In a preliminary report on the crash, the National Transportation Safety Board said Thursday that emergency braking maneuvers are not enabled while Uber's cars are under computer control, because it reduces the potential for erratic behavior.
Instead, Uber relies on a human backup driver to intervene. The system, however, is not designed to alert the driver.
In the March crash, the driver began steering less than a second before impact but didn't brake until less than a second after impact.
A video of the crash showed the driver looking down just before the woman was hit.
VIDEO: Deadly crash with self-driving Uber vehicle in Arizona