An Uber self-driving car hit and killed a woman in Tempe, Ariz., a tragic milestone that could lead to a major setback in the otherwise feverish development of driverless transportation.

The death of Elaine Herzberg, 49, who was crossing a street outside the marked crosswalk, appears to be the first time a self-driving car has been involved in the fatality of a person not inside the vehicle.

In 2016, a man was killed while behind the wheel of a Tesla that was driving itself in Autopilot mode when a truck cut across its path — the first such death related to a car in self-driving mode. Investigators found the driver failed to heed warnings to take back control of his car.

The Arizona accident is poised to become a focal point of a debate over the viability of self-driving cars, which promoters vow will save lives while skeptics contend it remains dangerously ill-prepared for prime time.