US police thought they had their man when they watched a taxi make an illegal U-turn right in front of them -- only to discover there was no-one in the driver's seat.

Officers in San Bruno, California pulled over the Waymo driverless car on Friday night, but -- despite its flagrant law breaking -- had to let it go.

"Since there was no human driver, a ticket couldn't be issued," police in the city, south of San Francisco, said on social media.

"Our citation books don't have a box for 'robot.'"

Describing the situation as "a first" for law enforcement, the police department said they had spoken to Waymo about the problem.

"Officers... contacted the company to let them know about the 'glitch.'"

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