Interstate Highway standards

Standards for Interstate Highways in the United States are defined by the American Association of State Highway and Transportation Officials (AASHTO) in the publication A Policy on Design Standards: Interstate System. For a certain highway to be considered an Interstate Highway, it must meet these construction requirements or obtain a waiver from the Federal Highway Administration.

These standards are, as of July 2007^[update], as follows:

The standards have been changed over the years, resulting in many older Interstates not conforming to the current standards, and yet others are not built to standards because to do so would be too costly or environmentally unsound.

Some roads were grandfathered into the system. Most of these were toll roads that were built before the Interstate system came into existence or were under construction at the time President Dwight D. Eisenhower signed the Federal Aid Highway Act of 1956. The most notable example is the Pennsylvania Turnpike, which originally had a very narrow median that later required the installation of a Jersey barrier due to heavy traffic loads.

Interstate 35E through St. Paul, Minnesota is an example of a freeway that was not grandfathered into the system that is nonetheless an exception to standards. The freeway was not opened until 1990, has a speed limit of 45 mph (70 km/h), and does not allow vehicles weighing over 9,000 pounds (4,100 kg) GVW. This is due to a number of lawsuits from wealthy surrounding homeowners, which heavily delayed and modified the project dating to the 1960s.

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