Self-driving cars now need a permit in California

For the first time, California's Department of Motor Vehicles knows how many self-driving cars are traveling public roads.

The agency is issuing permits Tuesday that let three companies test 29 vehicles on highways and in neighborhoods - with a human "safety driver" behind the wheel, in case the onboard computers make a bad decision.

With 25 vehicles, Google's souped-up Lexus SUVs are the biggest fleet. Mercedes and VW have two vehicles each.

Self-driving cars already have gone hundreds of thousands of miles in California.

They did because there was no law saying they couldn't.

The Legislature eventually decided to regulate the emerging technology, which navigates using sophisticated sensors and detailed maps.

Over the past year, the DMV wrote testing rules that require driver training and the reporting of incidents.