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:

Florida Indiana Maryland
Missouri Pennsylvania Texas
Utah Virginia

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:


By calendar year, the effectiveness of passenger car CHMSL (average percent reduction of police-reported rear impact crash rates in eight States) was:

The effectiveness of passenger car CHMSL did not have a statistically significant downward trend during 1989-95. The average effectiveness in 1989-95 was 4.3 percent. It may be concluded that the lamps reached their long-term effectiveness level of 4.3 percent in 1989.

Passenger car CHMSL were significantly more effective for the period 1987-88 than for 1989-95. The effect in 1987, 8.5 percent, was nearly double the long-term effect.

Effectiveness of passenger car CHMSL, and its confidence bounds, by calendar year:

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

There was little State-to-State variation in the effectiveness of passenger car CHMSL.


The long-term effectiveness of passenger car CHMSL is about equal in property-damage and nonfatal-injury crashes.

The lamps had little or no effect on fatal rear-impact crash rates at any time during 1986-95.

CHMSL are more effective in daytime than in nighttime crashes. They are more effective at locations away from traffic signals than at locations equipped with traffic signals. Since 1989, they have been more effective in preventing two-vehicle crashes than in preventing crashes involving three or more vehicles.

The lamps may be somewhat more effective in towaway than in nontowaway crashes. They may be somewhat more effective on wet roads than on dry roads. Effectiveness may be slightly higher in rural than in urban crashes.

In general, the simpler the accident scene, the more effective the CHMSL. The more a driver is distracted by other lights or traffic features, the less effective the CHMSL.

CHMSL effectiveness in the struck vehicle in a front-to-rear collision is about the same whether the driver of the striking vehicle is young or old, male or female.


Initial crash data from six States show that light trucks equipped with CHMSL have 5 percent lower rear-impact crash rates than light trucks without CHMSL. The reduction is statistically significant (confidence bounds: 0.3 to 9.4 percent).

Although the observed point estimate of effectiveness for CHMSL in light trucks (5.0 percent) is close to the lamps' long-term effectiveness in passenger cars (4.3 percent), the uncertainty in the light-truck estimate, at this time, does not yet permit the inference that the lamps are equally effective in cars and trucks.

These initial analyses did not show any significant variations in CHMSL effectiveness by light truck type (pickup, van, sport utility) or size (full-sized, compact).


At the long-term effectiveness level (4.3 percent reduction of rear-impact crashes), the public would obtain the following annual benefits when all cars and light trucks on the road have CHMSL:

Police-Reported Unreported Police-Reported
Plus Unreported
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
(1994 $)

The annual cost of CHMSL in cars and trucks sold in the United States is close to $206 million.

Since the value of property damage avoided, alone, far exceeds the cost of CHMSL, the lamps still are and will continue to be highly cost-effective safety devices.