Autonomous Braking vs. Anti-Lock Brakes

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For more than a century, countless new technologies have been made available to new car buyers to make driving safer. Some features, such as crumple zones and airbags, protect occupants in case of a crash. Other features minimize the chances of a crash in the first place.

Often when a hazard appears, a driver's first instinct is to slam on the brakes. In other instances, drivers may not brake enough or at all. Without anti-lock brakes and autonomous braking, either of these instances could lead to an accident. We will explore both of these revolutionary technologies below.

How Brakes Work

Quite simply, brakes use friction to turn a car's kinetic energy (energy of motion) into heat. Most cars use a hydraulic braking system. With a hydraulic braking system, when a driver applies the brake pedal, the pedal pushes fluid through a series of tubes to the brakes at each wheel. The fluid pushes against brake pads, or shoes, which in turn press against a disc or drum attached to each wheel. This friction slows the wheel, and thus the car.

However, there is friction in another place – between the tire and the road. If the available friction between the tire and the road, called traction, is less than the friction from the brakes, the tire can skid, or lock up. This can happen when there is ice or rain on the road, or even on dry pavement, if the driver is braking aggressively. When the brakes lock up and the tires skid, the car becomes quite difficult to control, occasionally leading to accidents.

Anti-Lock Brakes

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Most drivers are familiar with the term ABS, or Anti-Lock Braking System, but many aren't aware of exactly what it means. Nearly every car sold in the U.S. today has anti-lock brakes fitted as standard. Just two decades ago, however, this feature was either an extra-cost option or unavailable, depending on the car. Some cars had extra badges trumpeting that they were equipped with “ABS.”

Before anti-lock brakes became commonplace, drivers were taught to “pump” their brakes on slippery surfaces – to quickly press and release their foot on the pedal – to keep their brakes from locking. While effective, it was dependent on the skill level of the driver. Some newer, lesser-skilled drivers were unable to effectively pump their brakes, leading to a loss of control.

Anti-lock brakes take the “pumping” off the driver's hands (or, rather, feet) and hand it over to a computer. When the driver presses the brake pedal in an ABS-equipped car, the computer reads specialized sensors at each wheel and determines whether the wheel is turning or locked. The sensors also report the speed each wheel is turning. If the computer senses a lock-up, it can pulse the brakes, helping the driver maintain control.

Autonomous Braking

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ABS is effective if the driver uses the brakes properly. However, if the driver does not see a hazard in front, due either to sun glare, distraction, or for some other reason, the anti-lock brakes cannot help. Over the last few years, however, automakers have been using several different technologies to “see” the road ahead and automatically apply the brakes in an emergency situation. Many different names and acronyms have been used for these technologies, including autonomous braking, autobrake, automatic emergency braking, and automatic pre-collision braking.

Just as ABS uses sensors at each wheel to determine if and how quickly each wheel is turning, autonomous braking uses sensors to detect hazards in front of the car. Generally, an autonomous braking system will use a radar or laser that projects forward to “see” pedestrians, animals, or rapidly approaching rear bumpers. The system will calculate the amount of braking force needed to avoid the hazard. Sometimes the autonomous braking system will determine that the driver is braking, but not hard enough, and it will add pressure to the brakes to slow the car properly. If the driver makes no effort at all to brake, many of these systems will bring the car to a complete stop.

Autonomous braking is not available on all cars yet, although automotive safety organizations like the Insurance Institute for Highway Safety (IIHS) are starting to factor autonomous braking into their safety ratings. That has given automakers the incentive to start including autonomous braking as an option across their lineups. It has long been available in luxury cars like the Mercedes-Benz S-Class, but now mainstream brands such as Honda and Toyota are making it available in everything from the compact Honda Civic to the three-row Toyota Highlander SUV. It's also becoming more common to see autonomous braking come standard, as it does in the Volvo XC90. When shopping for your next car, look for options with terms such as “forward collision warning,” “forward collision avoidance,” “emergency braking,” or “city braking assist” to select an autonomous braking system.

Be aware that autonomous braking isn’t perfect, nor is anti-lock braking. While engineers are working on creating fully-autonomous cars, your eyes are still your best bet for keeping you safe. Make sure you keep your eyes on the road, scanning back and forth for hazards, and avoid distractions like text messages and email. 

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