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.
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.