It looks similar to a cable/fiber rollout where they have to onboard each region individually. I know they are currently doing this in the Atlanta metro.
Like cable/fiber, once they have good models of the business and what it costs to roll out, they have the freedom to accelerate and do regions in parallel. If the business works, I would expect them to scale the pace of rollout.
Now 150k trips per week (things are moving fast)
https://x.com/Waymo/status/1851365483972538407