Figuring out how to reduce congestion and emissions has long been a thorny issue for the car-centric United States. One possible solution - high-occupancy vehicle lanes (HOV), or carpool lanes - have their own thorny history. Despite pushes from the federal government for more HOV lanes in the 1970s and 1990s, the math of how HOV lanes work may mean they relieve traffic only a little - or even make it worse.