San Francisco - When police stop a car at night that doesn't have its headlights switched on, they usually know what to expect – most often a tired driver, or, potentially, a drunk one.

Two officers in San Francisco were not sure whom to blame, however, after pulling up behind a driverless taxi operated by Cruise at a red light early in April.

In a video posted on Instagram on April 2 by an eyewitness, one of the cops is seen peering through the car's windows while trying to open the driver's door, with no success.

As he returns to his car, the robot cab starts driving away in what looks set to become one of those heart-stopping police chases. But then the autonomous vehicle stops again right after the crossroads and obediently puts on its hazard lights.

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The cab had only been looking for a safer spot for the traffic check and wasn't trying to escape, a Cruise spokesperson clarified in comments to tech blog The Verge late on Sunday.

The officers pulled up behind the vehicle again and eventually contacted the company, which then fixed the problem, according to the spokesperson.

General Motors subsidiary Cruise launched its commercial robot taxi services in San Francisco in February. Its driverless vehicles are allowed to carry paying passengers at night without a safety driver.

Google's sister company Waymo also plans to build services with fully self-driving robotaxis in the city.