GM's self-driving taxi arm Cruise will have to answer to federal safety regulators who are now rightfully concerned about several incidents in which the robot cars decided to stop in the middle of streets and block human drivers from getting past, or randomly brake quickly and cause collisions.

In a new filing, the National Highway Traffic Safety Administration (NHTSA) says it is looking into reports of Cruise vehicles braking suddenly and/or stopping in the middle of city streets for no apparent reason. As the New York Times reports, the agency's Office of Defects Investigation is investigating two types of incidents: the type in which the cars become strangely disabled or disconnected from Cruise's servers, as was apparently the case in that creepy July incident in which a whole gang of the robot cars swarmed on Gough Street and blocked traffic for several hours; and the type in which the cars brake too quickly and sometimes cause rear-end collisions.

The NHTSA says that these seemingly defective behaviors of the cars "may strand vehicle passengers in unsafe locations, such as lanes of travel or intersections, and become an unexpected obstacle to other road users," and/or "These immobilizations may increase the risk to exiting passengers."

"Further," the agency writes, "immobilization may cause other road users to make abrupt or unsafe maneuvers to avoid colliding with the immobilized Cruise vehicle."

The investigation came about, the NHTSA, after discussions with Cruise, local law enforcement, and because of media coverage of the incidents.

As SFist learned shortly after the Gough Street swarm in July, that wasn't the first time something like that had happened. Other, earlier incidents had occurred in May and June in which Cruise's servers lost touch with the robo-taxi fleet altogether. Wired revealed this in an investigative report, which in part came out of a letter from an anonymous Cruise employee who blew the whistle with the California Public Utilities Commission (CPUC), suggesting the company had tried to hide the incidents to avoid bad press.

Now, the CPUC will have the final word on whether Cruise is going to get its permit to take paid passengers on rides all over the city at all times of day. As the Chronicle reports, while Cruise vehicles have been able to offer driverless taxi rides for much of this year in some parts of San Francisco, geofencing keeps the cars out of downtown and off of streets with steep inclines. And the cars have only been allowed to operate during overnight hours when the streets are less busy — between 10 p.m. and 6 a.m.

News of the federal probe arrived one day after the California Department of Motor Vehicles granted its permission to GM to all Cruise to offer 24/7, full-city service in SF. And now it seems like the CPUC will wait to see what the results of the investigation are before giving its blessing.

Previously: GM Cruise Robotaxis Blocked SF Streets More Frequently Than We Knew, One Night ‘Nearly 60’ Cars Stopped

Top image: @TaylorOgan via Twitter