55 MPH Speed Limit
The federally mandated 55 MPH speed limit failed at every goal: it was instituted to increase safety and save oil. It only saved 1/4th the oil they promised ($350M), but it cost $3.65B in enforcement. We went from 70% compliance of speed limits to 90% non-compliance, no difference in accidents or fatalities (in fact when we raised speed limits later, accident rates declined). It caused a bitter legal fight against the unconstitutional overreach and was eventually abandoned after many millions of dollars and decades of court time wasted on that.