You’re not American, are you? The number of roads marked 55 mph on which nearly every vehicle is moving 75 mph is very high. Driving under the speed limit would be hazardous to yourself and everyone else.
Other countries tend to follow the 85th percentile for setting speed limits, so driving under the speed limit is actually safe there. People in them do not realize that a road that would be 140 km/h in Europe is 90 km/h in the U.S.
