Antilock Brake Systems (ABS) are the first of a series of three braking technology developments. They are four-wheel systems that prevent wheel lock-up by automatically modulating the brake pressure when the driver makes an emergency stop. Traction Control Systems (TCS) are the second technology. They deal specifically with front-to-back loss of friction between the vehicle’s tires and the road surface during acceleration. Electronic Stability Control (ESC) systems are another breakthrough technology evolving from and incorporating the first two technologies – ABS and TCS. They are stability enhancement systems designed to improve vehicles’ lateral stability by electronically detecting and automatically assisting drivers in dangerous situations (e.g. understeer and oversteer) and under unfavorable conditions (rain, snow, sleet, ice). ESC systems have sensors that monitor the speed of each wheel, the steering wheel angle, and the overall yaw rate and lateral acceleration of the vehicle. Data from the sensors are used to compare a driver’s intended course with the vehicle’s actual movement to detect when a driver is about to lose control of a vehicle and automatically intervene in split seconds by applying the brakes to individual wheels and possibly reducing engine torque to provide stability and help the driver stay on course. For example, if a system detects that the rear wheels have begun to slide to the right and the vehicle is yawing counter-clockwise, it may momentarily apply the brake to the right front wheel, imparting a clockwise spin to counteract the yaw and stabilize the vehicle. It may then slow down the vehicle to a speed more appropriate for conditions. This technology appears to provide safety benefits by reducing the number of crashes due to driver error and loss of control, because it has the potential to anticipate situations leading up to some crashes before they occur and has the capability in some cases to automatically intervene to prevent them. The potential benefit should be primarily a reduction of single vehicle crashes that involve losing control and running off the road. These crashes include rollovers and collisions with fixed objects.
Sales of vehicles with ESC are increasing. In model year 2003, 7.4 percent of the light vehicle fleet were sold with an ESC system.
The study consisted of a series of analyses of crash data currently available from the State Data System (maintained by NHTSA) and the Fatality Analysis Reporting System (FARS) databases. This preliminary results report documented that, as of September 2004, NHTSA had analyzed data regarding the effectiveness of ESC in reducing single vehicle crashes, for model years 1997 to 2002, in make models where ESC was introduced in those years. The agency also looked at single- and multi-vehicle crash rates per 100,000 registration years to ensure that any reduction in single vehicle crashes is not offset by an increase (if any) in multi-vehicle crashes.
An analysis of the effectiveness of electronic stability control at reducing single vehicle crashes in passenger cars and SUVs (1997-2002 crash data from five States) suggested that single vehicle crashes were reduced by 35 percent for passenger cars and by 67 percent for SUVs.