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Tests by the American Automobile Association (AAA) reveal automatic emergency braking usually doesn't prevent a collision under certain circumstances.

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A common safety feature designed to stop drivers from backing into an approaching vehicle or a pedestrian doesn’t work as intended much of the time, new research shows.

In tests by the American Automobile Association (AAA), rear automatic emergency braking (AEB) with cross-traffic detection failed to prevent a crash between a reversing SUV and an approaching vehicle more than 97 per cent of the time.

Half of the time, the systems didn’t prevent an SUV from hitting a simulated child standing still behind the vehicle.

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“These systems are not magic,” said Austin Shivers, the AAA technical engineer who led the study. “You shouldn’t rely on them completely and you should understand how they work.”

AEB uses sensors to detect objects in the path of your car and, if you don’t stop after a warning, it applies the brakes automatically.

Some newer systems also have rear cross-traffic detection – so, when you’re backing out into traffic, they’re supposed to detect cars that are approaching from either side. But to detect a vehicle, the sensors need to see it.

In the first test, the team looked at how well AEB with cross-traffic detection would sense rear traffic when the vehicle was parked next to a larger vehicle that blocked the sensors’ view of oncoming traffic.

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They tested four mid-sized 2023 SUVs: a Hyundai Tucson Hybrid, a Lexus RX 350, a Mazda CX-30 and a Volkswagen Tiguan.

As the SUVs backed out of their spot, an inflatable test car approached from the side at 15 miles an hour (24 kilometres an hour).

The AEB systems detected the car and automatically applied brakes in 65 per cent of test runs, but it only prevented a crash in one out of 40 test runs, Shivers said. “The results for the cross-traffic scenarios weren’t great.”

The four SUVs all fared similarly, although some slowed more than others, he said.

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“The study really isn’t intended to focus on individual vehicles or compare performance – the intention is to look at where these systems are at,” Shivers said. It was a deliberately difficult test – but it’s a situation that most drivers will face.

“[The sensors] don’t have line of sight until you back up far enough so the back of your vehicle has backed out past the vehicle parked next to you,” Shivers said.

So, drivers need to realize that their rear AEB sensors are located at the rear of their vehicle – and they can’t see through parked cars or other obstacles.

“You have to help them help you. You need to back up a little, pause, and then give the sensors a chance to see oncoming traffic,” he said. “That’s not different than how you should be backing up when your car doesn’t have rear AEB.”

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Could the systems do a better job at seeing oncoming cars?

“These are new systems so, inevitably, there will be refinement in the future,” Shivers said. “Our message isn’t so much for the people designing these systems – we’re asking for test standards for the industry.”

Braking bad?

In the second test, the SUVs backed up toward a stationary inflatable target, shaped like a child, that was eight feet (2.4 metres) behind them.

“We expected this to be a fairly easy scenario [for the AEB technology] because it’s designed to detect objects directly behind the vehicle,” Shivers said. “That technology has been around for a decent amount of time now ... this is the one type of situation where you’d expect it to work.”

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But the systems only applied brakes 75 per cent of the time – and they hit the pedestrian target half of the time. It’s not clear why the sensors didn’t see the target, Shivers said.

“I think as far as consumers are concerned, these results are evidence that you still have to drive cautiously and pay attention,” he said.

AEB has been shown to reduce backup crashes. According to the U.S.-based Insurance Institute for Highway Safety (IIHS), cars with rear AEB, rearview cameras and parking sensors had 78-per-cent fewer backup crashes than cars without those technologies.

“So three out of every four of those crashes are going away,” said David Aylor, IIHS vice-president of active safety testing. “They’re the most common crashes. They’re not severe, but they happen all the time [for example] backing out at the grocery store and you back into a car behind you.”

Even if nobody is injured in these minor fender benders, they can get expensive, Aylor said.

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“Even at four or five miles (six to eight kilometres) an hour, you can end up with $5,000 worth of damage,” he said.

Still, you shouldn’t rely on these safety systems to keep you safe – they’re meant as a backup, Aylor said.

“The manufacturers really designed [AEB systems] to have the driver respond and that’s why you get a warning first,” he said. “Obviously. anytime you’re backing up, you need to be aware of your surroundings.”