Though I’ve heard people treat it differently in the US
If "we'll have too many cars on the freeway following the speed limit" ranks as a serious concern, I think we've really lost the plot.
I recently drove by a fatal accident that had just happened on the freeway. A man on the street had been ripped in half, and his body was lying on the road. I can't imagine the scene is all that unlike the 40 thousand other US road deaths that happen every year.
As a driver I'm willing to accept some minor inconvenience to improve the situation. As a rider I trust Waymo's more than human drivers.
I've lived in a couple of places where going the speed limit is a whole problem that can cascade outside of just yourself. There is an argument to be made that perhaps then the speed limit shouldn't be that low, but in driving safety is far more important than legality. It will be interesting to see how Waymo handles these realities when it gets to those areas.
Second, it is a widely known issue, that a slower mowing car is causing ripple-like delays far from the car itself. For example when a police car is driving inside the traffic flow. But if most of the cars are following the rules, like 95% of them, then one law abiding Waymo would fit just fine. In EU, with the deluge of speed traps and mobile patrols, most of the cars are driving under the limit, and honestly it feels fine. I'm originally from a country where +20 above was like a norm, and fast cars were +40 or more, so adjusting to EU took some effort. But now I don't even feel the need to speed, especially if it is 140km/h highway (86 m/h)