The automatic emergency braking systems showing up on an increasing number of vehicles may not work quite the way drivers think they do.

A study released Wednesday by AAA found that many buyers don't know that some systems are designed to prevent accidents, while others only attempt to lessen the damage done when a crash occurs. The Florida-based auto club says consumers need to be better educated on these systems, particularly as federal regulators and auto makers tout them as lifesaving devices.

Most auto makers have agreed to install systems within six years that automatically apply brakes when the car senses it is about to crash. Automatic braking systems are often being paired with lane-keeping assist and adaptive cruise control to make vehicles more capable of making decisions that drivers have traditionally made.

These advanced driver-assistance systems have been cited by the National Highway Traffic Safety Administration as technology that will help reduce the rising number of deaths attributed to accidents. The agency has said a pledge earlier this year from auto makers to install automatic emergency-braking systems on new cars by 2022 will make the technology standard three years faster than through a formal regulatory process, preventing additional crashes and injuries.

The National Safety Council on Tuesday said 2016 is on pace to be the deadliest year on the roads in nearly a decade, with 19,100 people killed through the first half. That increase comes after traffic fatalities unexpectedly rose in 2015 after several years of declines.

AAA conducted a series of tests on driving tracks and found systems that are created to prevent crashes altogether slowed the car nearly twice as much as others built to slow the car just enough to stop massive damage.

Greg Brannon, director of automotive engineering with AAA said two-thirds of 1,000 people surveyed said they thought all automatic braking schemes worked the same.

Driver assistance systems have come under increased scrutiny after the fatal May crash in Florida of Joshua Brown, who died while the Autopilot function of his Tesla Model S was engaged.

Safety advocates say drivers may be lulled into a false sense of security.

"None of these systems yet are (fully) autonomous," said Mr. Brannon. "You need to keep your hands on the wheel and your foot on the brake."

Automatic braking systems come on a wide range of vehicles.

The 2016 Honda Civic now comes with optional automatic brakes that will cost an extra $1,000 and engage at speeds of about 60 miles an hour or under when the car judges it is about to hit something. Honda Motor Co.'s automatic brakes will be available on Accords and Ridgelines in 2017, said spokeswoman Jessica Pawl.

Nissan Motor Co.'s 2017 Pathfinder sport-utility vehicle will have an optional automatic emergency-braking system that can engage whether or not the vehicle is towing a load.

Automatic brakes have been standard on Volvo Car Corp. cars since 2014, said spokesman Jim Nichols. The speed the vehicle is traveling is more of a factor in whether the car comes to a complete stop than the type of automatic brakes it is equipped with. Under 17 miles an hour, says Mr. Nichols, the car will come to a stop with Volvo's automatic brakes, but at higher speeds will slow the car to lessen damage.

"Essentially, Isaac Newton is kind of calling the shots in certain situations," he said.

Volvo also outfits some models with a camera-brake combination that can pick out what an object is from a database of hundreds of thousands of uploaded photos, be it an animal, person or another car.

Write to Jonathan Bach at jonathan.bach@wsj.com

(END) Dow Jones Newswires

August 24, 2016 11:25 ET (15:25 GMT)