U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Intersection Safety Case Study
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:
Red-light running is one of the most serious traffic safety problems facing Americans today. It is estimated that vehicles running red lights cause more than 200,000 crashes, 170,000 injuries and approximately 900 deaths per year.1 Some of these crashes occur because drivers are speeding or are distracted, other drivers may be unaware they are approaching an intersection, or they are unable to see the traffic control device in time to comply.
Clearance intervals-both lengthening yellow-change intervals as well as providing an all-red clearance interval-together with increasing the size of signal lenses, have been shown to improve intersection safety. Increasing the length of the yellow-change interval in accordance with the recommended Institute for Transportation Engineers (ITE) formula has been shown to significantly decrease the chance of red-signal violations. Providing red clearance intervals and increasing the yellow-change interval have been shown to decrease late exits from intersections.
The following case study showcases two successful low-cost strategies that measurably improved safety at 33 signalized intersections in Detroit and Highland Park, MI. The treatments included providing an all-red clearance interval and increasing the size of the signal lens for the red, yellow and green indications.
All of the intersection examples used in this report are from a corridor that extends between Detroit and Highland Park, MI. This case study examines the application of two treatment enhancements that reduced angle and injury crashes at signalized intersections:
Adding or extending an all-red clearance interval (a brief period when lights in all directions are red) provides additional time before conflicting traffic is released following the yellow-change interval. This short all-red clearance interval provides a greater time separation between opposing vehicular movements, clearing the intersection of vehicles between signal phases, thereby reducing the potential for a crash.
"The intersection safety improvements (which included larger signal heads and implementing the all-red clearance interval) showed a great reduction in injury and fatal crashes and also ultimately helped protect people of all ages. Michigan's experience demonstrates the benefits derived from a comprehensive approach to intersection safety."
Larger signal lenses make traffic signals more visible and provide motorists with more time to determine and respond to the color indication. Figure 1 provides a visual comparison of the 8-inch and larger 12-inch traffic signals. Agencies can use different sizes of signal lenses in the same signal face or signal head, as specified in the MUTCD Section 4D.15. In this case, the cities increased the lens size of all three lenses in the traffic signals for maximum conspicuity.
Figure 1: Relative size difference between 12-inch (top lens) and 8-inch signal lens (bottom lenses)
This case study examines 33 intersections in Detroit and Highland Park, MI with a high incidence of angle crashes (many with injuries). Crash reductions were based on a review of "before and after" data from these intersections during a minimum of 13 months, between 1997-20024. (The "before" and "after" observation periods ranged between 13-34 months depending on the intersection). The safety improvements were implemented in 1997.
Problem: The signalized intersections along the Woodward Avenue corridor (Route 1) which runs between Detroit and Highland Park, MI, were experiencing a high number of angle crashes, as well as a high number of total crashes.
Solution: In 1996, the American Automobile Association (AAA) of Michigan, in partnership with the cities of Detroit and Highland Park, initiated the Road Improvement Demonstration Program (RIDP) to improve intersection safety along this arterial corridor. The program's main purpose was to identify high-crash locations in these cities and develop and implement safety measures.
The cities decided to add an all-red clearance interval and increase the size of the signal lens for the red, yellow and green signals at the intersections along the Woodward Avenue corridor. The corridor is approximately 7.5 miles long (see Figure 2) with a posted speed limit that varies between 35 to 40 miles per hour (mph). The corridor consists of three to four lanes in the north- and south-bound directions and a center lane for left turns.
Figure 2: Site Map of the Woodward Avenue Corridor
The safety treatments implemented along the Woodward Avenue corridor reduced the targeted angle crashes as well as total crashes. After implementation of these countermeasures, 90.9 percent of the treated intersections experienced a reduction in angle crashes, 81.8 percent experienced a reduction in injury crashes, and 75.8 percent experienced reductions in total crashes. At the corridor level, the safety treatments had a positive effect in reducing the targeted angle crashes by 75.7 percent, injury crashes by 45.5 percent, and total crashes by 33.3 percent per year. See Table 1 for a more detailed tabular summary of these treatments.
Crashes Per Year
|Chicago/Aden Park Blvd||24||12||4.5||2||34||3.9||0||0.4||67.5||100||80|
|Buena Vista Street||24||4||1||1||24||7||3||0.5||-75||-200||50|
|Merrill Plaisance Street||24||6.5||3.5||3.5||13||0.9||0||0||86.2||100||100|
|Seven Mile Road||24||46||15.5||8.5||13||24||6.5||3.7||47.8||58.1||56.5|
|State Fair Avenue||24||28.5||9||4||13||15.7||3.7||0||44.9||58.9||100|
The city experienced no implementation issues with these countermeasures.
The cost to implement the safety improvements at all 33 intersections totaled $2.3 million (an average cost of $70,000 per intersection). Costs included replacement of the smaller (8-inch) signal lens with the larger (12-inch) lens, and providing the all-red clearance intervals.
The treatments were installed at all locations within six months. This included development of retiming plans, addition of all-red clearance intervals, and installation of the larger 12-inch signal lenses.
The combinations of enhanced treatments discussed have been effective in reducing crashes at these signalized intersections. AAA Michigan conducted a benefit-cost evaluation using National Safety Council (NSC) cost data and further determined the benefit-cost ratio of 11:1 for the specified treatments. An Empirical Bayes analysis was also performed at selected intersections in the City of Detroit for which suitable control sites were identified. Finally, the significant results of additional tests (Poisson, Chi-Square, and Paired t-tests) conducted by the study team helped to further conclude that the implemented countermeasures were effective in reducing the targeted crashes at a 95 percent confidence interval. These results are consistent with other similar studies of the effectiveness of these treatments.
The safety enhancements discussed in this case study were added to reduce angle and total crashes. The combinations of enhanced countermeasures installed in these Michigan signalized intersections cumulatively reduced angle crashes at the treated intersections by 75.7 percent, injury crashes by 45.5 percent, and total crashes by 33.3 percent. The average reductions in crashes achieved by the treatments are within the range of crash reductions for these treatments (15 percent for all-red clearance interval and 24 percent for 12-inch signal lenses for total crashes) mentioned in the Desktop Reference for Crash Reduction Factors (September 2007), published by the United States Department of Transportation (USDOT) Federal Highway Administration (FHWA).
1) Federal Highway Administration Office of Safety Red-Light Running web site (http://safety.fhwa.dot.gov/intersection/redlight).
2) Desktop Reference for Crash Reduction Factors, FHWASA-07-015, Federal Highway Administration, (Washington, DC: September 2007), p. 8, 15.
3) "Evaluating Impact on Safety of Improved Signal Visibility at Urban Signalized Intersections," Transportation Research Record, Transportation Research Board, The National Academies, 0361-1981, volume 2019, pp. 51-56, 2007.
4) Datta T.K. and Schattler K.L., Evaluation Studies for the AAA Road Improvement Demonstration Program, Wayne State University, Michigan, 2003.
5) Polanis, Stanley F., "Improving Intersections Safety through Design and Operations (Examples)," presented at ITE 2002 Spring Conference, Palm Harbor, FL, March 2002.
6) Schattler K.L. and Hill, C. L., "Change and Clearance Interval Design on Red-Light Running and Late Exits," Transportation Research Record, pp. 193-201, January 2003.
7) Making Intersections Safer: A Toolbox of Engineering Countermeasures to Reduce Red-Light Running: An Informational Report. Countermeasures to Reduce Red-Light Running, Federal Highway Administration and Institute of Transportation Engineers, 2003.(http://safety.fhwa.dot.gov/intersection/redlight/cameras/rlr_report/)
8) SEMCOG Traffic Safety Manual, Southeast Michigan Council of Governments, Third edition, 1998.
1 Federal Highway Administration Office of Safety Red-Light Running web site (http://safety.fhwa.dot.gov/intersection/redlight).
2 Calculated using Institute of Transportation Engineers (ITE) recommended guidelines ARI = (W+L)/V, where ARI is the length of the all-red interval in seconds, W is the intersection width (in feet), L is the length of the clearing vehicle (typically 20 feet), and V is the approach speed (in feet/second).
3 The signal lenses in some of the study intersections were also relocated to improve visibility.
4 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.
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
U.S. Department of Transportation
Federal Highway Administration