How Pre-Collision Systems Work

Types of Pre-collision Systems
Pre-collision systems offer a variety of indicators, including alarms and pre-crash braking.
Pre-collision systems offer a variety of indicators, including alarms and pre-crash braking.
AP Photo/Katsumi Kasahara

Some systems sound an alarm to notify drivers that a collision may be imminent -- a sound simply to alert the driver and get him or her ready to take evasive action. Other systems actually take control of certain aspects of the car. There are pre-crash brake systems, which apply additional pressure to the car's brakes to assist the driver in slowing the car down as fast as possible and potentially reducing damage caused by an accident. Some systems also connect the PCS unit to a pre-crash seat belt system, which can automatically tense passengers' seat belts before a crash. These are often referred to as seatbelt pretensioners. The development of these types of systems needs to be fine-tuned and highly accurate, since any malfunction could disrupt a drivers' attention and potentially cause an accident. Obviously, designers and manufacturers put pre-collision systems through rigorous testing to ensure that this doesn't happen.

One of the earliest uses of accident detection was the Mercedes-Benz Pre-Safe system in the 2003 S-class sedan, which the company touted as the "world's first production car equipped with an astonishing new system that can sense a possible collision a few seconds in advance and take pre-crash protective measures." The system used sensors to measure the car's steering angle and acceleration, but not its surrounding environment -- actions such as pre-tensing of the seat belts, automatic sunroof closing and raising of reclined seats were triggered during any emergency maneuver [source: Mercedes-Benz Canada].

More recent accident detection technologies use radar systems, like Toyota's Pre-Collision System. The company introduced its PCS in 2003 on a vehicle sold in Japan called the Harrier. In 2010, the system will be available on the Toyota Prius. The system uses millimeter-wave radar to determine when additional braking assistance is required as well as when to apply tension to the seat belts. Toyota has also added pre-crash seatback preparation for rear seat occupants. If a crash situation is imminent, reclined rear seats are automatically brought to an upright position.

Ford has also announced its own radar system, called "Collision Warning with Brake Support," for its latest models of the Ford Taurus, the Lincoln MKS sedan and the Lincoln MKT crossover. And Honda and Nissan also offer lane-deviation prevention and front collision avoidance systems on many of their domestic models.

For more information about pre-collision systems, hybrid cars and other related topics, follow the links the next page.

Related HowStuffWorks Articles


  • Automotive Industries. "Safety matters: advanced crash avoidance technology finds its way into production vehicles in Japan." Aug. 2004. (April 13, 2009)
  • DENSO Corporation. "Pre-crash Safety System." Oct. 22, 2003. (April 6, 2009)
  • "Ford's latest safety breakthrough - Collision Warning with Brake Support - Coming in 2009." April 6, 2009. (April 6, 2009)
  • Lemmen, Paul et al. "Development of a Pre-Crash System Using the VEHIL Test Facility." National Highway Traffic Safety Administration. March 8, 2005. (April 6, 2009)
  • Mercedes-Benz Canada. "Mercedes-Benz launches first-ever car with 'reflexes.'" Oct. 15, 2002. (April 6, 2009)
  • Merkelbach, Bettina. "Toyota Adds Front-Side Pre-Crash System and Seatbacks to Safety Technologies." ATZ online. March 2, 2009. (April 6, 2009)