|NHTSA Technical Report Number DOT HS 808 696||March 1998|
Charles J. Kahane, Ph.D. and Ellen Hertz, Ph.D.
Center High Mounted Stop Lamps (CHMSL) have been standard equipment on all new passenger cars sold in the United States since model year 1986 and all new light trucks since model year 1994, as required by Federal Motor Vehicle Safety Standard 108. The purpose of CHMSL is to safeguard a car or light truck from being struck in the rear by another vehicle. When brakes are applied, the CHMSL warns drivers of following vehicles that they must slow down. This report tracks the effectiveness of CHMSL, year by year, from 1986 through 1995. The statistical analyses are based on police-reported crash files from eight States. It was found that:
Center High Mounted Stop Lamps (CHMSL) have been standard equipment on all new passenger cars sold in the United States since model year 1986 and all new light trucks since model year 1994, as required by Federal Motor Vehicle Safety Standard 108. The purpose of CHMSL is to safeguard a car or light truck from being struck in the rear by another vehicle. When brakes are applied, the CHMSL sends a conspicuous, unambiguous message to drivers of following vehicles that they must slow down. NHTSA was especially encouraged to promulgate the CHMSL regulation in 1983 by three highly successful tests of the lamps in taxicab and corporate fleets, showing 48 to 54 percent reductions of "relevant" rear-impact crashes in which the lead vehicle was braking prior to the crash, as reported by the study participants. Since nearly two-thirds of all rear impact crashes involve pre-impact braking by the lead vehicle, these results are equivalent to a 35 percent reduction of rear-impact crashes of all types.
The Government Performance and Results Act of 1993 and Executive Order 12866 (October 1993) require agencies to reevaluate the effectiveness, benefits and costs of their programs and regulations after they have been in effect for some time. NHTSA has already published two effectiveness evaluations based on the early police-reported crash experience of cars with production CHMSL. In the first study, based on Summer 1986 data, CHMSL-equipped cars were 15 percent less likely to be struck in the rear than cars without CHMSL. In the second study, based on calendar year 1987 data from eleven States, the reduction in police-reported rear-impact crashes of all types was 11.3 percent.
These levels of crash avoidance were still high enough to assure an excellent ratio of benefits to costs. Nevertheless, the decline in effectiveness from the fleet tests to the evaluations was clear-cut, even taking into account that the data bases were not perfectly comparable (participant-reported vs. police-reported crash data). That raised questions: as more and more cars on the road have CHMSL, do drivers "acclimatize" to the lamps and pay somewhat less attention to them? Would effectiveness continue to decline? A 1996 study by the Insurance Institute for Highway Safety, showing an average 5 percent crash reduction for CHMSL during 1986-91, strongly suggested a continued decline.
The principal objective of this report is to assemble enough crash data to allow an accurate estimate of the effectiveness of passenger car CHMSL in each calendar year from 1986 through 1995. That would make it possible to track the trend in effectiveness over time, find out when and if that trend leveled out, and determine the long-term crash reduction for CHMSL. The analysis is based on police-reported crash data from the eight States that furnished their files to NHTSA throughout 1986-95 and have the data elements needed for the analysis:
In each State and calendar year of data, the ratio of rear impacts to non-rear impacts for model year 1986-89 cars (all CHMSL equipped) is compared to the corresponding ratio in 1982-85 cars (mostly without the lamps), after the ratios have been adjusted for vehicle age. Other objectives of this report are: (1) Compare the effectiveness of passenger car CHMSL in various crash types, environmental conditions, etc. (2) Obtain an initial estimate of the effectiveness of CHMSL for light trucks (pickup trucks, vans and sport utility vehicles). CHMSL only began to appear on some light trucks in model year 1991 and they have been required since model year 1994. There has been some question as to whether they would be as effective in light trucks as cars. (3) Estimate the crash and injury reducing benefits of CHMSL and assess their long-term cost effectiveness.
The most important finding of the evaluation is that, in the long term, passenger car CHMSL reduce rear impacts by 4.3 percent (confidence bounds: 2.9 to 5.8 percent). Even though that effectiveness is well below the levels in earlier studies, and CHMSL can no longer be considered a "panacea" for the rear-impact crash problem, the benefits of CHMSL still far exceed the modest cost of the lamps, and CHMSL will continue to be a highly cost-effective safety device. The principal findings and conclusions of the study are the following:
PASSENGER CAR CHMSL: YEAR-BY-YEAR TREND OF OVERALL EFFECTIVENESS
|CY Group||Rear Impact Reduction (%)||Confidence Bounds|
|1986||5.1||2.5 to 7.7|
|1987||8.5||6.1 to 10.9|
|1988||7.2||4.8 to 9.5|
|1989-95||4.3||2.9 to 5.8|
PASSENGER CAR CHMSL: LONG-TERM EFFECTIVENESS BY CRASH TYPE, ETC.
LIGHT TRUCK CHMSL
LONG-TERM BENEFITS AND COSTS
|Crashes avoided||92,000 - 137,000||102,000||194,000 - 239,000|
|Injuries avoided||43,000 - 55,000||15,000||58,000 - 70,000|
|Property damage and
associated costs avoided