Uber says its self-driving cars remains suspended in Arizona and Pittsburgh, Pennsylvania, following a crash over the weekend.

The company also grounded self-driving cars in San Francisco over the weekend but they resumed operating on Monday. The difference between the California program versus the ones in Arizona and Pennsylvania is that California does not allow passengers to ride in the vehicles.

No one was seriously injured. Uber says its vehicles have been grounded as it investigates.

An Uber spokeswoman told Bloomberg news that “we are continuing to look into this incident and can confirm we had no backseat passengers in the vehicle.”

Police in Tempe say the self-driving SUV was obeying the law and the driver in the other car who didn’t yield was cited for a moving violation after the Friday night crash.

An Uber statement Saturday says there were no passengers in the self-driving Volvo SUV at the time of the crash but there were two operators in the front.

Police say the Uber rolled over onto its side as a result of the collision.​

Multiple automakers and technology companies are testing fleets of self-driving vehicles. In nearly all cases, the cars have backup drivers who can take the wheel in an emergency.

Testing hasn’t been accident-free. Waymo — a division of Google — has been testing self-driving cars since 2009 and has driven them more than 2 million miles. Last year, Waymo reported 13 accidents involving its fleet in the state of California, which requires companies testing autonomous vehicles to report any accidents. Most of the accidents were minor and weren’t caused by Waymo’s vehicles. But in February 2016, a Waymo test car struck a public bus near the company’s headquarters in Mountain View, California. No one was injured.

Cruise Automation — a self-driving startup owned by General Motors Co. — and Nissan Motor Co. also reported fender benders involving self-driving cars in California within the last year.

Last year, a driver of a semi-autonomous Tesla — which is different from a self-driving car — was killed with the car’s Autopilot system engaged.