$500 billion (2008 dollars)
The longest East-West Interstate Highway is I-90, which travels from Boston to Seattle, a distance of 3,100 miles. The longest in the North-South direction is I-95. That Interstate extends from the Canadian border southward to Miami, a total of 1,927 miles.
While most Interstate Highways have exit signs, the scheme for identifying the exits varies from state to state. For instance, in most states the exit number also identifies the number of miles traveled on that Interstate. In those states the exit number (which is also the mileage count) will increase as one travels from the West to the East but for Interstates aligned in a North-South direction (or odd numbered), the exit number will increase as one travels from the South to the North. In the Northeast, the states label the exit numbers sequentially without any reference to the number of miles traveled. These states include Connecticut, Massachusetts, Rhode Island, New Hampshire, New York, Vermont, New Hampshire, and Delaware.
The initial construction of the Interstate, as proposed in 1956, was completed in September of 1991 and at the time had a total length of approximately 40,000 miles.
Actual Cost to build the Interstate Highway System was $114 Billion over 35 years ago, and $500 billion in 2008 dollars.
Today, more than half of the costs for construction and maintenance of Interstate Highways comes primarily from gasoline taxes and tolls; the balance being paid by the federal government. However, as the cost of maintaining the present Interstates as well as constructing new ones has grown significantly, it is highly likely that many primary Interstate Highways will be converted into turnpikes (toll roads) to help defray this growing expense.
Whatever the costs, however, the Interstate Highway System has proven to be instrumental in transforming the country, its economy, and American society. It has grown in proportion to our dependency on being mobile by providing us all with the means to travel longer distances quickly and inexpensively. Also, to take advantage of this greater mobility the Interstate Highway System made it possible for a whole range of markets, carriers, and industries to emerge. And finally, as the number of industries grew so did the number of auxiliary Interstate Highways which in turn instigated the suburbanization of our major cities.