Autonomous Emergency Braking (AEB)

Autonomous Emergency Braking (AEB) is an active safety feature designed to automatically apply a vehicle's brakes to avoid or mitigate an impact when a potential collision risk ahead is detected and the driver fails to respond in time. Operating typically in tandem with Forward Collision Warning (FCW), it serves as a core component of Advanced Driver Assistance Systems (ADAS) and is widely regarded as the "last line of defence" prior to a crash.

How It Works

The operation of an AEB system follows the standard "Perception - Decision - Execution" logic.

Perception Phase: The system relies on front-mounted sensors, such as millimetre-wave radars, cameras, or LiDAR, to scan the road ahead in real time, precisely measuring relative distances and speeds between the host vehicle and preceding traffic, pedestrians, or other obstacles. Mainstream setups often adopt a "radar + camera" fusion approach to strike an optimal balance between range-measuring precision and object recognition.

Decision Phase: The Electronic Control Unit (ECU) continuously calculates the Time to Collision (TTC) with detected obstacles based on real-time sensor data. When the TTC drops below a preset safety threshold, the system flags a collision risk. Many carmakers further divide the AEB function into several execution stages. Taking Bosch's system as an example, it typically encompasses four progressive stages: alert, pre-braking, partial braking, and full braking. For instance, the system will first issue visual and audible warnings whilst pre-filling the brake hydraulics; if the driver remains unresponsive, it applies a brief "brake jerk" at roughly 30% braking force to jolt the driver into alertness; once an impact becomes inevitable, the system commands maximum braking pressure and may simultaneously trigger pretensioners for the seatbelts.

Execution Phase: Once full emergency braking is commanded, the system employs brake-by-wire technology to build up maximum braking pressure and cut drive power in an instant (approximately 0.2 seconds), rapidly decelerating or bringing the vehicle to a complete standstill.

Distinction from Other Systems: AEB and the Electronic Stability Programme (ESP) serve distinct purposes. While ESP primarily prevents skidding and loss of vehicle control, AEB focuses on actively preventing or mitigating frontal impacts. Both systems work in synergy; for example, ESP steps in to maintain vehicle stability during full AEB intervention.

Adoption Status and Regulatory Trends

AEB is globally recognised as a critical technology capable of drastically cutting road accident rates. According to Euro NCAP data, vehicles fitted with AEB can reduce pedestrian fatalities by approximately 50% and rear-end collisions by 45%.

At present, key markets such as the European Union, the United States, and Japan have already implemented or drafted mandates requiring AEB as standard equipment. In China, the Ministry of Industry and Information Technology (MIIT) released the mandatory national standard "Performance Requirements and Test Methods for Advanced Emergency Braking System of Light-duty Vehicles" (GB 39901—2025) in January 2026, which is slated to take effect on 1 January 2028. From that date onward, all locally produced Category M1 (passenger vehicles) and Category N1 (light commercial vehicles) with a gross vehicle weight not exceeding 3.5 tonnes must come factory-fitted with compliant AEB systems as standard.

Although the AEB fitment rate for passenger vehicles in China has surpassed 60% as of 2025, the new national standard signifies that AEB will transition from being a "marketing selling point" reserved for premium or high-spec variants to a mandatory standard safety specification across the board, driving broader democratization of active safety tech. The updated standard also stipulates that AEB systems must effectively detect crossing targets at speeds between 20 and 60 km/h, backed by newly added simulation tests to validate reliability under complex driving scenarios.

Operating Limits and Limitations

Despite its vital safety role, AEB is not infallible. The system's performance is inherently tied to sensor detection capabilities, software algorithms, and target classification support. For instance, entry-level variants may feature basic AEB setups that only detect vehicles, lacking support for pedestrians or cyclists. Operational speed thresholds also vary significantly depending on sensor hardware configurations. Furthermore, adverse weather conditions such as heavy rain, snow, or thick fog can degrade sensor performance, potentially leading to missed detections or false interventions. As such, AEB remains a driver-assistive safety feature; motorists must remain attentive at all times and retain ultimate responsibility for safe vehicle operation.