Automatic emergency braking (AEB) uses radar, cameras or both to detect an impending collision with a vehicle — and increasingly with pedestrians and cyclists — and brakes automatically if the driver doesn’t react in time. It evolved from luxury-car systems of the mid-2000s, such as Mercedes-Benz’s Pre-Safe Brake, and became standard on most new U.S. cars through a voluntary industry commitment. A federal rule finalized in 2024 will require AEB, including pedestrian detection, on new light vehicles by 2029.

At a glance
Sensors Radar, cameras, sometimes lidar
Early systems Mid-2000s luxury cars
Voluntary adoption Most automakers made it standard by the early 2020s
Federal requirement Rule finalized in 2024, effective for new vehicles by 2029

How AEB works

  1. Sensors track objects ahead.
  2. If a collision appears likely, the car issues a forward collision warning.
  3. If the driver doesn’t respond, it pre-charges and then applies the brakes — partially, then fully.
  4. Some systems also steer to help avoid obstacles.

What it prevents

Studies have found AEB significantly reduces rear-end crashes. Pedestrian AEB, particularly at night, is a newer focus of testing and regulation.

Origins

Luxury cars introduced radar-based collision mitigation in the 2000s. Mercedes-Benz’s Pre-Safe Brake on the W221 S-Class is one example. Read the W221 S-Class and adaptive cruise control.

Limitations

AEB can be affected by weather, low light and unusual obstacles, and it doesn’t replace attentive driving. Chauffeurs keep generous following distances so emergency braking is rarely needed. Read how chauffeurs plan routes.

Types of AEB

  • Low-speed city AEB: designed for urban traffic, typically using cameras or short-range sensors.
  • High-speed AEB: uses radar to handle highway speeds.
  • Pedestrian and cyclist AEB: detects vulnerable road users, including at night.
  • Reverse AEB: brakes automatically when backing toward an obstacle or cross-traffic.
  • Junction AEB: detects vehicles when turning across traffic.

False alarms

Early systems sometimes braked for harmless objects, such as overhead signs or shadows (“phantom braking”). Newer sensor fusion — combining radar and cameras — has reduced false activations, but drivers should know how their car’s system behaves.