Over the past few years, driver assistance features such as automatic emergency braking and lane keep assist have become increasingly more common. But how effective are they? According to a recent study conducted by the Insurance Institute for Highway Safety (IIHS), these features still aren't substitutes for actual human drivers, and the results varied significantly from brand to brand.

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On a controlled test track, the researchers put the cars through four tests to evaluate their adaptive cruise control systems. In the first scenario, each vehicle was driven at 31 mph toward a stationary target to test automatic emergency braking performance with adaptive cruise control off. Only the Teslas failed to stop in time. When the same test was performed with adaptive cruise control on, all cars were able to avoid the target. Researchers noted, however, that the Volvo S90 braked later and more abruptly than the other cars. In the third test, the cars had to follow a vehicle that slowed to a stop, something all five test vehicles were able to do smoothly. When the test vehicles followed a car that changed lanes to reveal a stopped vehicle, the Volvo again slowed down more abruptly than the other four vehicles.

On the road, the safety systems showed more inconsistencies. All cars except the Model 3 failed to respond to stopped vehicles ahead of them, and the Mercedes-Benz E-Class only detected a stopped truck at a traffic light for a short time before losing sight of it. "At IIHS we are coached to intervene without warning, but other drivers might not be as vigilant," said Jermakian, in a release. "ACC systems require drivers to pay attention to what the vehicle is doing at all times and be ready to brake manually." IIHS researchers noted that the Model 3 had a tendency to brake unnecessarily and counted 12 different times where it did so for oncoming traffic and vehicles changing lanes. Jermakian noted that unnecessary braking could pose a safety risk in heavy traffic especially if it's more forceful. Due to the systems getting confused easily, the IIHS study concluded that adaptive cruise control systems require drivers to stay attentive behind the wheel.

The second active safety feature that IIHS looked at was lane keeping assist. Using real roads, the team put the cars through traffic, hills, and turns to evaluate each system's performance. All five cars offer steering assistance on roads with clear markings and can use the vehicle ahead as a guide at low speeds or when lane markings are blocked. Between the five vehicles, only the Model 3 stayed in its lane during all tests while the Model S overcorrected during one run, which caused it to cross the inside line of the turn. The Mercedes-Benz E-Class stayed in its lane nine out of 17 times but disengaged during one run and crossed the inside line twice. The Volvo S90 crossed the inside lane eight times while the BMW 5 Series only stayed in its lane three of the 16 runs and was the most likely to disengage instead of steer outside the lane.

The Model S, S90, and 5 Series all came up short when hills were added to the equation. The 5 Series had a tendency to steer toward or across the lane line and even disengaged steering assistance in certain conditions. As a result, it failed to stay in its lane in all 14 of the hill trials. Tesla's Model S was only able to stay in its lane in five of the 18 trials and had a tendency to swerve left and right when going uphill until it found the correct place in its lane and rarely warned the driver to take over as it was struggling to center itself. The Volvo S90 stayed in its lane for nine of the 16 trials, with the car crossing the lane line twice and disengaging steering assistance four times when it was cresting hills. The system automatically turned back on once lane markings were detected, however.

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But not all test vehicles were as confused by hills. The Mercedes-Benz E-Class stayed in its lane 15 out of 18 times and provided steering assistance without many erratic moves when the car wasn't able to see lane lines. Meanwhile, the Tesla Model 3 only drifted out of its lane once when it hugged the lane line. IIHS researchers noted that some of the vehicles had a tendency to follow slow-moving vehicles into the exit lane. This is due to the lane keeping assist system using the vehicle in front as a guide instead of the lane lines at lower speeds. The IIHS sees a big potential for lane keeping assist systems to save lives but the evidence isn't as pronounced as it is with adaptive cruise control and forward collision avoidance technologies. Lane keeping assist system have the potential to save 8,000 lives per year, according to the IIHS, while lane departure warning systems have been credited with an 11-percent drop in sideswipe and head-on collisions, as well as a 21-percent reduction in injury rates for those accidents.