The software fix is being classified as a voluntary recall by the National Highway Traffic Safety Administration, which said in an email that it received notification of the issue from Waymo and plans to publish related documents online Thursday morning.

“As we serve more riders in more cities, we will continue our safety first approach, working to earn trust with our riders, community members, regulators, and policymakers,” Waymo said in a statement.

The Phoenix incident which prompted the recall took place May 21, when a Waymo taxi hit a telephone pole in an alley while pulling over at low speed, according to the company. No one was harmed in the incident.

“We went to work immediately and determined that, in certain situations, our vehicles had an insufficient ability to avoid collisions with on-road narrow, permanent objects within the drivable surface,” the company said in an emailed statement.

Self-driving vehicles have been the subject of regulatory scrutiny over the past year. Last month the National Highway Traffic Safety Administration announced it would open investigations into Waymo and its Amazon-owned competitor, Zoox. NHTSA already had an investigation underway into Ford’s BlueCruise hands-free driving feature, and last year it issued an over-the-air recall of nearly every Tesla vehicle over problems with self-driving technology.

The Waymo inquiry covers nearly two dozen incidents involving its vehicles, including situations in which robotaxis hit stationary objects such as gates, chains, or parked vehicles, or disobeyed traffic safety signals. There have also been public reports of Waymo cars entering construction zones or driving in the wrong lane.