Robotaxis operated by Baidu’s Apollo Go stalled throughout Wuhan, China, in some cases trapping passengers for up to two hours, according to multiple media reports as well as numerous video and social media posts.

Baidu has not shared what caused the widespread outage. However, local police said it was a “system failure” that affected at least 100 robotaxis, Reuters reported. The unknown technical error caused robotaxis to suddenly freeze — and sometimes in dangerous spots such as the fast lane of roadways, according to Wired. Police are still investigating the matter.

Baidu did not respond to TechCrunch’s emailed request for comment.

Baidu is one of the largest robotaxi operators in China and has expanded to the Middle East. Last year, the company said it planned to deploy a fleet of more than 1,000 autonomous vehicles in Dubai over the next few years.

The recent Baidu outage is the latest incident to raise questions about the safety of robotaxis and their impact on local communities. And it’s not just isolated to China. A widespread power outage in California last December wiped out traffic lights, causing Waymo vehicles to become stuck.