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Electronic safety systems in today’s vehicles don’t always measure up to claims made for them, says a report published Tuesday by the Insurance Institute for Highway Safety (IIHS), a nonprofit supported by auto insurers.

“We zeroed in on situations our staff have identified as areas of concern during test drives with Level 2 systems, then used that feedback to develop road and track scenarios to compare vehicles,” IIHS senior engineer Jessica Jermakian said, according to the report.

Level 2 autonomy employs advanced driver assistance systems (ADAS), which as the phrase implies are meant to help drivers, not replace them. A Level 2 car has the two functions of lane keeping and adaptive cruise control, a feature that maintains a safe distance from the car in front.

The Insurance Institute isn’t yet at the point of ranking Level 2 systems for safety, although it does say that all of the five cars it has just tested have excellent emergency braking systems. That function is the sole criterion for Level 1 autonomy.

The tested cars were the 2017 BMW 5-series with Driving Assistant Plus; the 2017 Mercedes-Benz E-Class with Drive Pilot; the 2016 Model S and the 2018 Tesla Model 3 with Autopilot (using different software versions); and the 2018 Volvo S90 with Pilot Assist.

When starting at a speed of 50 kilometers per hour (31 mph) and with the adaptive cruise control turned off, the two Teslas were not able to brake in time to completely avoid hitting an obstacle. The other cars were able to stop well short of the obstacle. With the adaptive cruise control turned on, all the cars stopped in time.

When set to follow a lead vehicle that first slowed and then stopped, all five cars were able to keep their distance and to stop in time. When the lead vehicle moved out of the way to reveal a stopped vehicle in the lane, all five cars were able to avoid hitting it.

So far, so good. But these tests were conducted on the track; in traffic, things weren’t always so smooth. Engineers found that all of the cars except the Tesla 3 would sometimes be baffled by a stopped car. And even the Tesla 3 had a problem: excessively cautious braking.

“In 180 miles, the car unexpectedly slowed down 12 times, seven of which coincided with tree shadows on the road,” the report recounts. “The others were for oncoming vehicles in another lane or vehicles crossing the road far ahead.”

Such braking isn’t dangerous, the researchers note, but it could be annoying enough to induce drivers to turn off the adaptive cruise control. They’d then give up the safety edge that the function normally provides.

As for the other Level 2 function—keeping to the right lane—all the cars had a little trouble. In one set of tests, only the Tesla 3 consistently stayed within its lane; other models sometimes oversteered enough to require that the driver intervene. But in another set of tests, on hilly roads, even the Tesla 3 sometimes required the driver to intervene.

The Insurance Institute says that though lane keeping is a somewhat less potent safety function than adaptive cruise control, it could—if properly implemented—prevent 8,000 deaths a year.