What happens if these cars get in a situation where there is no legal way to move? For instance, the other day I was driving down the street and it was closed because of a Farmers market. Will they just sit there for hours? In gridlock, there is often no way to move without "blocking the box."
Presumably Waymo has this traffic data by leveraging existing Android monitoring technology and can reroute accordingly. If you've ever used Google Maps for directions and come up to an accident or something you know it will suggest a better route as it becomes available. These cars will likely not get themselves into high-congestion areas with no way out simply because they already know about the high congestion are…
I'm sure this Waymo project does not take the perfection of Google Maps data as a given.