(Reuters) -The U.S. National Highway Traffic Safety Administration (NHTSA) has opened a preliminary evaluation into Amazon.com's ( AMZN ) self-driving Zoox vehicles due to unexpected braking leading to rear-end collisions.

The auto safety regulator's Office of Defects Investigation (ODI) has received notice of two incidents involving the self-driving technology unit's vehicles equipped with the Zoox Automated Driving System and has started a probe into 500 such robotaxis, the NHTSA said on Monday.

In March, Zoox said it was expanding its vehicle testing in California and Nevada to include a wider area, higher speeds and nighttime driving, to stay ahead of Alphabet's Waymo robotaxis.

Zoox did not immediately respond to a request for comment.

(Reporting by Devika Nair and Akash Sriram in Bengaluru; Editing by Sonia Cheema and Devika Syamnath)