Picture this: you are driving at 35 km/h in town, you turn your head for a second to look at your passenger, and suddenly your car slams the brakes by itself. No pedestrian in front of you, just a metal manhole cover misread by the radar. Welcome to the world of AEB, the system that probably saves more lives than any other ADAS but also demands to be understood. According to the French Road Safety agency, AEB cuts low-speed rear-end collisions by 38 %.
What is AEB and how does it work?
Three sensors to see the threat
AEB (Autonomous Emergency Braking) relies on a long-range front radar, a wide-angle camera behind the windscreen and sometimes a lidar on premium models. The computer fuses these inputs 30 times per second to estimate the distance, relative speed and trajectory of every object ahead. If the collision risk crosses a threshold, the system warns then brakes.
Three steps: warning, pre-fill, full braking
The intervention is rarely a sudden full stop. Step 1: visual and audible alert. Step 2: brake circuit pre-fill, AEB pushes the pads against the discs for instant response. Step 3: maximum braking if the driver has not reacted, up to 1 g on some cars. The seatbelt may pre-tension, the windows roll up, the sunroof close.
When is AEB active and at what speed?
The classic operating range
On most mainstream cars, AEB is effective between 10 and 60 km/h for pedestrian and cyclist detection, and between 10 and 180 km/h for vehicle detection. The EU regulation mandates at least low-speed pedestrian and vehicle detection, but carmakers often go further. At night, performance drops when ambient lighting is poor.
Reverse and parking
Many models also bundle a rear AEB (RCTA - Rear Cross Traffic Assist) that brakes if a vehicle or pedestrian appears while you back up. Very handy when leaving a perpendicular parking slot. Note: this system does not exempt you from looking in the rear-view mirror, let alone turning your head.
Common AEB traps
False positives in town
Reflective manhole cover, ground-painted roadworks sign, badly mapped speed bump, car parked half on the road: situations where AEB may brake by mistake. If another car follows you at short distance, this unjustified brake can cause a rear collision whose legal responsibility remains blurry. Insurers handle these cases one by one.
False negatives on the motorway
Conversely, AEB can miss a stationary object in its own lane, especially at high speed. The front radar tends to filter out stationary returns to avoid confusing a parked car with a road sign. That is why several carmakers state in the manual that above 100 km/h, AEB is in no way a replacement for an attentive driver.
Temporarily disabling: useful or risky?
You can legally disable AEB through the menu, for example on a road under construction with messy markings. The EU regulation forces the system to reactivate at the next start, though. Bear in mind that disabling AEB and having a crash can be held against you by your insurer if the cause is obvious inattention.
AEB and the driving test: what to know
The candidate remains in command
AEB is not a substitute for anticipating brakes. If you systematically let the car brake for you, the examiner may judge that you do not master the safety distance. The Highway Code does not mention AEB: the core rule is still being able to stop within the distance you can see, with no aid at all. The Vienna Convention sets the same principle at international level.
If AEB triggers during the test
It is not eliminatory on its own. The examiner reads the context: if you reacted before AEB, that is a plus. If AEB braked because you missed the crossing pedestrian, it is a serious fault, possibly eliminatory depending on actual risk. Since the autonomous vehicle is not yet on public roads at large, the human stays fully responsible.
DevisPermis expert opinion
AEB is probably the most useful ADAS invented in the last 20 years, but also the most dangerous to misunderstand. Our advice: spend an hour in an empty car park to test its trigger threshold at 30 km/h (with your instructor present), so you know exactly when it takes over. Knowing the limit of the aid is knowing where your real responsibility as a driver starts. Many learners discover their car's AEB only during an actual emergency, which is far too late to react calmly.
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Frequently asked
Your questions on this topic
How does autonomous emergency braking (AEB) work?
Autonomous emergency braking (AEB) uses front radar and camera identifying obstacles, vehicles and pedestrians. The system first alerts (audio and visual at 1.5 seconds to collision), pre-tensions brakes (1 second), then triggers full braking (0.5 seconds). Operational between 5 and 250 km/h depending on models. AEB saves 38 percent of rear collisions per Euro NCAP.
Can AEB trigger braking by mistake?
Yes, AEB can trigger a false positive in less than 0.3 percent of cases per Euro NCAP, often by detection of bridge shadows, low signs or reflective objects. Risk increases in heavy rain or difficult lighting. In case of unwarranted braking without obstacle, report to dealer: camera recalibration (200 to 400 euros) or OTA update resolves most cases.
Should you disable AEB in mountains or snow?
Do not disable AEB in mountains or snow: the system stays active and reduces accident risk. However, on slippery ground, AEB braking distance may surprise. Adapt speed 30 to 40 percent below allowed limit. If sliding detected, ABS and ESP take over. Manual disabling stays possible for extreme off-road, but resets at startup.
Is AEB effective on pedestrians and cyclists?
Pedestrian/cyclist AEB has been mandatory since July 2022 (5-star Euro NCAP). It detects from 5 to 60 km/h, avoids collision under 40 km/h in 85 percent of cases per Thatcham Research. Night detection (infrared LED) stays limited: 35 percent efficiency reduction vs daytime. Lateral cyclists are less well detected than those crossing trajectory.
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