After a collision in Tempe, Arizona over the weekend, Uber says they are voluntarily suspending further testing of the self-driving vehicles.

An Uber Volvo XC90 was in self-driving mode with an operator in the driver’s seat on Friday evening when it was hit by another car whose driver failed to yield. The Uber vehicle rolled onto its side and neither driver was injured.

“We are continuing to look into this incident and can confirm we had no back-seat passengers in the vehicle,” said Uber spokeswoman, Chelsea Kohler. Further, she said that testing had been suspended in Pittsburgh and San Francisco at least for the day.

Currently Google, General Motors and Ford Motor Company are all testing autonomous vehicles in California. Uber, after successfully starting autonomous vehicle testing in Pittsburgh, was found to be testing in San Francisco without proper permits that disclose historical accident information.

The California Department of Motor Vehicles ultimately revoked Uber’s later registrations for autonomous vehicle testing after one vehicle failed to stop at a stoplight, driving straight through a crosswalk.

Uber then took their vehicles to Arizona, where the company was not required to apply for testing permits.

While companies work hard on self-driving technology, they seem to be finding that accounting for the error-prone nature of human drivers is a difficult variable. Google has found their autonomous vehicles in a number of accidents, typically the fault of other drivers.