Interstate Highway

The Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly called the Interstate Highway System, is a network of highways in the United States. The Interstate Highway System is a separate system within the larger National Highway System. With very few exceptions, Interstate highways are controlled-access freeways, allowing for safe high-speed driving when traffic permits. They are assigned a special level of funding at the federal level. Despite this federal funding, these highways are owned, designed, built and maintained by the state in which they are located, with the only exception being the federally-owned Woodrow Wilson Bridge on the Capital Beltway (I-95/I-495).

Terminology

While the name implies that these highways cross state lines, many Interstates do not. Rather, it is the system of interstates that connects states. There are interstate highways in Hawaii, funded in the same way as in the other states, but entirely within the populous island of Oahu. They have the designation of H-X, and connect military bases. Similarly, both Alaska and Puerto Rico have public roads that receive funding from the Interstate program, though these routes are not signed as Interstate Highways.

Related Topics:
State - Many Interstates - Hawaii - Oahu - Military base - Alaska - Puerto Rico

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Primary routes

The numbering scheme for the Interstate Highway System (as well as the U.S. Highway System) is coordinated by the American Association of State Highway and Transportation Officials (AASHTO), though their authority is occasionally trumped by a number written into Federal law. Within the continental United States, primary Interstates (also called main line Interstates or two-digit Interstates) are given one- or two-digit route numbers. Within this category, even-numbered highways are signed east-west, and odd-numbered highways are signed north-south. As you travel north, the east-west (even) Interstate numbers increase, and as you travel east, the north-south (odd) Interstate numbers increase. Numbers divisible by 5 are intended to be primary routes, carrying traffic long distances. For example, I-5 runs from Canada to Mexico along the west coast while I-95 runs from Miami north to Canada. In addition, I-10 runs from Los Angeles, California to Jacksonville, Florida while I-90 runs from Seattle to Boston. However, not all primary routes traverse long distances. I-45 runs from Galveston, Texas north to Dallas, Texas, a distance of only 284 miles. It is the only primary route that does not cross state lines (see List of intrastate Interstate Highways).

Related Topics:
U.S. Highway System - American Association of State Highway and Transportation Officials - Divisible - I-5 - Canada - Mexico - I-95 - Miami - I-10 - Los Angeles, California - Jacksonville, Florida - I-90 - Seattle - Boston - I-45 - Galveston, Texas - Dallas, Texas - List of intrastate Interstate Highways

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It should be noted that I-50 and I-60 do not exist, mainly because they would most likely have passed through the same states that already have US 50 and US 60. AASHTO rules discourage Interstate and US Highways with the same number to exist in the same state, although I-24 and US 24 exist at opposite ends of Illinois. Some planned Interstates do not follow this guideline - I-69 will enter Texas (which has US 69), I-74 will have a multiplex with US 74 in North Carolina, and I-41 will do the same with US 41 in Wisconsin.

Related Topics:
US 50 - US 60 - I-24 - US 24 - Illinois - I-69 - Texas - US 69 - I-74 - Multiplex - US 74 - North Carolina - I-41 - US 41 - Wisconsin

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Several two-digit numbers are shared between two roads at opposite ends of the country, namely I-76, I-84, I-86 and I-88. Some of these were the result of a change in the numbering system in the 1970s; previously letter-suffixed numbers were used for long spurs off primary routes; for example, western I-84 was I-80N, as it went north from I-80. In the 1970s, AASHTO decided to eliminate these; some became additional two-digit routes, while others became three-digit routes (see below). Only two pairs of these exist; I-35 splits into I-35W and I-35E through both the Dallas-Fort Worth and the Minneapolis-St. Paul areas.

Related Topics:
I-76 - I-84 - I-86 - I-88 - 1970s - I-84 - I-80 - I-35 - I-35W - I-35E - Dallas-Fort Worth - Minneapolis-St. Paul

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Strict adherence to the directional nature of the system results in some amusing oddities. For a ten-mile stretch east of Wytheville, Virginia, the driver can be traveling on both North I-81 and South I-77 at the same time (and vice versa) (see also Wrong-way multiplex).

Related Topics:
Wytheville, Virginia - Wrong-way multiplex

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For the sake of efficiency, some Interstates double up for short or sometimes long distances. The above is one example. Another notable example is I-90 and I-94, which double and then separate several times as they criss-cross the upper Midwest and Great Plains.

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Three-digit Interstates

Three-digit route numbers, consisting of a single digit prefixed to the number of a primary Interstate highway, are used to designate usually short spur or loop routes from their "parent" route, either directly or via another three-digit Interstate. A route that spurs from its parent and ends at an intersection with no other Interstates is given an odd first digit; a route that returns to its parent is given an even first digit. The number given to the first digit of a route that spurs from the parent and ends at another Interstate depends on the state; some consider these routes spurs and give them odd numbers, while others consider them loop-style connectors and give them even numbers.

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For instance, I-90 in New York has a full set of three-digit Interstates - I-190, I-290, I-390, I-490, I-590, I-690, I-790, I-890 and I-990. Due to the large number of these routes, they can be repeated in different places along the mainline; no two three-digit Interstates in the same state can share a number.

Related Topics:
I-90 - New York - I-190 - I-290 - I-390 - I-490 - I-590 - I-690 - I-790 - I-890 - I-990

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The Minneapolis-St. Paul area has a single loop around the entire Metro area, but the southern half of it is labeled I-494 while the northern half of it is labeled I-694.

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Charlotte, North Carolina has a single loop around the city that intersects with both I-77 and I-85, but the entire loop is known as I-485.

Related Topics:
Charlotte, North Carolina - I-77 - I-85 - I-485

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The Baltimore-Washington metropolitan area has several spur routes off of I-95. The area has I-195, I-295, I-495, I-795 and I-895. It also has two routes numbered I-395 (in Baltimore and Washington) and two I-695s (one is signed, the other is a secret designation), as well as an unsigned route called I-595. No I-995 exists anywhere.

Related Topics:
Baltimore - Washington - I-95 - I-195 - I-295 - I-495 - I-795 - I-895 - Baltimore - Washington - Signed - A secret designation - I-595

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New York City has numerous spur routes off of I-78 and I-95, but none of I-78's spur routes actually intersect with I-78.

Related Topics:
New York City - I-78 - I-95

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A three-digit spur off a letter-suffixed two-digit Interstate (see above) was given a number without a letter suffix, except for one case - I-184 in Idaho was I-180N.

Related Topics:
I-184 - Idaho

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Exceptions

:Main article: List of gaps in Interstate Highways

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Interstate 238 near Oakland, California is one of two major exceptions to the numbering scheme, as no Interstate 38 exists. (This number exists because Interstate 238 replaced a segment of California Highway 238, and no appropriate number was available.) The other exception is I-99 in Pennsylvania, which was written into law as I-99 by Pennsylvania Congressman Bud Shuster; I-99 (which is also U.S. Highway 220) is west of several Interstates that are numerically less than 99, and was the nearest available unused two-digit number.

Related Topics:
Interstate 238 - Oakland, California - California Highway 238 - I-99 - Pennsylvania - Bud Shuster - U.S. Highway 220

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Some proposed future Interstate routes have been given similarly non-conforming designations by their legislative proponents. For example, backers of the proposed Third Infantry Division Highway, a route in Georgia and Tennessee, have suggested it be named Interstate 3, in honor of the batallion for which the highway is named http://www.interstate-guide.com/i-003.html.

Related Topics:
Georgia - Tennessee - Interstate 3 - Batallion

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other notable examples

• I-82 lies fully north of I-84, but I-84 was I-80N when I-82 got its number.
• I-85 diverts west of I-75 (intersecting it near Atlanta, Georgia)

The following two-digit Interstates change direction from their normal (even=east-west, odd=north-south) direction:

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• I-26
• I-69
• I-76 (west)
• I-44

Two-digit interstates in Hawaii, as well as the "paper" interstates of Alaska and Puerto Rico, are numbered sequentially in order of funding, without regard to the rules on odd and even numbers.

Related Topics:
Hawaii - Alaska - Puerto Rico

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Business Loop and Business Spur Interstates are not subject to any of the Interstate standards. Their designation is simple - a Business Loop heads into a downtown area from its parent and returns to its parent; a Business Spur ends downtown, occasionally continuing from the end of the main Interstate. Business routes can split from either two- or three-digit Interstates, and can be repeated within a state. In a few cases, where an Interstate has been realigned, the old road has been designated a Business Loop because it is not up to standards.

Related Topics:
Business Loop - Business Spur - Interstate standards

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