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)
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.
Current 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.:
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.
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 for now, including TN 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.
Interstate Highway System Trivia
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.
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).
I-80 - Carlin Tunnel between Carlin and Elko, Nevada along Humboldt River near Milepost 276
I-80 - cut and cover tunnel in downtown Reno, Nevada, at the Virginia Street interchange; a Walgreens store is located prominently on top of the deck
I-80 - Yerba Buena Island Tunnel on San Francisco-Oakland Bay Bridge in California
I-84 - Toothrock Tunnel (Located between Bonneville Dam and Eagle Creek exits; built in the 1930s as an improvement to U.S. 30. The tunnel bypassed a narrow viaduct built on a cliff around 1916. When the freeway was built as part of Interstate 80N, the eastbound lanes were routed through the tunnel and a viaduct was built for the westbound lanes. The westbound viaduct skirts the side of the mountain below the 1916 viaduct, which was recently reopened as a bike path. [Thanks to Andy Ransom for this entry.])
I-90 - Mount Baker Tunnels in Seattle, Washington adjacent to the Mercer Island floating bridges
I-90 - "Snow Tunnel" over westbound lanes of Interstate 90 in Cascade Mountains
I-90 - Central Artery Tunnel (Big Dig) under downtown Boston, Massachusetts (connects directly to Tip O'Neill Tunnel, see below)
I-93 - Central Artery Tunnel (Big Dig) under downtown Boston, Massachusetts
I-94 - Lowry Hill Tunnel - downtown Minneapolis, Minnesota
I-195 - Fall River City, Massachusetts, under City Hall
I-210 - cut and cover tunnel in La Canada Flintridge near California 2 interchange
I-64, Sherman Minton Bridge over Ohio River - New Albany, Indiana, and Louisville, Kentucky
I-95, St. Johns River Bridge - Jacksonville, Florida
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; now this drawbridge is part of Ohio 65)
I-695 - Curtis Creek Bridge - Baltimore, Maryland
At-Grade Intersections and Traffic Signals
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-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 section between Business Loop I-85 and I-85 is at-grade; the southern end has a few traffic signals north of U.S. 29, even though the I-585 signs continue south to the U.S. 29 intersection.
I-676 in Philadelphia, Pennsylvania - the through connection between the Benjamin Franklin Bridge and the Vine Street Expressway requires passage through one traffic signal.
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-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-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)
Interstate 64 has almost never met a two-digit Interstate it does not like. From its inception in metropolitan St. Louis, Interstate 64 merges with every two-digit Interstate it passes, except for one: Interstate 65. All the other routes (Interstate 70, Interstate 55, Interstate 57, Interstate 71, Interstate 75, Interstate 77, Interstate 81, and Interstate 95) all share pavement with Interstate 64.