Interstate Highway standards

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The American Association of State Highway and Transportation Officials (AASHTO) defines standards for Interstate Highways in their publication A Policy on Design Standards - Interstate System. For a highway to be an Interstate, it must be built to these standards or obtain a waiver from the Federal Highway Administration.

These standards are (as of August 2003):

All access is to be controlled with interchanges and grade separations (including railroad crossings). See List of gaps in Interstate Highways for the few cases that violate this rule. Interchanges should provide full access; ramps are to be designed with the appropriate standards in mind. Minimum interchange spacing should be 1.5 km (1 mi) in urban areas and 5 km (3 mi) in rural areas; collector-distributor roads or other configurations that reduce weaving can be used in urban areas to shorten this distance.
- Access control (from adjacent properties) should extend at least 30 m (100 ft) in urban areas and 90 m (300 ft) in rural areas in each direction along the crossroad from the ramps.
Minimum design speed of 110 km/h (70 mph) in rural areas, with 100 km/h (60 mph) acceptable in rolling terrain, and as low as 80 km/h (50 mph) allowed in mountainous and urban areas.
- Sight distance, curvature and superelevation according to the current edition of AASHTO's A Policy on Geometric Design of Highways and Streets for the design speed.
Maximum grade determined by a table, with up to 6% allowed in mountainous areas and hilly urban areas.
At least two lanes in each direction, and more if necessary for an acceptable level of service in the design year, according to the current edition of AASHTO's A Policy on Geometric Design of Highways and Streets. Climbing lanes and emergency escape ramps should be provided where appropriate.
Minimum lane width of 3.6 m (12 ft).
Minimum outside paved shoulder width of 3.0 m (10 ft) and inside shoulder width of 1.2 m (4 ft). With three or more lanes in each direction, the inside paved shoulder should be at least 3.0 m (10 ft) wide. If truck traffic is over 250 Directional Design Hour Volume, shoulders at least 3.6 m (12 ft) wide should be considered. In mountainous terrain, 2.4 m (8 ft) outside and 1.2 m (4 ft) inside shoulders are acceptable, except when there are at least four lanes in each direction, in which case the inside shoulders should also be 2.4 m (8 ft) wide.
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.
Minimum median width of 11 m (36 ft) in rural areas, and 3.0 m (10 ft) 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 9.0 m (30 ft) in width should be decked over; otherwise barriers or guardrails should be installed to keep vehicles out of the hole.
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.
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.
Minimum vertical clearance under overhead structures (including over the paved shoulders) of 4.9 m (16 ft) in rural areas and 4.3 m (14 ft) in urban areas, with allowances for extra layers of pavement. Through urban areas at least one routing should have 4.9 m (16 ft) clearances. Sign supports and pedestrian overpasses must be at least 5.1 m (17 ft) above the road, except on urban routes with lesser clearance, where they should be at least 0.3 m (1 ft) higher than other objects. Vertical clearance on through truss bridges is to be at least 5.1 m (17 ft).
Horizontal clearance under or along a bridge shall be the full paved width of the rest of the road. Bridges longer than 60 m (200 ft) can be narrower, with a minimum of 1.2 m (4 ft) on both sides of the travel lanes.
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 3.6 m (12 ft) lanes with 3.0 m (10 ft) outside and 1.1 m (3.5 ft) inside shoulders. Long bridges are to have at least 1.1 m (3.5 ft) on each side of the travel lanes; bridge railing should be upgraded to current standards if necessary.
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 13.1 m (44 ft), which consists of two 3.6 m (12 ft) lanes, 3.0 m (10 ft) outside and 1.5 m (5 ft) inside shoulders, and .7 m (2.5 ft) 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 9.0 m (30 ft), with at least 0.6 m (2 ft) more than the approach for the sum of the shoulder widths, but at least 7.2 m (24 ft) total, and at least 0.5 m (1.5 ft) on each side for a safety walkway. If there is no safety walkway, a 1.0 m (3 ft) offset with a "safety shape" in the wall is acceptable.

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.

edit Primary Interstate Highways
4	5	8	10	12	15	16	17
19	20	22	24	25	26	27	29
30	35	37	39	40	43	44	45
49	55	57	59	64	65	66	68
69	70	71	72	73	74	75	76 (W)
76 (E)	77	78	79	80	81	82	83
84 (W)	84 (E)	85	86 (W)	86 (E)	87	88 (W)	88 (E)
89	90	91	93	94	95	96	97
99	238	H-1	H-2	H-3
Unsigned Interstate Highways
A-1	A-2	A-3	A-4	PRI-1	PRI-2	PRI-3
Lists Two-digit Interstates - Three-digit Interstates Gaps in Interstates - Intrastate Interstates Interstate standards - Proposed Interstates