WASHINGTON (Reuters) - The National Highway Traffic Safety Administration said Friday it has learned of nine additional incidents that raise concerns about the performance of Alphabet's Waymo self-driving vehicles.

Earlier this month, the U.S. auto safety regulator opened an investigation after 22 reports of its robotaxis exhibiting driving behavior that potentially violated traffic safety laws, or demonstrating other "unexpected behavior." In a letter to Waymo released Friday, NHTSA said it has learned of 9 additional similar incidents. The agency said several incidents under investigation "involved collisions with clearly visible objects that a competent driver would be expected to avoid."

(Reporting by David Shepardson)