Boston is the IDEAL place to test out autonomous driving. The narrow roads, lined as they are with trees, rocks, and parked cars, mean that when an autonomous car makes the wrong decision, the damage will be primarily to property, Waymo's and the other guy, and Waymo can just cut a check, learn some lessons, and move on.
(BTW, Boston's reputation as a place for aggressive driving is undeserved. Insurance companies pay a lot for fender repair for the reason above, making the city look bad, but for injuries against a person, insurers just pay the limit, which an injured pedestrian can run up in the first 24 hours in a hospital. Anything beyond that doesn't show up in car insurance statistics. Houston and Austin have far more aggressive drivers, for instance.)
It's also true that the old elevated central artery was really bad and pretty much required aggressive behavior because of how many ramps there were and the fact that you had to navigate a confusing network of crowded surface streets to access the harbor tunnels. I've admittedly lived in the area a long time but between the Big Dig and GPS, I don't think Boston is uniquely bad these days.
(BTW, Boston's reputation as a place for aggressive driving is undeserved. Insurance companies pay a lot for fender repair for the reason above, making the city look bad, but for injuries against a person, insurers just pay the limit, which an injured pedestrian can run up in the first 24 hours in a hospital. Anything beyond that doesn't show up in car insurance statistics. Houston and Austin have far more aggressive drivers, for instance.)