Interstate Design Standards

• Access must be controlled, with points of entry limited to interchanges with grade separation. (There are several exceptions to this rule.) Interchanges should be spaced one mile apart in urban areas and three miles apart in rural areas.
• All overpasses must have a 16.5-foot vertical clearance above the freeway, although 14-foot overpasses may be permissible within urban areas. This particular specification was created to allow for military apparatus (such as the huge atomic cannon) to pass cleanly under an overpass. Although military equipment of this size is not commonly transported on the Interstate Highway System (and the atomic cannon is no longer in use), the standard remains. If an overpass cannot be upgraded to accommodate 16.5 feet, then there must be exit and re-entry ramps that allow high profile vehicles to leave the freeway and rejoin the freeway on the other side of the overpass. Truss bridges and overhead signs must meet or exceed 17 feet. Standards may be reduced somewhat for tunnels or other long underpasses.
• Interstates must be constructed with at least two 12-foot lanes of traffic in each direction. If level of service requirements specify additional lanes for the particular section of freeway, then those additional lanes become part of the standards (standard since 1966).
• Right shoulders must be at least 10 feet wide; left shoulders must be at least four feet wide.
• Median width should be 36 feet in rural areas and 10 feet in mountainous or urban areas. Guardrail or jersey curb (K-rail) may be placed between lanes of traffic to reduce the required width of urban Interstate highways.
• Vertical curbs are prohibited unless they are at the edge of the right shoulder and are sloping in nature (no more than four inches tall).
• Design speed should generally be 70 miles per hour, with 60 miles per hour sections allowed in hilly terrain and 50 miles per hour sections allowed in urban areas or within mountainous regions.
• Maximum grade is six percent, with certain exceptions allowed in mountainous regions.
• Interstates are generally open to all traffic, unless a specific waiver exempting certain vehicles is granted.
• At-grade railroad crossings are not permitted on the Interstate Highway System (standard since 1966).

All of this information may be found in detail in the American Association of State Highway and Transportation Organization (AASHTO) publication "A Policy on Design Standards - Interstate System." It contains much more detail and explanation on these design standards commonly referred to as Interstate standards. These standards are also summarized on the webpage Celebrating the 50th Anniversary of the Eisenhower Interstate Highway System: Previous Interstate Facts of the Day by the Federal Highway Administration (FHWA) - see the dates April 11-14, 2006. There are plenty of exceptions to Interstate standards found on the existing system, some of which were allowed into the system despite the deficiencies. Other routes were added with a waiver on a certain standard.