Washington, DC (CNN Business) A federal investigation into a fatal crash involving an Uber self-driving car concluded the probable cause was a safety driver distracted by their phone. The National Transportation Safety Board investigation also determined that an inadequate safety culture at Uber (UBER) contributed to the March 2018 crash in Tempe, Arizona.

Investigators described it as an avoidable crash during a board meeting in Washington, DC, Tuesday. The investigators found that an alert vehicle operator would have had two to four seconds to detect and avoid pedestrian Elaine Herzberg, who was crossing a street when struck by Uber's self-driving vehicle.

The test driver behind the wheel of Uber's self-driving car was supposed to intervene if the autonomous driving software failed. But the driver was glancing away from the road during 34% of the fatal trip, including 23 glances in the final three minutes before the crash, according to the investigation. A camera in the car recorded the driver.

The NTSB found that Uber had no safety plan for its self-driving operation, or equivalent guiding document at the time of the crash. Uber's self-driving software wasn't designed to expect that pedestrians outside crosswalks may be crossing the street. The board also said Uber lacked appropriate oversight for vehicle operators.

"This is about one fatality, but it's about a lot more than that," NTSB chairman Robert Sumwalt told reporters afterward. "We felt by focusing on this we could have much broader ramifications for improving safety."

Read More