Meanwhile, the costs of building and maintaining highway and transit systems have increased significantly, and rising fuel efficiency has reduced gasoline tax revenues per mile driven. As a result, the purchasing power of the federal gas tax declined by about 30% from 1997 to 2011.
公路信托基金的资金来源是联邦汽油税，这也是联邦政府最为重要的交通融资源。自1993年以来，该税的名义标准一直是每加仑18.4美分。与此同时，建设和维护高速公路和运输体系的成本有了极大的增长，而能源效率的提高也降低了每英里汽油税收入。结果是，从1997年到2011年，该联邦税的购买力下降了 30%左右。