One of China’s biggest robotaxi firms suffered an outage, leaving passengers stranded on highways and apparently causing accidents.

Baidu operates self-driving cars in dozens of Chinese cities. Social media users in Wuhan reported seeing the cars suddenly halt on busy highways, often in the fast lane; WIRED reported that at least three accidents were seen. Some passengers were stuck for 90 minutes. US company Waymo’s autonomous cars report better safety records than human drivers, but centralized control means vulnerability to centralized problems; robotaxi firms are unwilling to say how often their cars require human intervention.

Their expansion continues, however. Two firms have begun testing robotaxis in London, and Waymo started offering services to a fourth US airport.