Waymo, the American company developing a robotaxi service, is now under investigation by the U.S. National Transportation Safety Board (NTSB). The probe follows reports that since the start of the school year, Waymo's autonomous vehicles have passed stopped school buses at least 19 times in Austin, Texas.

Under U.S. traffic laws, drivers in all states must stop when a school bus is standing with flashing red lights and an extended STOP sign. Passing is prohibited, even for oncoming traffic, unless the road is physically divided.

This isn't the first time Waymo has faced criticism over such incidents. In December, the company recalled over 3,000 vehicles to update software intended to prevent passing school buses. However, the Austin school district reported at least five new cases after the November update and demanded that Waymo halt its service near schools during student drop-off and pick-up hours. According to the district, Waymo refused.

Waymo states that no collisions occurred and that its Waymo Driver system is continuously being improved. The NTSB plans to visit Austin, with a preliminary report expected within 30 days.