FHWA-SA-09-012

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 this case 

study to answer the following questions:

•	 What isan inexpensivetreatmentoption toreducecrashes,

particularlyright-anglecrashes (many with injuries),during

late-night/early-morning hours?

•	 How manycrashesdidthetreatment reduce?

•	 Arethereanyimplementation issuesassociated with this treatment,

andif so, how can theybeovercome?

Removal of Signal Flashing 

Mode During Late-Night/ 

Early-Morning Operation

Intersection Safety Case Study

Photo by April Armstrong (used with permission).
2 FHWA | Removal of Signal Flashing Mode During Late-Night/Early-Morning Operation

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/earlymorning 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 WinstonSalem, 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.

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”.

“...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
FHWA | Removal of Signal Flashing Mode During Late-Night/Early-Morning Operation 3

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 latenight/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).

Locations

Months

Implementation 

Date

Total CrashesInjury CrashesRight-Angle Crashes

Months

Total CrashesInjury CrashesRight-Angle Crashes

TOTAL 329 198 68 36 327 136 27 4 30.9% 60.1% 88.8% Total Crashes Injury Crashes Right-Angle Crashes

Before After

Oct-01 35 31 11 3 35 15 0 0 51.6% 100% 100% 

Apr-02 39 8 4 3 37 6 1 0 20.9% 73.6% 100% 

Jul-02 37 7 4 3 37 4 0 0 42.9% 100% 100% 

May-04 36 16 10 7 36 9 6 1 43.8% 40% 85.7% 

Oct-01 46 32 10 4 46 26 5 1 18.8% 50% 75% 

Apr-02 51 16 7 4 51 16 4 1 0 42.9% 75% 

Indiana Ave. and 

Patterson Ave.

Sprague St. and 

Vargrave St.

Country Club

Rd. and 

Westview Dr.

Hawthorne Rd. 

and Miller St.

Marshall St.

and Salem Ave.

Sixth St. and 

Trade St.

Third St. and 

Metropolitan Dr.

Martin Luther 

King Dr. and 

Waughtown St.

May-01 45 31 11 4 45 22 4 1 29% 63.6% 75% 

Mar-02 40 57 11 8 40 38 7 0 33.3% 36.4% 100% 

Table 1: Summary of crash reductions after removal of signal flashing mode during late-night/early-morning operation

Percent Reduction 

In Crashes/Year
4 FHWA | Removal of Signal Flashing Mode During Late-Night/Early-Morning Operation

•	 Indiana Avenue and Patterson Avenue

Indiana Avenue has three eastbound lanes and two westbound 

lanes. Patterson Avenue is a two-lane street carrying northsouth traffic. The speed limit on both roads is 35 mph. The 

intersection served a total entering annual daily traffic (ADT) of 

approximately 6,700 during the “before” period, and 8,400 in the 

“after” period. The flashing operation during late-night/earlymorning hours was removed from the traffic signals on May 31, 

2001. After the return to steady operations during latenight/early-morning hours, this intersection experienced 

a crash reduction of 29 percent, reduced injury crashes by 

63.6 percent per year and reduced the targeted right-angle 

crashes by 75 percent per year.

•	 Sprague Street and Vargrave Street

Both of these streets have an approach speed of 35 mph with 

one lane in each direction. The intersection served a total 

entering ADT of approximately 11,000 during the “before” 

period, and12,000 in the “after” period. The flashing operation 

during late-night/early-morning hours was removed from the 

traffic signals on October 29, 2001. After the return to steady 

operations during late-night/early-morning hours, this 

intersection experienced a crash reduction of 18.8 percent, 

reduced injury crashes by 50 percent per year and reduced 

the targeted right-angle crashes by 75 percent per year.

•	 Hawthorne Road and Miller Street

Miller Street runs north-south and carries one lane of traffic in 

each direction, while Hawthorne Road carries east-west and 

carries two lanes of traffic in each direction. The speed limit on 

both streets is 35 mph. The intersection served a total entering 

ADT of approximately 12,000 during the “before” period, 

and12,000 in the “after” period. Late-night/early-morning 

flashing operations were removed from the traffic signals 

on October 30, 2001. After the return to steady operations 

during late-night/early-morning hours, this intersection 

experienced a crash reduction of 51.6 percent. Injury crashes 

and the targeted right-angle crashes were eliminated 

(from eleven and three, respectively, in the “before” period).

•	 Martin Luther King Jr. Drive and Waughtown Street

Martin Luther King Jr. Drive and Waughtown Street both 

have two lanes in each direction with a posted speed limit 

of 35 mph. The intersection served a total entering ADT of 

approximately 10,000 during the “before” period, and 9,200 in 

the “after” period. Flashing operations were removed from the 

traffic signals on March 14, 2002. After the return to steady 

operations during late-night/early-morning hours, this 

intersection experienced a crash reduction of 33.3 percent 

and reduced injury crashes by 36.4 percent per year. The 

targeted right-angle crashes were eliminated (from eight in 

the “before” period).

•	 Country Club Road and Westview Drive

Country Club Road has a speed limit of 35 mph with two 

lanes in each direction. Westview Drive has one lane in each 

direction, with a speed limit of 30 mph on the northbound 

approach and speed limit of 35 mph on the southbound 

approach. The intersection served a total entering ADT of 

approximately 13,000 during the “before” period, and 12,000 in 

the “after” period. Late-night/early-morning flashing operations 

were removed from the traffic signals on April 10, 2002. After 

the return to steady operations during late-night/earlymorning hours, injury crashes decreased by 42.9 percent per 

year and the targeted right-angle crashes decreased by 75 

percent per year (total crashes remained the same).

•	 Marshall Street and Salem Avenue

Both streets have one lane in each direction and a posted 

speed limit of 35 mph. The intersection served a total entering 

ADT of approximately 3,700 during the “before” period, and 

2,700 in the “after” period. Late-night/early-morning flashing 

operations were removed on April 10, 2002. After the return to 

steady operations during late-night/early-morning hours, 

this intersection experienced a crash reduction of 20.9 

percent, reduced injury crashes by 73.6 percent per year. The 

targeted right-angle crashes were eliminated (from three in 

the “before” period).

•	 Sixth Street and Trade Street

Sixth Street runs east-west and carries one lane of traffic each 

direction. On-street parking is permitted on Trade Street, 

which also has one lane in each direction. The speed limit on 

both roads is 35 mph. The intersection served a total entering 

ADT of approximately 6,100 during the “before” period, and 

6,000 in the “after” period. Late-night/early-morning flashing 

operations were removed from the traffic signals on July 02, 

2002. After the return to steady operations during latenight/early-morning hours, this intersection experienced a 

crash reduction of 42.9 percent and injury and the targeted 

right-angle crashes were eliminated (from four and three, 

respectively, in the “before” period).

•	 Third Street and Metropolitan Drive

Third Street has two lanes in each direction, while Metropolitan 

Drive is one way street with two lanes. The speed limit on 

both streets is 35 mph. The intersection served a total entering 

ADT of approximately 3,500 during the “before” period, and 

3,000 in the “after” period. Late-night/early-morning flashing 

operations were removed from the traffic signals on May 26, 

2004. After the return to steady operations during latenight/early-morning hours, this intersection experienced a 

crash reduction of 43.8 percent, reduced injury crashes by 

40 percent per year and reduced the targeted right-angle 

crashes by 85.7 percent per year.
FHWA | Removal of Signal Flashing Mode During Late-Night/Early-Morning Operation 5

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.

Photo by Ron Schaefer (used with permission).
6 FHWA | Removal of Signal Flashing Mode During Late-Night/Early-Morning Operation

Discussion

Implementation Issues

The city experienced no implementation issues with this 

countermeasure. 

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/earlymorning 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]. 

Photo by Vaughan W. Inman, Ph.D. (used with permission).
FHWA | Removal of Signal Flashing Mode During Late-Night/Early-Morning Operation 7

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/EarlyMorning 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.
FHWA-SA-09-012 February 2010

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