What it does
Most systems work in stages. First comes a forward collision warning: a chime, a flashing symbol or a red bar in the instrument cluster. If you do not respond, many cars pre-charge the brakes so the pads are ready to bite. If the gap keeps closing, the car brakes by itself, sometimes in two steps, a short tug and then full force.
When you do press the pedal but not hard enough, a related feature called brake assist can add pressure for you.
How it senses
A camera behind the windshield, near the rear-view mirror, reads the shapes of vehicles and, on many cars, people and bicycles. A radar unit behind the grille or front badge measures distance and closing speed. Some vehicles use both and compare them; some use only one. A few add lidar.
What sets it off
The system compares your speed with the speed of whatever is ahead and estimates the time left before contact. When that time drops below a threshold set by the maker, it warns; when it drops further, it acts. Braking hard yourself, steering away or pressing the accelerator firmly usually tells the system you are in control, and it backs off.
Where it stops
- A dirty, iced or snow-covered windshield or grille can blind it. Many cars show a message when that happens; some do not.
- Heavy rain, fog, spray and low sun straight ahead reduce what the camera can see.
- A vehicle cutting in close, a car stopped just past a bend, or an object low to the ground may be picked up late or not at all.
- Some systems only work within a speed range, and at higher speeds they may slow the car rather than stop it.
- Detection of pedestrians, cyclists and animals varies widely between models, and is weaker at night on many of them.
Check for your car
- Whether your car brakes for people and cyclists, or only for vehicles.
- The speed range it works in.
- Where the sensors sit, so you know what to keep clean.
- Whether it needs recalibrating after a windshield replacement or a front-end repair. Most camera systems do.
- How to change the warning distance, and whether switching the system off lasts until the next drive or only until restart.
This guide describes how the system generally works. Makers tune and name their systems differently, so your owner’s manual is the final word for your vehicle.
Next guideLane Departure and Lane Keeping
