Forward Collision Warning and Braking

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  • As a red car approaches the rear of a black car that is stopped at an intersection, graphic shows that first, the driver of the red car is warned that there is a possible collision.  If the driver takes no action and gets closer to the stopped car, the system activates the brakes automatically.

Forward collision warning systems sense when the vehicle ahead is slowing or stopped and alert the driver of the risk of a possible crash. While most systems use radar, some use a laser, or camera. The system monitors the relative speed and following distance to the vehicle in front. When your vehicle gets too close to the vehicle in-front, a signal (audible and/or visual) alerts the driver.

Some systems offer collision warning with brake support. If the driver does not react after the collision warning has been given, the brake support function prepares the brake system to react quickly, and the brakes are applied slightly. A light jolt may be experienced.

In the event of an imminent crash and the driver has not applied the brakes, some of the newer systems apply strong braking automatically to help reduce the impact of the crash. Many systems will also activate the seat belt pre-tensioners, pre-charge the airbag systems and the brakes.

iso symbol for crash mitigation systemOther systems may stop the vehicle completely to avoid the crash.

Some names for these systems include Pre-safe Braking, Collision Warning with Auto-Brake (CWAB), Collision Mitigation Braking System (CMBS)


  • The system warning range differs by make, but generally above 40 km/h. The warning distance can usually be selected by the driver.
  •  Some systems are designed to operate in stop-and-go traffic and can prevent a crash only at speeds up to 30 km/h.

Things to keep in mind

  • These systems are designed to aid in the driving task. They are not intended to replace the driver’s attention and judgement. The driver is still responsible for maintaining a safe distance and speed even when the collision warning system is in use.
  • The automatic braking function will not prevent a collision. The driver must apply the brakes for full braking effect.
  • Forward Collision Warning systems do not take control of your vehicle or prevent you from driving.
  • They do not work in all driving situations, traffic, weather and road conditions.
  • For systems with cameras, adverse weather conditions such as heavy snow, dense fog, will affect the functioning of the system. Strong sunlight, reflections from the road surface, etc., may reduce the camera’s ability to detect another vehicle. Warnings for stationary or slow moving vehicles may not be provided in dark conditions.
  • The radar sensor may not be able to detect other vehicles in heavy rain or snow, or stationary vehicles.
  • The radar sensor may detect the wrong vehicle or lose a detected vehicle in curves.
  • Warnings may not appear if the distance to the vehicle ahead is very small or if there are large inputs to steering wheel and brake pedal.
  • The system may give delayed or no warnings for stationary or slow-moving vehicles, if your vehicle’s speed is above, for example, 70 km/h.
  • Drivers are encouraged to read the owner’s manual to become familiar with the system and the conditions when system operation is different or is not guaranteed.
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