3

“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. 

Photo from iStock (used with permission).
4 FHWA | Permissive/Protected Left-Turn Phasing

Objective

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 1998–20021.

(The “before” and “after” observation periods 

ranged from 6 months to 29 months at the treated 

intersections).

Results

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.

•	 Wyoming Road and Seven Mile Road

Wyoming Road is a two-lane urban arterial with a speed 

limit of 30 miles per hour (mph). Permissive left-turn 

phasing existed at the intersections of Wyoming and Seven 

Mile Roads in the “before” condition. The “after” condition 

involved installing permissive/protected left-turn mode 

on the Wyoming Road approaches. The “after” data was 

gathered between March 2002 and August 20022

 . After 

the improvements, total crashes decreased from 36 to 26 

(an average crash reduction of 27.8 percent). Injury crashes 

decreased from 8.7 to 4.0 (an average crash reduction of 54 

percent). Targeted left-turn head-on crashes were eliminated 

(from 3.3 in the “before” condition).

•	 Eastern Avenue and Alger Street

Eastern Avenue is a north-south corridor with a speed limit 

of 30 mph. In the “before” period, this primarily two-lane 

roadway had exclusive left-turn lanes with a permissive leftturn mode at its intersection with Alger Street. In the “after” 

condition, improvements included installing permissive/

protected left-turn modes on Eastern Avenue approaches3. 

The “after” data was gathered from January 2001 to August 

2002. After the improvements, total crashes per year 

decreased from 14.2 to 12.7 (an average crash reduction 

of 10.6 percent). Injury crashes decreased from 1.5 to 0 per 

year (an average crash reduction of 100 percent). Targeted 

left-turn head-on crashes were eliminated (from 3.8 crashes 

in the “before” condition). 

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.
FHWA | Permissive/Protected Left-Turn Phasing 5

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.

•	 Burton Street and Kalamazoo Avenue

Burton Street is a major east-west arterial with the posted speed limit varying between 30 mph and 35 mph. Prior to modification, 

Burton Street had two lanes in either direction with a permissive left-turn mode. The “after” condition involved installing permissive/

protected left-turn phasing for all approaches4

. The “after” data was gathered between April 2000 and August 2002. After the 

improvements, total crashes per year decreased from 36 to 19.9 (an average crash reduction of 44.7 percent). Injury crashes per year 

decreased from 9.5 to 4.1 (an average crash reduction of 56.8 percent). Targeted left-turn head-on crashes decreased from 6.0 to 2.1 

(an average crash reduction of 65%). Figures 2 and 3 depict the “before” and “after” condition of this intersection.

Westbound Burton Street at Kalamazoo Avenue

* Total crashes include injury and left-turn head-on crashes, and have been converted to reflect annual averages.

Table 1: Summary of crash reductions after installation of the permissive/protected left-turn modes

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

Figure 2: “Before” condition Figure 3: “After” condition Photo courtesy of Michigan Department of Transportation (used with permission).

Photo courtesy of Michigan Department of Transportation (used with permission).

Locations

Months

Implem-entation 

Date

Total CrashesInjury CrashesLeft-turn head-on Crashes

Months

Total CrashesInjury CrashesLeft-turn head-on Crashes

Wyoming Road 

and Seven Mile 

Road, Detroit

Burton Street 

and Kalamazoo 

Avenue, Grand 

Rapids

TOTAL 72 86.2 19.7 13.1 55 58.6 8.1 2.1 32.0% 58.9% 84.0% Total Crashes Injury Crashes Left-turn head-on Crashes

Percent Reduction 

In Crashes/Year Before After

Mar-02 24 36 8.7 3.3 6 26 4.0 0.0 27.8% 54.0% 100%

Apr-00 24 36.0 9.5 6.0 29 19.9 4.1 2.1 44.7% 56.8% 65.0%

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%
6 FHWA | Permissive/Protected Left-Turn Phasing

Discussion

Implementation Issues

The cities experienced no implementation issues with this 

countermeasure.

Cost

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.

Effectiveness

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). 

Photo from iStock (used with permission).
FHWA | Permissive/Protected Left-Turn Phasing 7

References

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.
FHWA-SA-09-015 February 2010

For More Information

Ed Rice

Intersection Safety Team Leader, 

FHWA Office of Safety

202.366.9064

ed.rice@dot.gov

Tapan K. Datta, Ph.D., P.E.

Professor, Civil and Environmental Engineering,

Wayne State University-Transportation Research Group

313.577.9154

tdatta@eng.wayne.edu

Visit FHWA’s intersection safety web site to download this and 

other case studies highlighting proven intersection safety 

treatments from across the country:

http://safety.fhwa.dot.gov/intersection