Ride-hailing firm Uber is pausing tests of its self-driving cars in four North American cities while police and federal investigators probe a fatal accident involving one of them.

The death, in Tempe, Ariz., occurred shortly after 10 p.m. local time on Sunday, when a 49-year-old woman walking outside of a crosswalk to get from one side of a street to the other was struck by an Uber vehicle in self-driving mode, police said.

A human driver was behind the wheel, but the automobile had no other passengers, according to police, who are still probing the circumstances. The accident will also be investigated by a team from the National Transportation Safety Board, which examines highway crashes that can illuminate broad or developing safety issues.

Tests of Uber's self-driving cars, run through the company's Advanced Technologies Group, have been halted in Phoenix, Pittsburgh, San Francisco and the Canadian metropolis of Toronto, according to a person familiar with the matter.

The technology behind self-driving cars, which requires wireless communication behind moving vehicles and their surroundings to gauge relative positions, is still in its infancy though many automobiles already on the road offer limited features based on it.

Ford's assisted-parking service, for instance, can steer vehicles into spaces while drivers control the brakes and accelerator, though the automaker warns that it may not work in all conditions.

The city of Tempe, home to Arizona State University, has supported local trials of such technology because of its potential to improve highway safety by curbing risks such as distracted or intoxicated driving.

While it's imperative that such evaluations be conducted safely, said Tempe Mayor Mark Mitchell, past indicators show that all the companies participating were obeying applicable traffic laws.

"Our city leadership and Tempe police will pursue any and all answers to what happened in order to ensure safety moving forward," he said. "I support the step that Uber has taken to temporarily suspend testing in Tempe until this event is fully examined."

The company, best known for its ubiquitous ride-hailing smartphone app, said it's cooperating with investigators and offered condolences to the victim's family.

"Some incredibly sad news out of Arizona," Uber CEO Dara Khosrowshahi wrote on social media platform Twitter.



Some incredibly sad news out of Arizona. We’re thinking of the victim’s family as we work with local law enforcement to understand what happened. https://t.co/cwTCVJjEuz — dara khosrowshahi (@dkhos) March 19, 2018



Based in San Francisco, Uber has more than 16,000 employees and takes passengers worldwide on more than 10 million trips a day.

