[gjm11]

Traffic flows more smoothly when everyone is going at about the same speed. To achieve this without communication between vehicles, you need some "obvious" speed for everyone to try to drive at. In many contexts the speed limit fits this need pretty well.

There are situations where it doesn't. These are mostly also situations where the speed limit doesn't work terribly well as a limit either. For instance, bad weather or winding roads may mean that driving at or near the limit is unsafe. In that case, sure, it's not good for everyone to coordinate on driving at the speed limit, but it's also not good for everyone to think of the speed limit as the right upper bound for driving to be safe.

So I think that in most situations either (1) treating the speed limit as a rough target as well as a limit is mostly a good idea, or (2) the speed you try to use as a limit should be something other than the nominal speed limit.

[mitthrowaway2]

Vehicles can all synchronize speed independently based on a well-tuned control law while observing only the distance to the vehicle in front of them. So looking out the window is sufficient for that, if the drivers are not terrible.

If driving at the speed limit on a winding road is unsafe, the speed limit is probably too high for that road. But in practice, it's complex, winding roads that cause people to drive at reasonable speeds, rather than numbers posted on signs.