U.S. safety regulators have opened an investigation to determine if the emergency braking system on General Motors 2014 Chevrolet Impala is defective.

There are an estimated 61,000 Impala sedans in the United States.

The U.S. National Highway Traffic Safety Administration said it opened a preliminary evaluation after a driver complaint related to his car’s autonomous braking system, a safety feature that is becoming more popular on new vehicles. When a vehicle is in danger of striking a vehicle in front of it or another object, the system activates the brakes.

The agency said the driver reported being rear-ended after the car’s automatic braking system caused the car to brake suddenly without cause. The car was traveling at 40 miles per hour before the accident occurred, according to a report filed on the NHTSA website on Tuesday.

“The consumer alleges that the driver assist system inappropriately activated emergency braking, bringing the vehicle to a complete stop under what the driver considered to be full braking force,” the NHTSA report said.

This happened three of four times to the consumer, NHTSA reported.

NHTSA said it opened the preliminary evaluation to assess the frequency, scope and consequences of the alleged defect.