How Brake Assist Works

You probably think these are all you need to safely stop your car in an emergency situation, but brake assist can help your car react even more quickly to avoid a collision. See more pictures of brakes.

If you've ever driven a car on a featureless highway, you probably know the feeling of zoning out behind the wheel. You may have even found yourself wishing the car could drive itself.

Now imagine this scenario: You've chosen a beautiful day to visit distant friends. You and your car are eating up highway miles. With the satellite radio playing your favorite tunes and the open sunroof allowing you to get an in-car tan, life is good. In fact, you're gently lulled into a driving trance until you crest a hill -- only to see a parade of brake lights directly ahead. "I'm toast," you think, pressing the brake pedal but fully expecting to rear-end the car in front of you. Luckily, your car hauls itself to a halt, mere feet short of the closest vehicle. You let out a sigh of relief as you remember the car dealer mentioning something about brake assist the day you purchased your trusty steed.


Compared to modern electronic sensors, we humans have pretty slow reaction times. The purpose of brake assist and its related technologies is to help us stop our vehicles sooner and faster. Those critical seconds -- usually fractions of a second -- can mean the difference between a crash and a close call.

So, is brake assist a truly worthwhile safety feature, or is it merely a clever marketing gimmick designed to sell more cars and pump up sticker prices? Numerous studies show that brake assist is the real deal, with the potential to prevent thousands of fatal auto accidents each year.

In this article, we'll take a look at what allows brake assist to stop cars much more quickly and effectively than the unaided driver can do alone. With power brakes and anti-lock brakes on the scene, why would a driver need any more help stopping his or her car? Learn why by clicking to the next page.



The Reason for Brake Assist

Considering all of the other braking features already available on a typical car, why would a driver even want brake assist? What if the electronic gizmos that activate it get a bit too sensitive, leading to jerky stops every time the driver taps the brakes? Auto enthusiast magazines have long railed against these electronic "nannies" that take much of the skill -- and the thrill -- out of driving.

In a word, brake assist is about safety. Simply put, research shows that most people are too wimpy with the brakes in an emergency. According to Mercedes-Benz, 99 percent of drivers either failed to apply full brake pressure, or applied brake pressure too late, in an emergency stopping situation. When Mercedes introduced the technology to the market in the late 1990s, the company said brake assist helped shorten stopping distance by 45 percent. Even skilled drivers benefited from stopping distances that were 10 percent shorter [source: Mercedes-Benz]. In practical terms, shorter stopping distances mean fewer accidents.


As for the powerful braking action coming into play when you don't want it to, the engineers sought to make sure that doesn't happen. Brake assist is what automotive engineers call a driver-adaptive system. In other words, the electronics that control brake assist measure and monitor the driver's normal driving patterns, including application of the brakes. The system can actually tell the difference between slowing down at a traffic light and making a panicked stop when a child runs out into the street.

Automakers have been known to occasionally resist new safety features because of their added cost. However, it was Mercedes-Benz along with parts supplier TRW/LucasVarity that invented brake assist and started offering it on their cars [source: Mercedes-Benz]. The technology first appeared in the consumer market in 1996, with the introduction of the Mercedes-Benz S-Class and SL-Class models. In 1998, the company made the feature standard on all its vehicles. Since then, several companies have offered their own version of brake assist, including Acura, Audi, BMW, Infiniti, Land Rover, Rolls Royce and Volvo.


Providing Power for Panicked Stops

Brake Assist can help sleepy nighttime drivers react properly when sudden danger arises.
Matt Cardy/Getty Images News

Let's go through another brake assist-aided stop. This time, we'll tackle it step by step.

You're traveling down a deserted country road on a moonless night, with the forest seemingly closing in on either side. Your high beams throw off barely enough light for you to comfortably drive the speed limit. Suddenly, a family of deer comes loping onto the pavement a few hundred feet ahead.


As your foot instinctively clamps down on the brake pedal, a sensor immediately knows, by the speed and pressure of your foot on the pedal, that this is an emergency. Within a fraction of a second, the brake assist system signals the brakes to direct maximum clamping power to the brake calipers. The pedal pulses as its anti-lock braking system (ABS) kicks in, working in concert with the brake assist. The vehicle remains under control as it decelerates and stops far short of the crossing deer. No animals are harmed - - and your insurance premium escapes equally unscathed.

There's more than one type of brake assist. Volvo offers the City Safety system, which automatically brakes in urban stop-and-go traffic [source: Volvo]. Mercedes-Benz came up with an additional feature known as Distronic Plus. Toyota has developed a system that combines a vehicle's brake assist system with navigation data, so that the brake assist engages during panicked stops at traffic signals [source: Toyota].

Active Brake Assist is a newer technological development that loads the brakes with hydraulic pressure milliseconds before an impending crash. This helps deliver more stopping power to the brakes even sooner. Bosch, the German parts supplier, calls its version Predictive Brake Assist. It's designed to communicate with the vehicle's Adaptive Cruise Control radar sensor to recognize situations that could develop into an accident. Beyond a certain triggering threshold, the system applies light brake pressure that the driver won't even notice as it prepares the vehicle to stop suddenly. Beyond another threshold of proximity, the system activates the full brake assist mechanism.

Why is all this necessary? Are drivers just that slow to react on the road? Bosch explains this innovation as necessary because, "even in critical situations, only about a third of drivers react appropriately and hit the brakes hard enough." As a result, the company adds, "the hydraulic brake-assist system is not triggered" [source: Robert Bosch GmBH].

Does brake assist actually make driving safer? Go to the next page to find out.


Collision Free Cars

Brake assist has proven so effective in reducing accidents that the European Commission (EC) plans to make the technology mandatory on all new vehicles sold in Europe. The EC has estimated the technology could save 1,100 pedestrian lives each year if all cars in Europe were equipped with this feature [source: eSafety Support]. Like stability control and anti-lock brakes, brake assist appears to be one of those technologies that was once reserved for higher-end cars, yet will eventually become a standard feature on all vehicles.

How much safer might U.S. roads be if brake assist technology became mandatory? According to the U.S. Insurance Institute for Highway Safety (IIHS), more than 400,000 crashes occur every year in which the driver reacted -- that is, he or she tried to stop or maneuver out of the way. These crashes result in about 3,000 deaths each year [source: United States Institute of Insurance Highway Safety]. There's no precise way to tell how many of those crashes could have been avoided if the driver stopped sooner, but it appears that brake assist can reduce accidents when it works in tandem with other cutting-edge safety technologies.


There's some evidence out there to support this as well: The IIHS "Future Vehicles" report studied five relatively new car safety features, including brake assist, forward collision warning, lane departure warning, blind spot detection and adaptive headlights. If all cars were equipped with all five of these features, they could potentially prevent 3.4 million crashes a year - - including 20,777 deaths -- according to the Institute's study [source:].

You don't have to be an automotive visionary to realize that with a little fine-tuning or perhaps some more processing power, these technologies collectively could eventually lead to cars that drive themselves. That could cut down drastically on collisions. That sounds great, but are people ready to surrender control of their vehicle to a computer in exchange for greater safety? In many cultures, cars and trucks are still associated with the freedom of personal mobility and a sense of personal control. What is fairly certain, though, is that technologies like brake assist, made possible by quick-thinking electronics, will continue to make driving a much safer proposition.

For more information on brake assist and other vehicle safety features, please see the links on the next page.


Lots More Information

Related HowStuffWorks Articles

  • "2007 Mercedes-Benz S-Class First Drive." (Accessed April 10, 2009)
  • Brooks, Glenn. "Germany: World first safety technology for Mercedes-Benz Travego coach at Hanover IAA." April 8, 2008. (April 7, 2009)
  • Consumer Reports. "Guide to safety features." Nov. 2007. (April 5, 2009)
  • Continental Teves Inc. "Brake Assist Systems." (April 8, 2009)
  • eSafety Support. "Commission proposes mandatory fitting of advanced vehicle safety systems." May 27, 2008. (April 12, 2009)
  • "IIHS Reports on Emerging Safety Technologies." (April 8, 2009)
  • Mercedes-Benz. "Mercedes-Benz Announces New Brake Assist System." March 17, 1997. (Accessed April 5, 2009)
  • Robert Bosch GmbH. "Predictive Brake Assist -- Prepared for braking, before you are." (Accessed April 13, 2009)
  • Tellem, Tori. "Top 10 High-Tech Car Safety Technologies." (April 4, 2009)
  • Toyota. "Toyota Advances Brake Assist with Navigation Link; Latest Technology Further Supports Driver." Friday, Feb. 8, 2008. (April 8, 2009)
  • United States Institute of Insurance Highway Safety. "EC Proposes Brake Assist Systems in New Cars to Reduce Fatalities." October 9, 2007. (April 10, 2007)
  • Volvo. "City Safety: Volvo introduces a unique system for avoiding collisions at low speeds." (April 10, 2009)