The Insurance Institute for Highway Safety recently ran a test on which cars have the best autonomous driving capabilities.

No, they didn't test robo-taxi services like Alphabet's (GOOGL) (GOOG) Waymo or General Motors' (GM) Cruise unit. Instead, the IIHS tested five luxury cars that many consider to have the top autonomous driving features.

They included the Tesla (TSLA) Model 3 and Model S, the Mercedes (DDAIF) E-Class, the BMW 5-Series and Volvo S90. However, there is an important distinction to note with the Tesla vehicles. The Model 3 is equipped with Autopilot version 8.1, while the Model S was running on version 7.1 and one camera.

Spoiler alert: The Model 3 did much better than the S. With Tesla's Autopilot version 9 expected to come out in the next several weeks, there's no telling what type of improvement these results will yield.

Autonomous Driving: Lane Assist

In any regard, the IIHS put the five vehicles through a number of tests, which include lane-keeping ability and emergency braking. For the lane-keeping test, each car was tested six times on six different tracks -- three hilly highways and three curvy highways. The IIHS wanted to put these cars through tough tests, seeing how they do when the lane markings disappear from sight. Touching or crossing the lane was considered a fail.

The Model 3 performed the best, passing on 35 of its 36 runs. Notably, the car touched its lane but did not lose the lane on its one strike. The Model S didn't fair as well, but was a mixed bag. The vehicle failed a whopping 12 out of 18 times on the hilly test, but passed on 17 of 18 runs on the curvy track (the Model 3 went 18/18 on the curve run).

Interestingly, the Mercedes E-Class did better on the hills vs. curves, scoring 83% and 50%, respectively. BMW put up a goose egg on the hills, failing on each run, while only passing on 3 fail-proof runs on the hill course. Further, the BMW 5-Series disengaged a whopping 16 times out of its 36 runs.

The Volvo S90 disengaged four times during its runs and only kept its lane half the time in each test.

In this regard, Tesla's Model 3 was a big-time winner compared to others and a sign that Tesla really is advancing rapidly in the autonomous driving game. 

Autonomous Driving: Braking

Adaptive cruise control allows vehicles to accelerate and slow down based on the vehicle in front of them. With only emergency braking turned on and with ACC turned off, not all cars performed the same.

There was good and bad news with this test for Tesla, starting with the latter. While driving at 31 mph toward a stationary object, both the Model S and Model 3 braked too late and hit the object. The E-Class, 5-Series and S90 all stopped in time.

With ACC turned on, though, neither Tesla hit the object and actually began braking before its Mercedes and BMW counterparts. All four cars decelerated gently, making for a comfortable stop, while the S90 hit the brakes forcefully with ACC engaged.

Other tests included following a lead driver who decelerates and another where the lead driver abruptly changes lanes and in doing so, reveals a stationary inflatable vehicle ahead of the test car.

In the first instance, all five vehicles "decelerated smoothly," according to IIHS. In the second scenario all but the S90 decelerated smoothly, with none of the vehicles hitting the object.

Autonomous Driving Conclusions

Following vehicles doesn't seem to be an issue, but encountering stopped vehicle can be. All of the vehicle except the Model 3 failed at some point in this regard when testing on public road with ACC engaged, although the Model 3 proved to be a bit overly cautious. Seven of its 12 braking incidents were for tree shadows. 

While we would generally want a cautious car over an aggressive one, unnecessary braking can cause accidents, along with driver frustration.

The conclusion?

"We're not ready to say yet which company has the safest implementation of Level 2 driver assistance, but it's important to note that none of these vehicles is capable of driving safely on its own," says David Zuby, IIHS chief research officer.

That's a strong stance on the matter and big reality check for the autonomous driving industry. 

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