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We hear the words "autonomous emergency braking" being thrown about quite often. The general assumption about this kind of system is that it, well, autonomously brakes the vehicle in an emergency. But how much braking does it actually do, and does the public trust it? Enter the American Automobile Association's latest study.

AAA ran more than 70 trials of five different autonomous emergency braking (AEB) systems, using a "soft car" target that doesn't damage the vehicle being tested. The results showed that not every system is equal, and that the public generally doesn't understand that there are different tiers of AEB functionality.

The "lite" version of AEB doesn't claim to stop the car fully -- rather, it will apply the brakes for the purpose of limiting crash severity. With this type of AEB, AAA found that it reduced speed by an average of 40 percent, and about 33 percent of test scenarios with speed differentials under 30 mph ended up with the AEB-lite vehicle avoiding the crash entirely. Move that speed differential up to 45 mph, and these systems avoided crashes just 9 percent of the time.

When it comes to full-on AEB systems -- the kind that promise to actually prevent accidents -- AAA found an average speed reduction of 79 percent. With the 30 mph speed differential, the systems averaged 60 percent crash avoidance. Push the delta up to 45 mph, and crash avoidance dropped to 40 percent. No automakers were named in the study's release.

Alongside its testing, AAA also polled US drivers about these systems. About 40 percent of those polled want AEB on their next vehicle, with men more likely to opt for the system than women. Only 41 percent of drivers without AEB would trust the system to work, compared to 71 percent of drivers that have an AEB-equipped car.

That distrust is likely to fade soon, as 22 different automakers have agreed to make AEB standard equipment by 2022. That covers about 99 percent of vehicles on sale today, and it could very well prevent the thousands of fatalities the US suffers each year from rear-end collisions, the exact kind of crash AEB attempts to avoid. In the meantime, if you are looking for a car with AEB, make sure you understand the system fully, as it may not provide the full protection you're after.