I'm not sure why you would say there's no significant inclement weather in Atlanta. The flooding this week was not super common, but also not unheard of. It rains here a LOT in the summer
Agreed, this happens here every year, it's why we built O4W park the way it is, and built many other drainage structures similarly. We have a real runoff problem. Waymo picked a great city to train the cars on weird weather and weirder roads. :D
The part of that people aren't considering is that it's very common to get brief, intense thunderstorms that dump a lot of rain quickly. They won't flood the whole city obviously but there's _always_ pockets that have very short-lived, localized flooding on the roads. So it's not a "oh what are the odds of that happening" kind of thing.