A self-driving Uber car which killed a female pedestrian in Arizona was speeding when the crash occurred, police investigators revealed.

Elaine Herzberg, 49, was crossing the street on Monday when she was hit by a self-driving Uber 4x4 in Tempe, Arizona. She was taken to a local hospital where she died of her injuries.

Tempe Police said the car was travelling at around 40 mph in a 35 mph zone and preliminary investigations suggest the SUV failed to slow down before hitting Ms Herzberg.

Rafael Vasquez, a 44-year-old safety tester from Uber, was behind the wheel of the car at the time but the vehicle was in autonomous mode.

The incident is the first known pedestrian death involving a fully autonomous vehicle.

Driverless cars rely on a system of cameras and sensors along with software to detect other vehicles, pedestrians and obstacles as they navigate around cities, and the initial police findings will raise concerns over the safety of current tests being carried out by hundreds of cars across the US.

Tempe police spokesman Sergeant Ronald Elcock said impairment did not appear to be a factor Mr Vasquez or Ms Herzberg, who was walking her bicycle across the road when she was knocked down.