WASHINGTON, May 24 (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."