Freeways are much, much less forgiving of abrupt speed changes and braking, which is something waymo (used to) have quite an issue with. Moving to freeways shows they are confident that won't be an ongoing issue.
Yes I was driving on a freeway last week and traffic went from 80mph to dead stop about as fast as I could stand on the brakes. I didn't hit the car in front of me with only a few feet to spare, and fortunately the driver behind me was also paying attention. The jam eventually cleared, and there was absolutely no indication of what caused it.
>> and there was absolutely no indication of what caused it
That's the Accordion Effect.
With enough cars on the road, one little tap of someone's brakes flashes their brake lights and it ripples upstream until the 80mph column of cars is forced to go to down to zero ASAP.