WASHINGTON (Reuters) -U.S. auto safety regulators said on Tuesday they have opened an investigation into the performance of Alphabet's Waymo self-driving vehicles after reports of its robotaxis exhibiting driving behavior that potentially violated traffic safety laws.

The National Highway Traffic Safety Administration said its preliminary evaluation into an estimated 444 Waymo vehicles follows 22 reports of 22 incidents including 17 collisions.

The agency said in some of those cases the automated driving systems "appeared to disobey traffic safety control devices" and some crashes occurred shortly after the automated driving systems "exhibited unexpected behavior near traffic safety control devices."

Waymo did not immediately respond to a request for comment. This is the latest in a series of investigations opened by NHTSA into performance of self-driving vehicles after initiated probes into General Motors Cruise and Amazon.com's Zoox .

In February, Waymo recalled 444 self-driving vehicles after two minor collisions in quick succession in Arizona, saying a software error could result in automated vehicles inaccurately predicting the movement of a towed vehicle.

NHTSA said all 22 incidents included either self-driving crashes or driverless vehicles that exhibited driving behavior that potentially violated traffic safety laws.

The incidents include collisions with stationary and semi-stationary objects such as gates and chains and collisions with parked vehicles.

NHTSA also cited incidents "such as vehicles driving in opposing lanes with nearby oncoming traffic or entering construction zones."

The auto safety agency will investigate the Waymo 5th Generation automated driving system performance "in the incidents identified in this resume and similar scenarios, as well as to more closely assess any commonalities in these incidents."

The investigation, which is the first stage before the agency could demand a recall if it believes the vehicles pose an unreasonable risk to safety, will evaluate Waymo vehicles performance "in detecting and responding to traffic control devices and in avoiding collisions with stationary and semi-stationary objects and vehicles."

Waymo said in March it was beginning to offer free driverless robotaxi services to select members of the public in Los Angeles after receiving approval from a state agency to start its ride-hailing program, Waymo One, in Los Angeles and some cities near San Francisco.