SAN FRANCISCO :Alphabet's Waymo robotaxis have driven more than 100 million miles without a human behind the wheel, doubling the mileage in about six months, a top company official said, as it speeds up deployment in U.S. cities amid rising competition. Waymo's push comes as Tesla expands its self-driving taxi service after a small trial with about a dozen of its Model Y SUVs in a limited area of Austin, Texas last month. While Tesla CEO Elon Musk has said Tesla will scale up the service