ABS and Accident Reconstruction
by Russell C. Lindsay, P.E.
published in PARC Views - Spring 1994 (V.1, No.1)
History of Antilock Brakes
Early versions of antilock braking systems (ABS) were available as early as the 1971 Chrysler Imperial, which had the first four wheel ABS. Recent advances in electronic technology, market demand, and other factors have made it economical for manufacturers to include fast-acting and reliable antilock braking systems, or ABS, on an increasing number of automobile and truck models.
Why ABS? - The Physics of Braking
Under normal braking, pedal pressure is converted to a retarding force on the rotation of the wheels, thus slowing the vehicle until the tire-to-road friction limit is exceeded. Once this limit is reached, the wheel will then rapidly decelerate to a "locked" condition. In addition to braking forces, tires and wheels are also expected to generate cornering forces, providing stability (rear wheels) and steerability (front wheels). However, once the wheel becomes locked this control is lost. To regain control the driver must recognize the situation and moderate his braking to "unlock" the wheel(s). A delay of only a fraction of a second in driver response can have catastrophic consequences, especially in a lightly-loaded truck or van. For this reason, drivers are often advise to "pump" the brake pedal in icy conditions.
The concept of antilock braking systems is quite simple. The idea is to automatically prevent wheels from "locking up" during braking, preserving vehicle stability and steerability regardless of the driver's skilla nd experience. ABS accomplishes this goal by rapidly modulating the brake pressure, just as a skilled driver might do. However, ABS is usually designed to work for each wheel (or axle) independently, as if the driver had a separate brake pedal for each wheel. And ABS response times are generally faster than the human counterpart.
The ABS controller constantly monitors each wheel via a wheel speed sensor. The controller compares the wheel speed with an estimate of the vehicle speed (calculated based on engine and transmission data). If the wheel speed and vehicle speed differ by more than the allowable "percent slip," the ABS controller activates the brake pressure modulator. Most systems can modulate brake pressure to each wheel individually. Once wheel and vehicle speeds return to within the allowable "slip" range, the modulator is deactivated. This cycle, of activating and deactivating the brake pressure modulator, repeats as long as the driver is pressing the brake pedal hard enough to cause a locked wheel skid.
As you might expect, any system that modulates, or temporarily reduces, braking forces could result in longer stopping distances. In fact, ABS does cause increased stopping distances on some surfaces. Recent tests by the National Highway Traffic Safety Administration (NHTSA), however, suggest that on most surfaces ABS-equipped vehicles can stop in about the same distances as non-ABS equipped vehicles, depending on vehicle loading and type. The exception was loose gravel and similar surfaces, on which a locked wheel gains extra friction because of a "plowing" effect. Loose material builds up in front of the tire, increasing the drag on the vehicle.
The same NHTSA tests also verified the stability and steerability benefits of ABS. Of the seven vehicles tested, all were essentially uncontrollable during a panic stop on surfaces simulating a partially ice-covered roadway when the ABS was disabled. This uncontrollability was especially evident when the vehicles were lightly loaded. With ABS on, however, the same seven vehicles were almost always under complete directional control.
Impact on Accident Reconstruction
Calculating speeds from skidmarks has long been a basic tool of the accident reconstructionist. The natural question, then, is how do we handle ABS vehicle crashes? Will there even be any skid marks if ABS prevents "locked wheel" skidding? The definitive answers are still unknown. ABS technology is evolving rapidly, and the wide variety of systems makes generalizations dangerous. However, I will offer some observations based on my experience driving and testing recent ABS-equipped vehicles, and observing the tire marks left by such vehicles.
First, most ABS versions do allow some degree of "slip" between the tire and road, at least intermittently. However, since ABS prevents extended "lockup," long, dark, continuous skid marks are becoming a thing of the past. My experience suggests that, while ABS-equipped cars sometimes do leave visible tire marks on the road, most often these marks are significantly lighter than those left by non-ABS vehicles. The potential problem is that these less obvious, short-lived, and often intermittent tire marks may be overlooked. This oversight may lead to the erroneous conclusion that the driver did not apply the brakes, or that (because of ABS) there is nothing on which to base a "braking distance" estimate. I use the term braking distance since ABS vehicles do not skid.
In many respects, ABS vehicles are no different from non-ABS vehicles, since neither type always produces visible skid marks. Testing has shown that up to 25 percent of the energy of a moving car can be dissipated by the braking system before visible skid marks are produced. This is equivalent to traveling 36 feet after applying the brakes before leaving visible skid marks (assuming an initial speed of 55 mph and a 0.7 skid coefficient). Wet road surfaces, especially during heavy rains, can enhance this effect and could result in a vehicle leaving no visible skid marks over a considerable braking distance. Also, skilled drivers can stop quickly without "locking up" the wheels, and without the aid of ABS.
Fortunately, speed estimates in a thorough accident reconstruction are rarely based solely on skid marks. Tire marks usually provide only one piece of the reconstruction puzzle.
The point is that ABS has the potential to greatly increase overall vehicular safety. And it may make the job of the accident reconstructionist more difficult. But timely, thorough investigation - and a sharp eye - can result in speed estimates from "braking distance" which are just as valid as those based on "skid distance."
So keep your eye out for those light "dotted line" tire marks, and pay attention to whether the vehicle is equipped with ABS.
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