Under a new test program by the Insurance Institute for Highway Safety (IIHS), seven vehicle models equipped with optional autobrake and forward collision warning systems drew the highest rating of “superior” for their front crash prevention equipment. These models include the Cadillac ATS sedan and SRX SUV, Mercedes-Benz C-Class sedan, Subaru Legacy sedan and Outback wagon, and Volvo S60 sedan and XC60 SUV.

Moderately priced and luxury midsize cars and SUVs were the first to be evaluated in the new IIHS test program. IIHS based its rating system on research by the Highway Loss Data Institute (HLDI) indicating that forward collision warning and automatic braking systems help drivers avoid front-to-rear crashes.

The institute rated models with optional or standard front crash prevention systems as either “superior,” “advanced” or “basic,” depending on whether they offered autonomous braking, or autobrake, and, if so, how effective it was in tests at 12 and 25 mph. Vehicles rated “superior” had autobrake and could avoid a crash or substantially reduce speeds in both tests. Vehicles rated as “advanced” had to have autobrake and avoid a crash or reduce speeds by at least 5 mph in one of two tests. To qualify for a “basic” rating, a vehicle had to have a forward collision warning system that met National Highway Traffic Safety Administration performance criteria.

For a NHTSA endorsement, a system must issue a warning before a specified time in five of seven test trials under three scenarios. The agency identifies vehicles with compliant systems as part of its online ratings.

Six models earned an “advanced” rating when equipped with autobrake and forward collision warning. These included the 2014 Acura MDX SUV, Audi A4 sedan and Q5 SUV, 2014 Jeep Grand Cherokee SUV, Lexus ES sedan and the 2014 Mazda 6 sedan.

In addition, the Volvo S60 and XC60 earned an advanced rating when they weren’t equipped with an option called Collision Warning with Full Auto Brake and Pedestrian Detection. The S60 and XC60 were the only models in the new test program with standard autobrake. Called City Safety, the system brakes to avoid a front-to-rear crash in certain low-speed conditions without warning the driver before it takes action.

Twenty-five other vehicles earned a “basic” rating. Three models available with forward collision warning earned higher ratings when equipped with autobrake. They were the 2014 Acura MDX and the Cadillac ATS and SRX. Thirty-six models either didn’t offer a front crash prevention system, or they had a system that didn’t meet NHTSA or IIHS criteria.

"Front crash prevention systems can add a thousand dollars or more to the cost of a new car. Our new ratings let consumers know which systems offer the most promise for the extra expense," said David Zuby, IIHS chief research officer.

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Front crash prevention is part of a larger group of crash avoidance features spreading through the U.S. vehicle fleet. Marketed under various trade names, system capabilities vary by manufacturer and model. Most are offered as optional add-ons. In general, current front crash prevention systems fall into two categories: forward collision warning and front crash mitigation or prevention with autobrake.

Forward collision warning alerts a driver when the system detects that the vehicle is about to crash into the vehicle in front, but the system doesn't slow down or stop the vehicle. Some forward collision warning systems are combined with an autobrake system to reduce vehicle speeds in a crash, but they aren't designed to avoid the collision. Acura's Collision Mitigation Brake System is an example.

Other autobrake systems can slow down or completely stop the car to avoid some front-to-rear crashes if its driver doesn't brake or steer out of the way in response to a warning. Like the Acura system, these will reduce the speed of those crashes they can't prevent. Cadillac's Automatic Collision Preparation and Volvo's Collision Warning with Full Auto Brake and Pedestrian Detection combined with City Safety are examples.

Another design difference involves whether the vehicle ahead is stopped or moving. All of the front crash prevention systems that earned a “superior” or “advanced” rating from IIHS were capable of braking for a stopped or slower-moving vehicle. Some other systems are designed to brake for a stopped car ahead only if sensors first detect the car moving before it stops. The 2013 BMW 3 series sedan is available with this type of system. It drew a “basic” rating for front crash prevention.

"The point of autobrake systems is to help inattentive drivers avoid rear-ending another car," Zuby explained. "It's clear that the ability to automatically brake for both stopped and moving vehicles prevents the most crashes."

To gauge how autobrake systems from different manufacturers perform, IIHS conducted a series of five test runs at speeds of 12 and 25 mph on the track at the Vehicle Research Center in Ruckersville, Va. In each test, an engineer drove the vehicle toward a stationary target designed to simulate the back of a car. Sensors in the test vehicle monitored its lane position, speed, time to collision, braking and other data. The IIHS protocol is similar to the procedure the European New Car Assessment Programme uses to evaluate autobrake systems, which the group plans to begin rating in 2014.

IIHS awards as many as five points in the autobrake tests, based on how much the systems slow the vehicle to avoid hitting the inflatable target or lessen the severity of the impact. In the case of an unavoidable collision, lowering the striking vehicle's speed reduces the crash energy that vehicle structures and restraint systems have to manage. That reduces the amount of damage to both the striking and struck car and minimizes injuries to people traveling in them.

"We decided on 25 mph because development testing indicated that results at this speed were indicative of results at higher speeds — and because higher-speed tests would risk damaging the test vehicles," Zuby explained. "As such, we expect crash mitigation benefits at higher speeds as well."

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In addition to points in the autobrake tests, vehicles earn one point if they have a forward collision warning system that meets NHTSA criteria. That means vehicles can earn a maximum of six points total for front crash prevention. Models with one point earn a “basic” rating. A total of 2 to 4 points qualifies vehicles for an “advanced” rating, and 5 to 6 points qualifies vehicles for a “superior” rating.

The highest-scoring cars and SUVs have autobrake and substantially reduce speeds in both the 12 and 25 mph tests. Most of these systems prevent the 12 mph collision.

Subaru's EyeSight performed best. It helped the Legacy and Outback avoid hitting the target at both test speeds. Next best was Cadillac's Automatic Collision Preparation. The system helped the ATS and SRX avoid hitting the target in the 12 mph test and reduced the ATS's speed by 15 mph and the SRX's speed by 19 mph in the 25 mph test.

"We want to help get the most effective systems in as many vehicles as soon as possible. That means a speed mitigation system like Subaru's EyeSight that can prevent crashes at low and moderate speeds," Zuby said. "At the same time, we want consumers to know that forward collision warning alone can help them avoid crashes, and it's a feature that's available on more models than autobrake."

Besides the 2013 BMW 3 series, another midsize model advertised with autobrake also earned a “basic” rating. In tests of the Infiniti JX SUV, there was only minimal braking at 12 and 25 mph. The Toyota Prius v wagon, which claims to have autobrake, had minimal braking in IIHS tests and currently fails to meet NHTSA criteria for forward collision warning. It doesn't qualify for an IIHS front crash prevention rating, IIHS said.

The front crash prevention ratings complement IIHS’s long-standing crash test program assessing how well passenger vehicles protect people in a range of crash configurations. In its crashworthiness program, the institute rates vehicles “good,” “acceptable,” “marginal” or “poor,” based on performance in moderate overlap front, small overlap front, side, roof strength and head restraint evaluations.

For more information about the test results, click here.