Uber has halted its self-driving trials across the US after one of its autonomous vehicles was involved in a fatal accident in Tempe, Arizona. The accident may prove to be a defining moment for the autonomous-car industry.

More details: The accident happened last night when a woman walked into the road in front of a vehicle traveling in self-driving mode. Uber says a safety driver was behind the wheel but no passengers were in the car. It isn’t clear yet whether the car’s self-driving system could have avoided the accident. Uber says it is cooperating with the investigation.

What it means: Experts have long worried that a negative public reaction to the first serious accidents involving self-driving vehicles could cool enthusiasm for the technology and lead to onerous regulations.

Past accidents: This is the first time a self-driving car has killed a pedestrian. But there have been accidents involving semi-automated systems. Most prominently, the driver of a Tesla Model S was killed in May 2016 when his car crashed into a truck while driving in Autopilot mode.