Police in a Silicon Valley suburb were flummoxed last weekend after pulling over a self-driving Waymo robo-taxi for making an illegal turn, then finding no driver they could issue with a ticket.

The incident happened in San Bruno, where cops set up a checkpoint to catch intoxicated drivers. As cars lined up, one vehicle, a self-driving Waymo, pulled an illegal U-turn and began to drive away.

The cops fired up their lights and the Waymo pulled over automatically as the robo-cab operator trains its cars to do. But the police officers who pursued the robo-taxi couldn't hand out a ticket, since the driverless taxi had nobody inside.

"Since there was no human driver, a ticket couldn’t be issued (our citation books don’t have a box for 'robot')," said the San Bruno Police Department on its Facebook page. "Hopefully the reprogramming will keep it from making any more illegal moves. It was a first for both officers."

Human drivers who make illegal turns risk a $234 fine, plus court fees if drivers contest a ticket. San Bruno’s police admitted they currently lack the authority to bill the vehicle's operator. However, changes are in progress to make that possible, the force said.

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