The Interstate Highway System has become central to transportation in the United States. It extends over 46,000 miles in length and includes 210,000 lane miles. The System carries over 24 percent of all vehicle miles traveling in the nation, while making up just 2.5 percent of total lane miles. Funding for the Interstate Highway System has been a major part of total highway funding since 1954 when interstate highway construction began. From 1954 through 2001, federal funding for interstates total over $370 billion (2001 dollars)--46 percent of all apportionments administered by the Federal Highway Administration (FHWA) during this period. Congestion on interstate highways has increased over the last decade; the physical condition of interstate highways has generally improved, and the level of safety has remained steady. Some of the factors states expect to negatively affect the conditions of their interstate highways in the future include increases in passenger and freight traffic, aging infrastructure, and financial constraints. FHWA's estimates of future annual interstate highway investment requirements vary depending on the goal transportation officials have for performance of the interstate system. In 2000, GAO evaluated the model that FHWA uses to forecast interstate and other highways' pavement preservation and highway capacity requirements and found that this model can be useful as a general guide for assessing relative investment requirements over time. However, the model has some limitations; it does not fully account for uncertainties associated with its method, data, and assumptions.