"Complaints allege that while utilizing... features including adaptive cruise control, the vehicle unexpectedly applies its brakes while driving at highway speeds. Complainants report that the rapid deceleration can occur without warning, at random, and often repeatedly in a single drive cycle," the NHTSA said of Tesla’s Model 3 and Model Y cars manufactured since last year.
The NHTSA said it has received 354 complaints over the past nine months about the phenomenon characterized as "phantom braking" by consumers.
It said Tesla had identified the problem as being associated with its advanced driver assistance system, also known as Autopilot, that allows its vehicles to brake and steer automatically within designated lanes.
The NHTSA said its Office of Defects Investigation has opened a preliminary evaluation "to determine the scope and severity of the potential problem and to fully assess the potential safety-related issues."
Earlier this month, US media reported that Tesla cars were unexpectedly slamming on their brakes in response to imagined hazards - such as oncoming traffic on two-lane roads - which has prompted their terrified owners to lodge a surge of complaints with NHTSA.