Directions

• Even Numbers travel east-west (I-4, I-8, I-10, I-12, etc.)
• Odd Numbers travel north-south (I-5, I-15, I-17, I-19, etc.)

Significance

• Routes ending in “0” are major east-west routes (I-10, I-20, I-40, I-70, I-80, I-90)
• Routes ending in “5” are major north-south routes (I-5, I-15, I-25, I-35, I-55, I-65, I-75, I-85, I-95)

Orientation

• Lower numbered routes are generally located in the south and west
• Higher numbered routes are generally located in the north and east
• As a result, the Interstate Highway system forms a grid with numerical designations increasing gradually from low to high – both from west to east and south to north.
• Interstate Highways were numbered so that they would not conflict with the preexisting U.S. Numbered System; additionally the intent was that no Interstate Highway and U.S. Route would share the same number within the same state.
• I-24 and U.S. 24 currently both exist in Illinois.
• Plans for extensions to I-49, I-69, and I-74 will result in those routes meeting and intersecting their U.S. Highway counterparts.
• Interstate 50 and Interstate 60 were not assigned so as to avoid conflicts with U.S. 50 and U.S. 60 in the central part of the country.
• Some Interstate Highways are Unsigned, including a variety of spur and loop routes that are often superfluous or confusing designations in addition to the primary route number.
• Loop routes and through routes that generally connect to an Interstate highway at either end have an even first digit:
• Full 360-degree beltways include I-270 OH, I-275 KY/OH/IN, I-285 GA, I-410 TX, I-465 IN, I-495 MD/VA, I-610 TX, I-695 MD.
• I-275 in KY, IN, and OH is the only three-digit Interstate currently to serve three states.
• I-495 (Capital Beltway) serves Virginia and Maryland, but a portion of the route passes through the District of Columbia while crossing the Potomac River at the Woodrow Wilson Bridge.
• I-271 and I-480 merge together briefly east of Cleveland, marking the only location where two three-digit Interstate routes merge together.
• The following routes have an even first digit but do not end at another Interstate highway on one end: I-264 VA, I-295 NY, I-478 NY, I-495 NY, I-678 NY, I-635 TX, I-878 NY.
• The following routes are examples of intercity routes, acting in a regional capacity: I-280 CA, I-476 PA, I-495 MA, I-680 CA, I-684 NY/CT
• I-238 in CA acts as a spur of I-80, connecting I-880 and I-580 in the San Francisco Bay Area. At the time it was commissioned, no more I-x80 route numbers were available for use, so the designation of adjacent CA 238 was used to number I-238.
• Some even-prefixed three-digit routes serve states that are not served by their parents, including I-275 OH/KY/IN and I-287 NJ/NY.
• Spur routes and city routes that may or may not connect to an Interstate highway at one end have an odd first digit:
• I-585 in SC does not connect to its parent (I-85) currently; it begins at Business Loop I-85 and ends in Downtown Spartanburg.
• The following routes are examples of intercity routes, acting in a regional capacity: I-380 PA, I-385 SC, I-505 CA.
• Some odd-prefixed three-digit routes serve states that are not served by their parents, including I-129 NE/IA and I-535 MN/WI.
• Some Interstates have state route extensions, including I-15 in San Diego (continues as CA 15), I-265 in New Albany (continues as IN 265), I-381 in Bristol (continues as VA 381), I-481 in Syracuse (continues as NY 481), I-690 in Syracuse (continues as NY 690), and I-794 in Milwaukee (continues as WI 794/Lake Parkway).
• Some proposed future Interstate corridors have state route designations, or were previously numbered as such. Some examples include I-840 near Nashville and CA 905 in San Diego.
• Still other state routes have numbers that may make it look like a future Interstate corridor but are not, including VT 191 in Newport and IL 394 near Chicago Heights.

• The Interstate Highway shield was designed by Richard Oliver of Texas as a black and white shield; the red, white, and blue version was approved by the American Association of State Highway Officials (AASHO) in 1957. It is trademarked.
• The Federal-Aid Highway Act and Department of Transportation Act passed by Congress in 1966 mandated that all Interstates be four lanes. Previously daily traffic counts determined whether an Interstate would be two or four lanes.¹
• Four state capitals are not served by the Interstate Highway System as of 2006: Juneau, Alaska; Dover, Delaware; Jefferson City, Missouri; and Pierre, South Dakota.
• It is not true that one mile of every five miles of Interstate highway must be straight enough to allow planes to land on it. This is an urban legend.
• Previously with one exception (I-65), Interstate 64 overlaps with every two-digit interstate it meets from Wentzville, Missouri to Hampton Roads, Virginia. I-64 overlaps with I-55, I-57, I-71, I-75, I-77, I-81 and I-95. Prior to 2014, I-64 also overlapped with I-70 between St. Louis, Missouri and East St. Louis, Illinois.

Sources:

1. "Timeline outlines Montana Department of Transportation history." Montana Standard, June 16, 2013.

• 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.

Shortest Routes:

• I-878 New York – shortest unsigned route
• I-375 Michigan – shortest signed route

Longest Routes:

• I-90
• I-80
• I-40
• I-70
• I-10

• Highest Point: Interstate 70 at Eisenhower Tunnel in Colorado, 11,192 feet above sea level
• Lowest Point (excluding tunnels): Interstate 8 near El Centro in California, approximately 52 feet below sea level

Suspension Bridges

• I-29 and I-35 – Paseo Bridge over the Missouri River, Kansas City, Missouri – replaced in 2010 with a cable-stay bridge
• I-74 – Iowa-Illinois Memorial Bridge – Quad Cities – replaced with a basket-handle, true-arch twin bridge fully opened to traffic on December 2, 2021.
• I-75 – Mackinac Bridge, Michigan
• I-76 – Walt Whitman Bridge spanning the Delaware River, Pennsylvania-New Jersey
• I-80 – San Francisco-Oakland Bay Bridge linking San Francisco with Yerba Buena Island in California
• I-80 – San Francisco-Oakland Bay Bridge connecting Yerba Buena Island with Oakland, California. The single-anchored suspension span fully opened to traffic on September 2, 2013.
• I-80 – Alfred Zampa (Carquinez) Bridge (westbound only), California
• I-95 – George Washington Bridge, New Jersey-New York
• I-278 – Robert F. Kennedy (Triborough) Bridge, Bronx and Queens New York
• I-278 – Verrazzano-Narrows Bridge, connecting Staten Island with Brooklyn in New York City
• I-295 – Delaware Memorial Bridge, Delaware-New Jersey
• I-676 – Benjamin Franklin Bridge, Pennsylvania-New Jersey

Cable Stay Bridges

• I-29 and I-35 – Christopher S. Bond Bridge – Kansas City, Missouri
• I-93 – Leonard P. Zakim and Bunker Hill Bridge – Downtown Boston, Massachusetts (Big Dig)
• I-95 – New Haven Harbor Crossing Pearl Harbor Memorial Bridge or the “Q Bridge” across the Quinnipiac River at New Haven, Connecticut. Two extradosed cable stayed bridges, opened in 2012 and 2013.
• I-275 – Sunshine Skyway – St. Petersburg, Florida
• I-278 – Goethals Bridge – Staten Island, New York, fully opened on May 21, 2018.
• I-278 – Kosciuszko Bridge – Brooklyn, New York, fully opened on August 28, 2019.
• I-280 – Veterans Glass City Skyway – Toledo, Ohio, opened June 24, 2007
• I-287 – Governor Mario N. Cuomo Bridge, Tarrytown, New York, fully opened on September 12, 2018.
• I-295 – Varina-Enon Bridge, spanning the James River southeast of Richmond, Virginia
• I-295 – N.B. Broward (Dames Point) Bridge, crossing the St. Johns River in Jacksonville, Florida
• I-310 – Hale Boggs Bridge, crossing the Mississippi River Bridge between Luling and Destrehan, Louisiana

Tied Arch Bridges

• I-24, Paducah Bridge over the Ohio River near Metropolis, Illinois
• I-24, Tennessee River Bridge in Livingston County, Kentucky
• I-39, Abraham Lincoln Bridge over the Illinois River in Illinois
• I-43, Leo Frigo Memorial (Tower Drive) Bridge over the Fox River in Green Bay, Wisconsin
• I-65, Mobile and Tensaw River bridges in Alabama
• I-87, Thaddeus Kosiusko Bridge over the Mohawk River in New York
• I-255, Jefferson Barracks Bridge over the Mississippi River between Missouri and Illinois
• I-275, bridge between Kentucky and Indiana over Ohio River
• I-279, Fort Duquesne Bridge in Pittsburgh, Pennsylvania
• I-279 and I-376, Fort Pitt Bridge in Pittsburgh, Pennsylvania
• I-280, Mississippi River Bridge near Quad Cities between Illinois and Iowa
• I-471, Ohio River bridge (“Big Mac Bridge”) connecting Newport, Kentucky, with Cincinnati, Ohio
• I-794, Hoan Bridge in Milwaukee, Wisconsin along the lakefront

Steel Through Arch Bridges

• I-40, bridge over Mississippi River near Memphis, Tennessee
• I-57, Mississippi River Bridge – Cairo, Illinois
• I-64, Sherman Minton Bridge over Ohio River – New Albany, Indiana, and Louisville, Kentucky
• I-95, St. Johns River Bridge – Jacksonville, Florida

Drawbridges

• I-5 – Interstate Bridge over Columbia River – Oregon-Washington
• I-64 – Elizabeth River Bridge – Virginia
• I-95 and I-495 – Woodrow Wilson Bridge over Potomac River, Virginia-D.C.-Maryland (original span opened in 1961; two replacement spans each with six lanes opened in 2006 and 2008 – the new span only requires 65 openings per year rather than the 260 openings required annually by the old bridge)
• I-110 – Back Bay Bridge in Biloxi, Mississippi
• I-264 – Elizabeth River in Norfolk, Virginia
• I-278 – Bronx River Bridge near I-895 – New York City
• I-280 – Passaic River Bridge – Newark, New Jersey
• I-280 – Maumee River Bridge – Toledo, Ohio. Replaced on June 24, 2007, with the cable-stay Veterans Glass City Skyway; the drawbridge became a part of State Route 65.
• I-695 – Curtis Creek Bridge – Baltimore, Maryland

• I-10 in West Texas between milepost 66 and Van Horn (Exit 138) with ranch roads in remote locations.
• I-25 in Wyoming has right in right out (RIRO) intersections with ranch roads south of Buffalo.
• I-40 west of Albuquerque, New Mexico has an at-grade intersection with a ranch road between milepost 134 and 135.
• I-40 in western North Carolina has several at-grade intersections with unpaved roads (including access points for Hurricane Road and an access road to Walters Dam)
• I-70 in Breezewood, Pennsylvania – to make the connection from I-70 to the Pennsylvania Turnpike, through traffic must use U.S. 30 through Breezewood, passing through several signalized intersections before returning to controlled access freeway.
• I-78 in Jersey City, New Jersey – between the western end of the New Jersey Turnpike Extension and the entrance to the Holland Tunnel, I-78 passes through several signalized intersections.
• I-80 at Columbia, New Jersey. Right in right out (RIRO) intersection with Hainsburg Road from the westbound roadway.
• I-180 in Cheyenne, Wyoming – the entire route is at-grade expressway with five signalized intersections: two at I-80, two at city streets, and one at Business Loop I-80.
• I-585 in Spartanburg, South Carolina – the northern section between Business Loop I-85 and I-85 is partially at-grade.
• I-676 in Philadelphia, Pennsylvania – the through connection between the Benjamin Franklin Bridge and the Vine Street Expressway navigates through a signalized intersection.

• I-15 – managed lanes segment of Escondido Freeway in San Diego
• I-30 – Dallas-Fort Worth Tollway (formerly tolled; originally signed as I-20)
• I-35 – Kansas Turnpike
• I-39 – Northwest Tollway – Rockford, IL
• I-44 – Oklahoma: H.E. Bailey Turnpike, Turner Turnpike, Will Rogers Turnpike
• I-64 – West Virginia Turnpike
• I-65 – Kentucky Turnpike (formerly tolled)
• I-70 – Pennsylvania Turnpike (on shared alignment with Interstate 76), Kansas Turnpike
• I-75 – Alligator Alley (Florida); Mackinac Bridge (Michigan)
• I-76 (Eastern) – Ohio Turnpike, Pennsylvania Turnpike, Walt Whitman Bridge
• I-77 – West Virginia Turnpike
• I-78 – Delaware River Bridge (westbound only), Holland Tunnel (eastbound only)
• I-80 – Delaware River Bridge (westbound only), Ohio Turnpike, Indiana Toll Road, Tri-State Tollway, Carquinez Bridge (northbound only), San Francisco-Oakland Bay Bridge (westbound only)
• I-84 – Hudson River Bridge (eastbound only)
• I-87 – New York Thruway
• I-88 – Illinois East-West Tollway
• I-90 – Massachusetts Turnpike, New York Thruway (Mainline and Berkshire Section), Ohio Turnpike, Indiana Toll Road, Chicago Skyway, Illinois Northwest Tollway
• I-93 – F.E. Everett Turnpike
• I-94 – Tri-State Tollway
• I-95 – Fort McHenry Tunnel, John F. Kennedy Memorial Highway (including both the Maryland Northeastern Expressway and the Delaware Turnpike), Pennsylvania Turnpike (future), New Jersey Turnpike, George Washington Bridge (eastbound), New England Thruway, Connecticut Turnpike (formerly tolled), New Hampshire Turnpike, Maine Turnpike
• I-910 – Crescent City Connection/Greater New Orleans Bridges
• I-335 – Kansas Turnpike
• I-355 – North-South Tollway
• I-470 – Kansas Turnpike
• I-275 – Sunshine Skyway Bridge
• I-276 – Pennsylvania Turnpike
• I-476 – Pennsylvania Turnpike Northeast Extension
• I-676 – Ben Franklin Bridge (westbound only)
• I-278 – Triborough Bridge, Goethals Bridge (eastbound only), Verrazano Narrows Bridge (westbound only)
• I-478 – Brooklyn Battery Tunnel
• I-678 – Whitestone Bridge
• I-580 – Richmond-San Rafael Bridge (westbound only)
• I-680 – Bencia-Martinez Bridge (northbound only)
• I-185 – Southern Connector (Greenville, SC)
• I-287 – New York Thruway section, including Tappan Zee Bridge (eastbound only)
• I-190 – Niagara Section of the New York Thruway
• I-294 – Tri-State Tollway
• I-295 – Delaware Memorial Bridge (southbound only)
• I-295 – Throgs Neck Bridge
• I-395 – Connecticut Turnpike (formerly tolled)
• I-495 – Falmouth Spur of the Maine Turnpike
• I-495 – Lincoln Tunnel between New Jersey and New York – former route (now NJ 495/NY 495)
• I-495 – Queens Midtown Tunnel under East River in New York City – former route (now NY 495)
• I-695 – Francis Scott Key Bridge
• I-895 – Harbor Tunnel (Baltimore)

• I-44 (Laclede County, Missouri) – first contract to be let for an Interstate Highway using Interstate Highway funds (08/02/1956)
• I-70 (St. Charles County, Missouri) – first contract to be awarded and to start construction (08/13/1956)
• I-70 (west of Topeka, Kansas) – contract awarded 08/31/1956, construction with new contract began 09/26/1956, construction completed and road opened 11/14/1956
• I-70/76 (Pennsylvania Turnpike from Irwin to Carlisle) – first section of Interstate Highway to be constructed (but not with Interstate Highway funds); road opened on 10/01/1940 but was not signed as Interstate until 1960s
• Finally, Nebraska was the “first state in the nation to complete its mainline Interstate System at a cost of
  $435 million. Work began in 1957 and the final link was dedicated October 19, 1974.” (Source: Nebraska Department of Roads Fast Facts http://www.nebraskatransportation.org/docs/facts-roads.pdf)