By David Shepardson and Akash Sriram

WASHINGTON (Reuters) -The National Highway Traffic Safety Administration said on Monday it has opened a preliminary investigation into about 2,000 Waymo self-driving vehicles after reports that the company's robotaxis may have failed to follow traffic safety laws around a stopped school bus.

The probe of the Alphabet unit is the latest scrutiny of self-driving systems by regulators reviewing how driverless technologies interact with pedestrians, cyclists and other road users.

NHTSA opened the investigation after a recent media report aired video of an incident in Georgia in which a Waymo did not remain stationary when approaching a school bus with its red lights flashing and stop arm deployed.

The report said the Waymo vehicle initially stopped then maneuvered around the bus, passing the extended stop arm while students were disembarking.

Waymo's automated driving system surpassed 100 million miles of driving in July and is logging 2 million miles per week, the agency said. "Based on NHTSA’s engagement with Waymo on this incident and the accumulation of operational miles, the likelihood of other prior similar incidents is high," the agency said.

A Waymo spokesperson said the company has “already developed and implemented improvements related to stopping for school buses and will land additional software updates in our next software release.”

The company added "driving safely around children has always been one of Waymo's highest priorities." The company said Waymo "approached the school bus from an angle where the flashing lights and stop sign were not visible and drove slowly around the front of the bus before driving past it, keeping a safe distance from children."

NHTSA said the vehicle involved was equipped with Waymo's fifth-generation Automated Driving System and was operating without a human safety driver at the time of the incident.

Waymo has said its robotaxi fleet numbers more than 1,500 vehicles operating across major U.S. cities, including Phoenix, Los Angeles, San Francisco and Austin.

In July, NHTSA closed a 14-month investigation into a series of minor collisions "with clearly visible objects that a competent driver would be expected to avoid" and "unexpected behavior" from Waymo self-driving vehicles, after 22 reports about its robotaxis driving in ways that potentially violated traffic safety laws. The probe ended after two recalls.

(Reporting by Akash Sriram in Bengaluru and David Shepardson in Washington; Editing by Tasim Zahid)