WASHINGTON, May 13 (Reuters) - The U.S. National Highway

Traffic Safety Administration (NHTSA) said Monday it has opened

an investigation into Amazon.com's ( AMZN ) self-driving Zoox

vehicles due to unexpected braking leading to two rear-end

collisions that injured motorcyclists.

NHTSA said it had opened its preliminary evaluation after

two crashes involving the self-driving technology unit's

vehicles equipped with the Zoox Automated Driving System that

resulted in minor injuries to motorcyclists and started a probe

into 500 Zoox robotaxis, NHTSA said on Monday.

Each incident involved a Toyota Highlander equipped with the

Zoox automated driving system. Zoox did not immediately respond

to a request for comment.

In March, Zoox said it was expanding its vehicle testing in

California and Nevada to include a wider area, higher speeds and

nighttime driving, as it competes with Alphabet's

Waymo robotaxis. Amazon ( AMZN ) acquired Zoox in 2020 for $1.3 billion.

NHTSA said both crashes occurred during daytime lighting

conditions and within the operational design limits of the Zoox

system. NHTSA said its initial investigation confirmed "each of

the Zoox vehicles was operating with the ADS engaged in the

moments leading up to each collision."

The investigation will evaluate the Zoox Automated

Driving System performance particularly relating to the

collisions as well as "the behavior in crosswalks around

vulnerable road users, and in other similar rear-end collision

scenarios."

NHTSA in March 2023 opened a probe into the

self-certification by Zoox in 2022 of a robotaxi without

traditional driving controls.

The investigation, which is pending, is to determine

whether the Zoox "certification basis depended upon unilaterally

developed test procedures or determinations that certain

standards were inapplicable due to the unique configuration of

the vehicle."