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Standards for Interstate Highways 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, it must meet these construction requirements or obtain a waiver from the Federal Highway Administration.
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Standards
These standards are (as of August 2003):
Access control (from adjacent properties) should extend at least 100 ft (30 m) in urban areas and 300 ft (90 m) in rural areas in each direction along the crossroad from the ramps.
Maximum grade. Maximum grade is determined by a table, with up to 6% allowed in mountainous areas and hilly urban areas.
Minimum lane width. Minimum lane width of 12 ft (3.6 m).
Shoulder width. Minimum outside paved shoulder width of 10 ft (3.0 m) and inside shoulder width of 4 ft (1.2 m). With three or more lanes in each direction, the inside paved shoulder should be at least 10 ft (3.0 m) wide. If truck traffic is over 250 Directional Design Hour Volume, shoulders at least 12 ft (3.6 m) wide should be considered. In mountainous terrain, 8 ft (2.4 m) outside and 4 ft (1.2 m) inside shoulders are acceptable, except when there are at least four lanes in each direction, in which case the inside shoulders should also be 8 ft (2.4 m) wide.
Pavement sloping. Pavement cross slope of at least 1.5% and preferably 2% to ensure proper drainage on straight sections. This can be increased to 2.5% in areas of heavy rainfall. Shoulder cross slope should be between 2% and 6% but not less than the main lanes.
Land slopes within the clear zone should be at most 4:1 and preferably 6:1 or flatter. Roadside barriers should be used for slopes of 3:1 or steeper, in accordance with the current edition of AASHTO's Roadside Design Guide.
Median width. Minimum median width of 36 ft (11 m) in rural areas, and 10 ft (3.0 m) in urban or mountainous areas. To prevent median-crossing accidents, guardrail should be installed in medians in accordance with the current edition of AASHTO's Roadside Design Guide, based on traffic, median width and crash history. When possible, median openings between parallel bridges less than 30 ft (9.0 m) in width should be decked over; otherwise barriers or guardrails should be installed to keep vehicles out of the hole.
Recovery areas. No fixed objects should be in the clear recovery area, determined by the design speed in accordance with the current edition of AASHTO's Roadside Design Guide. When this is not possible, breakaway supports or barriers guarding the objects shall be used.
Curb slope. Vertical curbs are prohibited. Sloping curbs are to be at the edge of the paved shoulder, with a maximum height of 100 mm (4 in). The combination of curbs and guardrail is discouraged; in this case the guardrail should be closer to the road than the curb.
Vertical clearance. Minimum vertical clearance under overhead structures (including over the paved shoulders) of 16 ft (4.9 m) in rural areas and 14 ft (4.3 m) in urban areas, with allowances for extra layers of pavement. Through urban areas at least one routing should have 16 ft (4.9 m) clearances. Sign supports and pedestrian overpasses must be at least 17 ft (5.1 m) above the road, except on urban routes with lesser clearance, where they should be at least 1 ft (0.3 m) higher than other objects. Vertical clearance on through truss bridges is to be at least 17 ft (5.1 m).
Horizontal clearance under or along a bridge shall be the full paved width of the rest of the road. Bridges longer than 200 ft (60 m) can be narrower, with a minimum of 4 ft (1.2 m) on both sides of the travel lanes.
Bridge strength. New bridges are to have at least MS 18 (HS-20) structural capacity. Weaker bridges that can continue to serve the route for 20 more years are allowed to remain.
Additionally, existing bridges can remain if they have at least 12 ft (3.6 m) lanes with 10 ft (3.0 m) outside and 3.5 ft (1.1 m) inside shoulders. Long bridges are to have at least 3.5 ft (1.1 m) on each side of the travel lanes; bridge railing should be upgraded to current standards if necessary.
Tunnel clearance. Tunnels should in theory be equivalent to long overcrossings, but because of cost the standards can be reduced. Vertical clearance is the same as under bridges, including the provision for an alternate routing. Width should be at least 44 ft (13.1 m), which consists of two 12 ft (3.6 m) lanes, 10 ft (3.0 m) outside and 5 ft (1.5 m) inside shoulders, and 2.5 ft (.7 m) safety walkways on each side. If necessary to meet the dimensions of the approach, this can be shifted left or right. A reduced width is acceptable due to high costs. In this case, the minimum width is 30 ft (9.0 m), with at least 2 ft (0.6 m) more than the approach for the sum of the shoulder widths, but at least 24 ft (7.2 m) total, and at least 1.5 ft (0.5 m) on each side for a safety walkway. If there is no safety walkway, a 3 ft (1.0 m) offset with a "safety shape" in the wall is acceptable.
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Exceptions
The standards have been changed over the years, resulting in many older Interstates not being built to the current standards. Other roads were grandfathered into the system, and yet others are not built to standards because to do so would be too costly or environmentally unsound. Even though a handful of Interstate highways have substandard elements, many freeways with non-Interstate designations are up to Interstate standards. Freeways in urban cores with jersey barriers are allowed in the system if median width is a constraint.
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