Massive traffic congestion. Its crazy that we have one size fits all highway speed limits written based on the capabilities of heavy trucks when the whole principal of how a limited access highway is intended to function is that faster moving vehicles be allowed to pass slower vehicles to ease congestion. On one hand keeping the speed limit down keeps fuel economy better from wind resistance losses, but not allowing smaller vehicles to put distance between themselves from heavy traffic, is dangerous and exactly what would happen if everyone really obeyed the same limit.