It was an Uber vehicle, not Waymo ('Google'.)

The vehicle's sensors actually did 'see' Elaine Herzberg, but the software was tuned to filter 'false positives' such as plastic bags blowing across the road.

"If the software is programmed to be too cautious, the ride will be slow and jerky, as the car constantly slows down for objects that pose no threat to the car or aren't there at all. Tuning the software in the opposite direction will produce a smooth ride most of the time—but at the risk that the software will occasionally ignore a real object."

[ arstechnica ]