Special Investigation Report

Vehicle- and Infrastructure-based Technology For the Prevention of Rear-end Collisions

NTSB Number SIR-01/01
NTIS Number PB2001-917003

Summary: In 1999, the most recent year for which data are available, more than 6 million crashes occurred on U.S. highways, killing over 41,000 people and injuring nearly 3.4 million others. Rear-end collisions accounted for almost one-third of these crashes1 (1.848 million) and 11.8 percent of multivehicle fatal crashes (1,923). Commercial vehicles2 were involved in 40 percent of these fatal rear-end collisions (770), even though commercial vehicles only comprised 3 percent of vehicles and 7 percent of miles traveled on the Nation's highways. Between 1992 and 1998, the percentage of rear-end collisions involving all vehicles increased by 19 percent. In 1999, 114 fatal crashes in work zones involved rear-end collisions, about 30 percent of the multivehicle fatal work zone crashes. Of these, 71 collisions (62 percent) involved commercial vehicles.

In the past 2 years, the National Transportation Safety Board investigated nine rear-end collisions in which 20 people died and 181 were injured (three accidents involved buses and one accident involved 24 vehicles).3 Common to all nine accidents was the rear following vehicle driver's degraded perception of traffic conditions ahead.4 During its investigation of the rear-end collisions, the Safety Board examined the striking vehicles and did not find mechanical defects that would have contributed to the accidents. In each collision, the driver of the striking vehicle tested negative for alcohol or drugs. Some of these collisions occurred because atmospheric conditions, such as sun glare or fog and smoke, interfered with the driver's ability to detect slower moving or stopped traffic ahead. In other accidents, the driver did not notice that traffic had come to a halt due to congestion at work zones or to other accidents. Still others involved drivers who were distracted or fatigued.

Regardless of the individual circumstances, the drivers in these accidents were unable to detect slowed or stopped traffic and to stop their vehicles in time to prevent a rear-end collision. According to a 1992 study by Daimler-Benz, if passenger car drivers have a 0.5-second additional warning time, about 60 percent of rear-end collisions can be prevented. An extra second of warning time can prevent about 90 percent of rear-end collisions.5

As the Safety Board reported in 19956 and further discussed at its public hearing, Advanced Safety Technologies for Commercial Vehicle Applications, held August 31 through September 2, 1999, existing technology in the form of Intelligent Transportation Systems (ITS) can prevent rear-end collisions. ITS, capable of alerting drivers to slowed or stopped traffic ahead, have been available for several years but are not in widespread use. The technology to alert drivers to traffic ahead includes adaptive cruise control (ACC), collision warning systems (CWSs), and infrastructure-based congestion warning systems. ACC detects slower moving vehicles ahead and closes the throttle and applies the engine brake to slow the host vehicle to a comparable speed.7 CWSs detect slower moving vehicles ahead and warn the driver of the host vehicle about the object ahead so the driver can take appropriate action. Infrastructure-based congestion warning systems use variable message signs to give drivers detailed information about the location of traffic queues. In the nine accidents investigated by the Safety Board, one (and sometimes more) of these technologies would have helped alert the drivers to the vehicles ahead, so that they could slow their vehicles, and would have prevented or mitigated the circumstances of the collisions.

The Safety Board addressed implementation of such systems for commercial vehicles in its 1995 special investigation of collision warning technology and recommended that the U.S. Department of Transportation (DOT) sponsor fleet testing of CWSs for trucks.8 On August 10, 1999, the Board classified the recommendation "Closed-Unacceptable Action" due to inaction by the DOT on testing of the CWS for trucks at that time. (See the "Related Report and Consequent Recommendations" section of this report for further information.)

Because of the lack of progress in deploying rear-end CWSs, the Safety Board addressed the issue at its summer 1999 public hearing focusing on advanced safety technologies for commercial vehicle applications to determine what had been done since its 1995 report. (See "Public Hearing" section of this report for further information.) At the hearing, representatives of Eaton VORAD Technologies, L.L.C. (Eaton VORAD); U.S. Xpress Enterprises, Inc. (U.S. Xpress); Greyhound Lines, Inc. (Greyhound); and the DOT provided information regarding the CWS and the status of various tests and deployments. As became clear during the public hearing, private industry is beginning to deploy vehicle-based safety systems. The CWS and ACC developed by Eaton VORAD are available as an option on trucks produced by all major manufacturers in the United States. Automobile manufacturers in Europe and Japan have begun to offer ACC on their high-end models, and Lexus and Mercedes are doing the same on their 2001 luxury vehicles in the United States.

According to a March 2000 TRW press release, industry analysts predict the market for ACC, CWSs, and headway control will grow from $11 million in 1998 to $2.4 billion in 2010. In 1999, the DOT commenced operational tests of ACC and CWSs for both cars and trucks. Several States also have projects under way to deploy infrastructure-based technology that alerts drivers to the location of the end of the queue in work zones or congested areas.9

The work being done by private industry and the Government is encouraging, but the pace of testing and of standards development for all vehicles and of deployment for commercial vehicles is cause for concern, given the increasing number of rear-end collisions and the number of fatalities when commercial vehicles are involved. Therefore, the Safety Board is again addressing subjects related to ITS, both vehicle- and infrastructure-based, for the prevention of rear-end collisions. The Safety Board has explored the issues involved in deploying technological solutions in this special investigation report, which focuses on some of the challenges, including implementation, consumer acceptance, public perception, and training associated with the deployment of such systems.

1  According to the 1999 Fatal Analysis Reporting System, rear-end collisions accounted for 29.5 percent of all crashes that year. Sometimes referred to as a frontal collision, a rear-end collision occurs when the following vehicle strikes the rear of the lead vehicle.

2  Heavy (over 10,000 pounds) trucks and motorcoaches.

3  The accidents occurred in Moriarty, New Mexico; Sweetwater, Tennessee; Trenton, Georgia; Sullivan, Indiana; Tinnie, New Mexico; Wellborn, Florida; West Haven, Connecticut; Elk Creek, Nebraska; and Eureka, Missouri.

4  Driver inattention is a major causal factor in about 91 percent of rear-end crashes, as reported in: U.S. Department of Transportation, ITS Joint Program Office, Program Area Descriptions: Motor Vehicle Crashes-Data Analysis and IVI Program Emphasis (November 1999).

5  D.R. Ankrum, "Smart Vehicles, Smart Roads," Traffic Safety 92(3) (1992): 6-9.

6  National Transportation Safety Board, Multiple Vehicle Collision With Fire During Fog Near Milepost 118 on Interstate 40, Menifee, Arkansas, January 9, 1995, and Special Investigation of Collision Warning Technology, Highway Accident Report NTSB/HAR-95/03 (Washington, DC: NTSB, 1995).

7  Within limits, most systems can only slow the vehicle by 25 percent, after which driver intervention (braking) is required.

8  NTSB/HAR-95/03.

9  In November 1999, subsequent to the public hearing, the DOT began a field operational test of CWSs on trucks and is currently testing CWSs on automobiles in cooperation with the General Motors Corporation.