SAN FRANCISCO — About 2 a.m. on March 19, Adam Wood, a San Francisco firefighter on duty, received a 911 call and raced to the city’s Mission neighborhood to help a male who was having a medical emergency. After loading the patient into an ambulance, a black-and-white car pulled up and blocked the path. It was a driverless vehicle operated by Waymo, an autonomous car company that Google’s parent, Alphabet, owned. With no human driver to instruct to move out of the way, Wood spoke through a devic