Car Advice

Autonomous Emergency Braking (AEB) Explained

Every so often a new safety technology comes along that has the potential to make a big difference to the road toll. Autonomous Emergency Braking (AEB) explained.

AUSTRALIA, AND VICTORIA, in particular, has become something of a pioneer for automotive safety technology. Victoria was the first state to mandate electronic stability control (ESC), the computer-controlled system that brakes individual wheels to keep the car pointing where you want it.

Next on the must-have list is AEB, or Autonomous Emergency Braking. Put simply, the car detects a potential collision ahead and if you don’t brake, it does. There’s a few sub-types of AEB – city systems focus on low speed crash avoidance, cruise systems work at higher speeds and because pedestrians are a special case, there are also pedestrian detection systems.

Cars with AEB sense potential collision objects using systems such as radar or stereo cameras. Once the object is detected the car determines the distance to the object and the car’s speed. As the car approaches the object it warns the driver of a potential impact with beeps and dash flashes, and it pre-charges the brakes, which means applying a little braking pressure; basically taking up the slack in the system. If the driver ignores the warning the system applies the brakes without driver intervention, hence the name of ‘autonomous emergency braking’. The driver can still steer, accelerate and do whatever else they want, taking instant control as you would in the case of cruise control for example. There is very little time between the warning and the start of automatic braking, perhaps only a second or two.

That is the theory, and in practice it works, but only some of the time, so let’s be clear on one point – you absolutely cannot rely on AEB to do your braking for you. Consider it an extra, just-in-case, backup to your own driving, but it’s valuable because that one time it works it could save a life, yours or someone else’s.

The reason you can’t fully trust AEB is because the technology isn’t fully mature and suffers from two big problems. Firstly, it doesn’t take enough factors in to account to determine stopping distances, and secondly, it isn’t all that great at recognising a problem.

AEB pretty assumes a best-case scenario – dry road, car in good condition, but there are so many factors that could change that ideal scenario. If the road is sloped, wet and slippery, or made of dirt, tyres are incorrectly inflated or worn, the car is heavily loaded or towing, or fitted with a space-saver spare then all these changes will increase the stopping distance and AEB won’t necessarily figure that out. I have tested a system that worked well enough on a bitumen road, but on dirt we hit the dummy target every time.

The second problem is danger recognition. AEB uses technology such as radar and lidar – sending out radio waves and lasers respectively and seeing what gets reflected back – or cameras. All three have pros and cons. Cameras can be fooled by driving into sunlight or at night, and generally anything other than a well-lit day is a problem to some degree or other. Radar and lidar have trouble recognizing objects, and all three find narrow, small, objects difficult to detect. My testing of such systems also showed that they need a bit of time to recognise a problem, so for example a kid dashing out in front of the car might not be recognised, but one standing still in the middle of the road for a while would be. Part of the problem is that the system has to be very sure there is a problem to avoid false alarms, so it is designed not to activate for situations that are marginal. AEB does not work in reverse either, and it may only work in a narrow band of speeds.

AEB is not the same as EBA, which is Emergency Brake Assist. EBA detects that you’ve made a panic stop by the speed and force with which you hit the brakes, and helps you apply maximum braking pressure. AEB does the braking for you, without you needing to touch any pedals.

Another technology related to, but not the same as AEB, is adaptive cruise control (ACC). This works just like normal cruise except that it uses technologies like lidar to work out how far away the car in front is, and then maintains a certain distance whether the other car speeds up, or slows down, but not exceeding the maximum speed you set. It is brilliant, and one of my favourite new-car technologies. Not all cars with ACC have AEB, but all cars with AEB should have ACC as the basics of the hardware are shared. There’s also related systems that detect what’s in front of the car when it’s parked, and prevent you driving off into the wall. Yes, that can and indeed does happen.

Like all such technologies, AEB cannot be retrofitted onto older cars, you either buy it new or nothing. This is because there’s a lot of integrated systems to design and calibrate and the costs to do that would be prohibitive on older models. But buying it can be confusing, because as usual manufacturers have invented different names for the same technology, just as they did with electronic stability control. Yes, there are minor differences between manufacturers but not as many as they’d have you believe as there’s only a few supplies of technologies like this, among them Bosch with its Australian test and development base. Names for AEB often sound like Pre-Crash, or Collision Mitigation, or Emergency Braking. Just not EBA.

So, bottom line, should you care? Yes. There are many, many safety technologies but only a few get special attention from the likes of the European safety body, Euro NCAP and its Aussie equivalent, ANCAP. Those bodies have focused just on curtain airbags and stability control, and now AEB. This is because the statistics and analysis indicate AEB could reduce accidents rates by 27%, as nose-to-tail crashes are the most common after reversing, and reversing accidents don’t often injure or kill.

There’s no disadvantage to AEB in day-to-day driving, and the chances of a false positive are small. Off-roaders will need to disable the system as they drive hills and through overhanging foliage, and sports car drivers doing track days would do likewise as it may be set off as you bunch up going into tight corners. In other words, for normal on-road driving it’s a Good Thing that should be left enabled. But remember, a car with AEB is not ‘self-driving‘ although it may feel like that sometimes. You’re still in control and accountable for the car’s actions.

Here’s what we say…

The other two major safety technologies, curtain airbags and stability control are very different to AEB. Airbags require very little driver knowledge as all you need to do is keep things out of the way of the bag in case it deploys. Stability control would probably never be noticed by many drivers, and when it does come on it may well scare them as it’d be clear the car was going out of control.

AEB is different, because it’s quite possible that drivers may begin to rely on it. But not to do the braking in normal situations – AEB is pretty vicious as it’s a full-on emergency stop, the warnings cannot be missed and it leaves braking right to the last nanosecond which is unsettling. Instead, drivers may succumb to the temptation of playing with smartphones as they drive through suburbia or crawl through city traffic, falsely safe in the knowledge that if anything did happen, the car will take car of it.

As an example, let’s say you’re in stop/start traffic and you’re using AEB with ACC to let the car do the work of speeding up, stopping, slowing down. This is working well for you because there’s a car immediately ahead, and the systems don’t have any trouble recognising such a big lump of metal. But then a cyclist or motorcyclist slots in front, and then the car fails to recognise the smaller, narrower object. If you aren’t paying attention, then BANG. Or perhaps the car in front pulls out, leaving you behind a truck that’s carrying a long set of steel pipes overhanging the tray. Again, you can’t rely on the systems to detect such objects, and that’d hurt you more than the truck. Or maybe there’s a 4WD in front, towing a long, low car trailer. Or you round a corner and head into a fuzzy sunset, confusing the sensors. There’s also no steering with AEB, so if you don’t pay attention you could be over a kerb.

So while I welcome AEB, as we must applaud any technology that has the capability to save lives, we must also be cautious that this particular technology has the very real potential to be abused and further serve to distract drivers. I would like to see the advertising reflect this danger so that we don’t swap one set of problems for another.


  • Alan

    My ACC loses the car in front in heavy rain combined with heavy traffic causing a lot of road spray. I’d assume that AEB wouldn’t “see” either. Under those circumstances, the ACC will accelerate, assuming there is no car in front – then as it gets closer and closer again sees it and backs off again – but I take manual control instead.

    My AEB has operated a few times, most of which I’ve been glad, several times when someone has cut into the gap which didn’t actually exist in front of me.

    But Semis and (worse) B-Doubles need to understand they need to sit back much further than they do at present – and stop telling my AEB off with his air-horns – sorry, mate, it wasn’t me that did it, and GET OFF MY TAIL next time.

  • Hi Wayne, what bad things happen in the wet? The ’email’ referenced in your NRMA link was an Australianised American chain email that circulates on the dangers of cruise control in the wet… I think it might be time for some myth busting. Thanks Isaac

    • Wayne

      disregard the “email”… take note of the last paragraph on the linked page. it should also be remembered that while ACC is very good, it is not perfect. for example: if you have your cruise set to 100kph in the wet and then hit a patch of stationary water, your driving wheels may loose some grip and compensate by speeding up… then in that possibly split second when full grip is obtained, maybe on a bend, or close to a following vehicle, you find yourself travelling a heck of a lot quicker… panic, hit the brakes and loose control. now, you might want to take the risk of using your ACC in the wet, however having had the experience of the above happening (driving a MY14 Subaru Liberty), I make sure my ACC is in the “off” position when driving in the wet. cheers

Robert Pepper

Robert Pepper

Robert Pepper is the editor of PM4x4, an offroad driver trainer and photographer interested in anything with wings, sails or wheels. He is the author of four books on offroading, and owns a modified Ford Ranger PX which he uses for offroad touring. His other car is a Toyota 86 which exists purely to drive in circles on racetracks. Visit his website: www.l2sfbc.com or follow him on Facebook https://www.facebook.com/RobertPepperJourno/