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According to the IIHS, automatic emergency braking technology may prevent half of all front-to-rear car crashes. There doesn’t seem to be a downside, so the U.S. just announced automakers need to include it in every car within five years. Most automakers have already pledged to make the safety software a standard feature, instead of charging extra for it. But recent tests prove it is far from all-powerful.

For years, the IIHS only tested AEB on new vehicles at 12 and 25 mph. Its test was parking a car in the road, then driving toward it. But this year, the Institute made the test tougher. It increased the speeds to 31, 37, and 43 mph. And it added tests with a motorcycle and semi truck trailer parked in the road.

The first segment tested was small SUVs. Based on how early they warned the driver, and if they were able to stop in time, the IIHS only gave the Subaru Forester a “Good” score. Here’s how every vehicle stacked up.

Make / ModelCrash Prevention Rating
2023-24 Subaru ForesterGood
2023-24 Honda CR-VAcceptable
2023-24 Toyota RAV4Acceptable
2023-24 Ford EscapeMarginal
2023-24 Hyundai TucsonMarginal
2023-24 Jeep CompassMarginal
2023-24 Chevrolet EquinoxPoor
2023-24 Mazda CX-5Poor
2023-24 Mitsubishi OutlanderPoor
2023-24 Volkswagen TaosPoor

The truth is that even the Subaru hit the motorcycle when traveling faster than 37 mph. The government’s 2029 target of 90 mph AEB may be a long way off.

Even the best AEB is most effective at low speeds. And it is best with car-shaped objects. Because it is tuned to not stop when climbing a big hill, larger vehicles can trip it up. And if it can’t see a motorcyclist at high speeds, imagine how invisible a pedestrian is. These systems are a good driver aid, but don’t think for a second that they will prevent every crash.

Wreckage of a motorcycle smashed into a silver Mercedes sedan, a police officer in the foreground
Motorcycle crash | Gary Coronado / Los Angeles Times via Getty Images

Even as AEB becomes more common, the number of pedestrian deaths continues to set records in many places. Recent studies found that in 90% of crashes, both participants were in some way distracted–whether by a phone or new tech such as big in-car infotainment screens.

AEB becomes dangerous if drivers begin to use it as an excuse to take their eyes off the road. In its investigation of Tesla’s driver aid software, the NHTSA found that in many fatal crashes, obstacles were visible for 10 seconds–or longer. This is plenty of time for a human driver to avoid them. But the drivers were overly reliant on the vehicle software, with deadly results.

Read more potential downsides to self-driving cars, or see how the IIHS has administered its latest AEB tests in the video below: