Source: Federal Highway Administration.

Length of the Interstate Highway System, 1959-2006

From its inception, the Interstate highway system expanded substantially, but at a declining rate as the system reached its planned size (46,000 miles). By 1991, the system was considered completed, almost to the initial plan, with a total cost of about 129 $US billion. Between 1954 and 2001, 370 $US billion were invested by the federal government in the construction and the maintenance of the system. However, the Interstate is facing diminishing returns due to high construction and maintenance costs, which is forcing many state governments to consider privatization of several highway segments. Construction costs went from 4 million $US per mile in 1959 to 20 million $US in 1979. Still, the system has returned more than $6 US in economic productivity for each dollar it cost, placing it at the core of American economic productivity in the second part of the 20th century.