On the stretch of motorway that I frequent in Italy, the speed limit is mostly 130 km/h, but the majority of people drive at about 100 to 110 km/h (including me).

But there are also people who drive in the left lane, who will tailgate you at 1 or 2 meters because you're doing 130 km/h. These people are idiots, but you get these sorts of people everywhere.

On American freeways, you don't have a choice, every lane is doing about 10 mph over the limit (or in LA way under) and it is disruptive or dangerous not to. These freeways tend to be running at full capacity so it actually makes sense since it improves capacity.

Road capacity does not increase with speed above 50 km/h on urban roads or 70 km/h on highways. Following distance scales with speed, so more speed can actually mean fewer cars per unit of time.

In theory, braking distance scales quadratically with speed. In practice, people leave less room on highways, because they rely on others driving predictably, but spacing still increases faster than linear.