Last weekend, about 10 Cruise vehicles stopped functioning in the middle of a busy street in San Francisco’s North Beach neighborhood, blocking traffic for 15 minutes. Drew Pusateri, a spokesman for Cruise, said in a statement that the cars had difficulty connecting to the Cruise employees who might have guided them out of the way because of a spike in cellular traffic caused by a music festival in the city’s Golden Gate Park about four miles away.

Several other Cruise vehicles also stalled in streets near the park.

One week ago, the C.P.U.C. allowed both companies to charge for rides around the clock anywhere in San Francisco. The C.P.U.C. and the D.M.V. are the two agencies governing autonomous vehicles in California. A company has to obtain a permit from the D.M.V. before it applies for driverless deployment permits — the kind that Cruise and Waymo received last week — from the utilities commission.

The motor vehicles authority said in a statement that it is “investigating recent concerning incidents involving Cruise vehicles in San Francisco.” The agency asked Cruise to cut the number of vehicles operating in San Francisco “until the investigation is complete and Cruise takes appropriate corrective actions to improve road safety.”

“The DMV reserves the right, following investigation of the facts, to suspend or revoke testing and/or deployment permits if there is determined to be an unreasonable risk to public safety,” the agency said in its statement.

San Francisco officials have complained since January that autonomous vehicles have been interfering with emergency vehicles. Before this week, officials documented 55 incidents where a driverless car abruptly stopped or interfered with emergency vehicles, including one instance with firefighters who were battling a house fire.