cars

U.S. streets have been filled with traffic since the country’s founding — first with wagons and livestock and now with more than 250 million autos and trucks. Building more and wider roads can reduce congestion, but the benefits are generally temporary: Vehicles soon fill new lanes, and the cycle starts all over again. The massive highway boom after World War II did speed cross-country travel, but it also added suburban congestion to the list of pressing national problems.

U.S. highways swarm with vehicles of every size, from 1,800-pound economy cars to tractor-trailers that can reach 80,000 pounds. Until recently, passenger vehicles had been becoming increasingly heavy — between 1980 and 2006, average curb weight rose 26%.