LOS ANGELES (AP) — A small fleet of cars that maneuver through traffic using an array of sensors and computing power is driving on California roads and highways.

Since September, seven companies have received permission from the state Department of Motor Vehicles to test these cars of the future in public. Four of the 48 vehicles have gotten into accidents, according to the state Department of Motor Vehicles.

Here are some things to know about self-driving cars:

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THE COMPANIES

The companies rolling with the most cars are not traditional automakers — they're run by Silicon Valley billionaires.

According to the DMV, Google has 23 Lexus SUVs, while electric car maker Tesla Motors has 12 vehicles. The other companies have long legacies in the car industry. Audi, Mercedes-Benz and Nissan have three vehicles each; parts suppliers Bosch and Delphi each have two.

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THE HUMANS

The DMV is still drafting rules for when the cars are ready for the general public to use. When that will be depends on whom you ask — Google executives have said their timeframe is the next few years. Others talk about truly self-driving cars say decades, not years.

For now, the DMV reviews the qualifications of the people required to be in the driver's seat — just in case.

In all, 255 people are licensed, according to the DMV.

As with the number of cars, Google has the most what it calls "safety drivers" — the tech giant has 159 of them, the DMV said.

The agency said Bosch has 12; Delphi has nine; Mercedes-Benz has 12; Nissan has 17; and Tesla has 16. Audi has 30 licensed drivers, according to the DMV, but the automakers said in an email it has 27.

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HOW FAR THEY HAVE DRIVEN

Google said its cars have gone "the equivalent of over 15 years of typical human driving" since September. That would be about 140,000 miles. Google was driving its cars on public roads before state lawmakers passed a law regulating testing. "Since the start of our program 6 years ago, we've driven more than 700,000 miles autonomously, on both freeways and city streets, without causing a single accident," spokeswoman Katelin Jabbari said.

The other companies wouldn't say how many miles their test cars have driven since September.

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OTHER STATES

Nevada, Michigan and Florida also have passed laws permitting self-driving cars on their roads. Because other states have not expressly outlawed the cars, companies may test elsewhere — though the four states with rules have just about every driving condition and terrain.

Regulators in Nevada, Michigan and Florida said they were not aware of any self-driving car accidents since they formally welcomed the cars onto their roads over the last few years.