Anti-lock braking systems, or ABS, are among the most important advances in driver safety of recent decades, but they can also give drivers a false sense of security in severe condition. Driving an ABS-equipped vehicle decreases the likelihood of many kinds of accidents, but having ABS doesn't solve every driving problem. Snow and ice present two of the biggest challenges to ABS effectiveness, and drivers are well-served to learn about handling these conditions.
How ABS Works
While ABS can restore braking and steering in many conditions, it can actually increase stopping distances on snow or ice. To understand why requires an understanding of how ABS detects a skid. When you apply your brakes, ABS senses braking system pressure, then checks to ensure that all wheels are turning. If a wheel locks up, ABS releases the brakes on that wheel until it turns, then applies the brakes again. This process is repeated until all four wheels stop turning, indicating to the ABS that the car has stopped. In effect, ABS "pumps" the brakes automatically, and does so very rapidly.
Effects of Snow on ABS
The National Highway Traffic Safety Association (NHTSA) explained in a comprehensive report that ABS increases stopping distances on surfaces covered with gravel, snow, or other loose materials. In such situations, a locked tire digs into the snow or gravel, pushing it forward and forming a wedge in front of the tire, which brings the vehicle to a stop. Since ABS works by preventing the skid, that wedge never forms, though the driver's ability to steer may be restored.
Effects of Ice on ABS
On roads that are partially covered by ice, ABS can help the driver stop and steer the vehicle more effectively, provided he keeps the brake pedal depressed, and does not pump the brakes. Under severe conditions when the entire road is covered with ice, all four wheels may lock simultaneously. Unless at least one wheel is turning, the ABS will react as if the vehicle has stopped. When this occurs, the ABS is defeated, and the driver will need to go back to the pumping technique.
ABS and Driving Habits
A Purdue University report on ABS and airbag effectiveness concluded that such safety devices contribute to a perception of insulation from danger, causing "offsetting behavior." In vehicles with ABS or airbags, drivers may tend to feel protected, causing them to drive too fast, follow too closely or ignore road conditions. By doing so, they may overestimate the ability of these systems to protect them, and will increase the likelihood of accident and injury. The NHTSA report showed that the likelihood of some types of accidents actually increased in vehicles with ABS.
Ice and Snow Precautions
ABS is no substitute for common sense. When snow-covered roads or icy conditions prevail, it's best to reduce speed, allow for greater stopping distances, and remain sensitive to how the vehicle is reacting to the road surface. Being aware of how ABS may be bypassed in such conditions will enable you to react appropriately if and when the time comes. With an assist from ABS, good driving habits and a respect for road conditions will get you safely to your destination.