The Insurance Institute for Highway Safety (IIHS) has launched a rear crash prevention ratings program to help drivers identify models equipped with technology that stop or reduce low-speed backing crashes, the institute has announced.

Two of six popular 2017 model vehicles — Subaru Outback crossover and Cadillac XT5 luxury SUV — earned the highest rating of superior when equipped with optional rear autobrake, parking sensors and rear cross-traffic alert.

The other four models evaluated by IIHS engineers all earned the second-highest rating of advanced when equipped with the same optional gear. These include the BMW 5-Series sedan, Infiniti QX60 luxury SUV, Jeep Cherokee compact SUV, and Toyota Prius hatchback.

The new rating program uses a three-tier scoring system. Models with optional or standard rear crash prevention systems are rated superior, advanced or basic. Ratings are determined by whether the vehicles have rear autobrake and, if so, how it performs in a series of car-to-car and car-to-pole tests with different approach angles. While rear autobrake carries the most weight, the availability of parking sensors and rear cross-traffic alert are also factored into the overall scoring.

While all three technologies help prevent crashes, research shows that rear autobrake provides the biggest advantage.

Testing for the new program is rigorous. In order for a vehicle to achieve a superior rating, it must have a rear autobrake system that can avoid a crash or substantially reduce speeds in many of the test scenarios, which involve multiple runs at about 4 mph. Systems are assigned points based on the number of runs that either avoid or barely hit the target, reducing speeds to less than 1 mph. For an advanced rating, a vehicle must have rear autobrake and avoid a crash or reduce speeds in some of the scenarios. Vehicles that only have parking sensors and/or rear cross-traffic alert earn a basic rating.

Rear autobrake systems are not very common, but the new IIHS rating program may change that as automakers experience pressure to up their game and earn a top score.

Presently, the feature is optional on only 5% — and standard on less than 1% — of 2018 model passenger vehicles, according to estimates from Highway Loss Data Institute (HLDI). Rear cross-traffic alert is optional on 43% and standard on 11% of 2018 models. Rear parking sensors are optional on 59% and standard on 33% of 2018 models. Rearview cameras are standard on 89% and optional on 10% of 2018 models.