Published On: July 2nd, 2009

by Jerry Hall and Loretta Hall

In one sense, the Interstate System is small. Roughly 2.5 percent of the public road mileage in the United States is eligible for federal-aid highway funding, and the Interstate System represents only 4.8 percent of those eligible roads. In most senses, though, the Interstate System is impressively large. Along its 46,800-mile length, it includes 55,500 bridges, 82 tunnels consisting of 104 bores, and about 14,750 interchanges. It carries about 728 billion vehicle-miles of travel annually.

The total travel on the Interstate System during its first 50 years was roughly equivalent to a trip to the moon for every person living California, New York, Texas, and New Jersey. Summing all of these trips into one, they are equivalent to 3 light years through space, covering three-fourths of the distance to our nearest neighboring star, Alpha Centauri.

The longest Interstate route, the 3,081-mile-long I-90, Connects Seattle to Boston. The route passing through the largest number of states (16) is I-95, which traverses 1,920 miles from Miami to the Canadian border in Maine; at a cost of $8 billion, it is also the most expensive route. The state with the most Interstate routes is New York, with 29 routes totaling 1,675 miles. The shortest route is I-97, which connects Baltimore and Annapolis; it never leaves Anne Arundel County.

Two western states have the largest total Interstate lengths: Texas with 3,233 miles and California with 2,456 miles. The next largest total mileage are in three states: Illinois, Ohio, and Pennsylvania. Compared with the combination of Texas and California, those three states have a total of 97 percent of the combined mileage, but only one-third the land area and two-thirds the population.

The northernmost points on the Interstate System are at the Canadian border: I-5 near Blaine, Washington;  I-15 at Sweetgrass, Montana; and I-29 at Pembina, North Dakota. The southernmost point is on Interstate H-1 in Honolulu. For the continental US, the southernmost point is at the end of I-95 in Miami. The easternmost point is at the other end of I-95 at the Canadian border near Houlton, Maine. The westernmost point is the end of H-1 in Kapolei, Hawaii. For the 48 contiguous states, the westernmost point is on I-5 near Wolf Creek, Oregon.

The highest elevation on the Interstate System (11,158 feet) is at the Eisenhower Tunnel, where I-70 crosses the Continental Divide, 60 miles west of Denver. The lowest elevation (107 feet below sea level) is in the Fort McHenry Tunnel, where I-95 crosses under Baltimore Harbor. The lowest dry-land elevation (-52 feet) is on I-8 near Seeley in California’s Imperial County, about 15 miles from the Mexican border.

Between 1958 and 1989, the total cost estimate for the Interstate System increased 37 percent. Inflation accounted for an increase of 13.8 percent, and initial underestimation of the new system amounted to an increase of less than 10 percent. Nearly half of the higher estimate (10.7 percent of total cost) was due to system additions and new requirements for aspects such as safety, relocation, and environmental accommodation.

Actual construction accounted for 82 percent of the cost of the Interstate System (the rest went to activities such as right-of-way acquisition and preliminary engineering). Building the system took about 2.4 billion man-hours of time. Construction used 300 million cubic yards of concrete (including 2.5 billion tons of aggregate), 27 million tons of bituminous material such as tar and asphalt, and 1.5 billion board feet of lumber (mostly for concrete forms and bridge pilings). About 1.8 million acres of right-of-way have been acquired; right-of-way fencing amounts to enough to circle the globe two times.

The first four-lane, border-to-border Interstate route was completed in October 1966 when Oregon opened its final section of I-5. The Interstate route shaved two hours off the time needed to drive through Oregon, compared with the former main route, US 99.

The last traffic signal to be removed from the path of an Interstate route was ceremonially buried in Wallace, Idaho on September 14, 1991. A new viaduct eliminated the need for the signal on a cross-street.

With all the careful planning involved in creating the Interstate System, some things just worked out nicely because of chance. For example, one of the two Interstate routes passing through Philadelphia just happens to be designated as I-76.

In 1919 future President Dwight Eisenhower participated in a US Army convoy that drove from Washington, DC to Oakland, California in 62 days. In 2006 (the 50th anniversary of the Interstate system), a reenactment of that trip (traveling in the opposite direction) took 14 days, including stops for anniversary celebrations in 18 cities. According to, the travel time for a non-ceremonial version of that trip is now 42 hours and 8 minutes.

Jerry Hall, a professor of Civil Engineering at the University of New Mexico, has served District 6 as president and international director. Loretta Hall, a member of the Construction Writers Association, is a freelance writer concentrating on engineering and construction. They can be contacted at and, respectively.

This is the fourteenth in a series of articles tracing the development of the Interstate Highway System.

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