WASHINGTON -- The federal interstate highway system is showing its age, and, faced with the cost of repairing all those bumps and cracks, some states want to ask motorists to pay tolls on roads that used to be free. When the interstate system was created in 1956, a federal per-gallon gasoline tax was enacted to provide a stream of revenue for the Highway Trust Fund. The federal government paid 90 percent of the construction costs, with the states making up the rest. That model worked for decades, but no longer. Americans are driving less because of the economy and higher...