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Ride-sharing firm Uber has suspended its test of self-driving cars in San Francisco after regulators revoked the registration of the vehicles.
Passengers had recently been given the option of a booking a self-driving vehicle..
But authorities had threatened legal action if Uber did not obtain a special permit to test the cars.
Uber argues that because they came with a safety driver and are not fully autonomous, the permit was not needed.
"We're now looking at where we can redeploy these cars, but remain 100% committed to California and will be redoubling our efforts to develop workable statewide rules," Uber said.
San Francisco is Uber's second trial city for the new technology. In Pittsburgh, the company has been running its driverless trial since September and has not required special permits.
Not yet fully autonomous
Although the cars are promoted as "self-driving", they must constantly be monitored by an actual driver who can take control any time the software should fail.
Uber says its technology is not sophisticated enough to have cars continuously drive by themselves - and therefore the permit demanded in San Francisco is not needed.
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According to the regulators though, 20 other companies testing self-driving technology in California - including Google, Tesla and Ford - do so with the special permit which requires the firms to report any accidents to the authorities. This potentially undermines the company's reputation, as the reports are available to the public.
Just before the city's Department of Motor Vehicles threatened Uber with legal action over the missing permit, a video of an Uber self-driving vehicle running a red light in San Francisco had been uploaded to YouTube.
The trials are designed for the firm to identify flaws and glitches in the technology behind the autonomous driving.