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FHWA Home / Safety / Intersection / Removal of Signal Flashing Mode During Late-Night/Early-Morning Operation

Intersection Safety Case Study

Removal of Signal Flashing Mode During Late-Night/Early-Morning Operation

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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:

Introduction

Operating traffic control signals in the flashing mode can be an efficient form of signal operation when traffic volumes are low by reducing delay, fuel consumption, vehicle emissions, and the use of electricity. As such, programming traffic signals to operate in a flashing mode is typically limited to late-night/early-morning hours1. Use of a flashing yellow signal indication on the major street approaches and a flashing red signal indication on the minor street approaches is the standard practice for most intersections with adequate sight distance. Results of field studies, however, have indicated that crash rates may increase around traffic signals using flashing operations during late-night/early-morning conditions2[7].

Objective

The following case study showcases a successful and effective low-cost strategy that measurably improved safety at eight signalized intersections in Winston-Salem, NC. The treatment consisted of removing the flashing operation from traffic signals during late-night/early-morning hours.

Treatment Summary

All the intersection examples used in this report are from Winston-Salem, NC. Existing intersection treatments met minimum MUTCD requirements. The city changed the traffic signal operations at the intersections from flashing mode to steady (stop-and-go) mode during late-night/early-morning hours.

Evaluation Methodology

This case study examines eight intersections in Winston- Salem, NC, with high incidences of right-angle crashes (many with injuries) during late-night/early-morning hours. Crash reductions were based on a review of "before and after" data from these intersections during a minimum of approximately six years, between 1997-20073. (The "before" and "after" observation periods ranged between 35-51 months, depending on the intersection). Right-angle crashes occurring at the intersections both before and after the changes during late-night/early-morning hours were targeted. In addition, total crashes and injury crashes were also examined and compared.

"...use of flashing signal operation requires careful application and additional monitoring. We have found that it is better to have positive control rather than leaving the driver to decide when it is safe to proceed into the intersection."

Stan Polanis
Director of Transportation,
City of Winston-Salem, NC



Results

Problem: Eight urban intersections with traffic signals operated in the flashing mode for late-night/early-morning operations were experiencing high crash rates, particularly right-angle crashes (many with injuries). Previous studies suggested a higher risk of this type of crash associated with traffic signals operating in flashing mode during the late-night/early-morning hours.

Solution: The city sought to reduce right-angle crashes by removing the flashing operation from traffic signals and returning to normal (steady) signal operations during late-night/early morning hours.

Table 1 summarizes the "before and after" crash analysis at the treated intersections. Following the table is a brief discussion of the results at each intersection. (Note that targeted right-angle crashes shown in the table occurred during the late-night/early-morning hours. Total and injury crashes shown may have occurred at any time of day or night).

Table 1: Summary of crash reductions after removal of signal flashing mode during late-night/early-morning operation
Locations Implementation Date Before After Percent Reduction in Crashes Per Year (Injuries/Year)
Months Total Crashes Injury Crashes Right- Angle Crashes Months Total Crashes Injury Crashes Right- Angle Crashes Total Crashes Injury Crashes Right-Angle Crashes
Indiana Ave. and
Patterson Ave.
May-01 45 31 11 4 45 22 4 1 29% 63.3% 75%
Sprague St. and Vargrave St. Oct-01 46 32 10 4 46 26 5 1 18.8% 50% 75%
Hawthorne Rd. and Miller St. Oct-01 35 31 11 3 35 15 0 0 51.6% 100% 100%
Martin Luther King Dr. and Waughtown St. Mar-02 40 57 11 8 40 38 7 0 33.3% 36.4% 100%
Country Club Rd. and Westview Dr. Apr-02 39 8 4 3 37 6 1 0 20.9% 73.6% 100%
Marshall St. and Salem Ave. Apr-02 39 8 4 4 37 6 1 0 20.9% 73.6% 100%
Sixth St. and Trade St. Jul-02 37 7 4 3 37 4 0 0 42.9% 100% 100%
Third St. and Metropolitan Dr. May-04 36 16 10 7 36 9 6 1 43.8% 40% 85.7%
Total -- 329 198 68 36 327 136 27 4 30.9% 60.1% 88.8%

The removal of the late-night/early-morning flashing mode cumulatively reduced total crashes by 30.9 percent, injury crashes by 60.1 percent, and right-angle crashes by 88.8 percent. Fatal crashes were eliminated at four intersections.


Discussion

Implementation Issues

The city experienced no implementation issues with this countermeasure.

Photo of an intersection where the late-night/early-morning flashing mode has been removed.
Photo by Vaughan W. Inman, Ph.D. (used with permission).

Cost

The costs for changing the traffic signals from flashing mode to normal mode were low: approximately $500 per intersection.

Time Frame

The removal of the flashing operations took less than four hours per intersection. The city also conducted public outreach 30 days prior to removal.

Effectiveness

Removing the late-night/early-morning flashing mode at these urban intersections was effective at reducing total crashes, injury crashes, and right-angle crashes, and in some cases eliminated injury or right-angle crashes between the "before" and "after" analysis periods. The National Cooperative Highway Research Program (NCHRP) conducted an Empirical Bayes analysis of the Winston-Salem Study which is detailed in its report on Accident Modification Factors for Traffic Engineering and ITS Improvements[8]. The NCHRP results indicated that late-night/early-morning angle crashes were reduced by approximately 34 percent and this was significant at the 10% significance level. These reductions are substantially lower than the 88.8 percent reduction presented in this case study based on simple "before and after" analysis of the treated intersections. However both studies (the NCHRP report and the "before and after" analysis) confirm that removal of flashing operations during late -night/early-morning hours reduces crashes.

Summary of Results

Overall, the removal of the late-night/early-morning flashing mode from eight urban intersections in Winston-Salem, NC cumulatively reduced total crashes by 30.9 percent, injury crashes by 60.1 percent, and right-angle crashes by 88.8 percent. At four of the eight intersections, right-angle crashes at night were eliminated between the "before" and "after" periods. The reductions in crashes achieved by the treatments exceed the overall crash reduction factors of 75 percent for right-angle crashes and 29 percent for overall crashes mentioned in the Desktop Reference for Crash Reduction Factors (September 2007), published by the United States Department of Transportation (USDOT) FHWA[2].

References

1) Justin A. Bansen. Evaluation of Traffic Operations at Intersections in Malfunction Mode. Georgia Institute of Technology, 2006.

2) Desktop Reference for Crash Reduction Factors, FHWASA-07-015, USDOT FHWA September 2007, p.13.

3) Gaberty, Mathew J. II and James C. Barbaresso. A Case Study of the Accident Impacts of Flashing Signal Operations along Roadways. ITE Journal, July 1987.

4) Barbaresso, James C. Relative Accident Impacts of Traffic Control Strategies During Low-Volume Nighttime Periods. ITE Journal, August 1987.

5) Kacir K.C., G. Hawkins Jr., R.J. Benz and M.E. Obermeyer. Guidelines for the Use of Flashing Operation at Signalized Intersections. ITE Journal, October 1995.

6) Kacir, K.C., G. Hawkins, Jr., R.J. Benz, M.E. Obermeyer, and R.E. Bartoskewitz. Evaluation of Flashing Traffic Signal Operation. Research Report 1297-2F. Project No. 1297. Texas Transportation Institute, College Station, TX. November 1993.

7) S.F. Polanis, "Right-Angle Crashes and Late-Night/Early-Morning Flashing Operation: 19 Case Studies," ITE Journal, (Washington, DC: Institute of Transportation Engineers, April 2002).

8) Accident Modification Factors for Traffic Engineering and ITS Improvements, NCHRP Report 617, 2008.




1 2009 MUTCD, Section 4D.28 to Section 4D.31

2 Source: S.F. Polanis, "Right-Angle Crashes and Late-Night/Early- Morning Flashing Operation: 19 Case Studies," ITE Journal, April 2002. Note that one possible exception to this is very low-volume conditions such as those where the major street volume is less than 200 vehicles per hour or the ratio of the two-way volume for the major versus minor streets is greater than 3 to 1.

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




For More Information

Ed Rice
Intersection Safety Team Leader,
FHWA Office of Safety

202.366.9064
ed.rice@dot.gov

Stanley F. Polanis
Director of Transportation,
City of Winston-Salem, NC

336.747.6867
stanp@cityofws.org

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U.S. Department of Transportation
Federal Highway Administration

February 2010

Page last modified on September 4, 2014.
Safe Roads for a Safer Future - Investment in roadway safety saves lives
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