Tuesday, February 5, 2008

Interstate highway
The Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly called the Interstate Highway System, is a network of highways (also called freeways) in the United States that is named for the president that was in office when the system was created. The Interstate Highway System is a separate system within the larger National Highway System. The entire system, as of 2004, had a total length of 46,837 miles (75,376 km) [1].
While Interstate Highways usually receive substantial federal funding and comply with federal standards, they are owned, built, and operated by the states in which they are located. The only exception is the federally-owned Woodrow Wilson Bridge on the Capital Beltway (I-95/I-495).
The system serves nearly all major U.S. cities. Unlike counterparts in most other industrialized countries, many Interstates pass through downtown areas. This has helped to facilitate the emergence of automobile-oriented postwar suburban development patterns, a phenomenon also known as urban sprawl.
The system is prominent in American daily life. The distribution of virtually all goods and services involves Interstate Highways at some point. Residents of American cities commonly use urban Interstates to travel to their places of work. The vast majority of long-distance travel, whether for vacation or business, uses the national road network;


Main article: Interstate Highway standards Speed limits
Further information: Contraflow lane reversal
In addition to being designed to support automobile and heavy truck traffic, interstate highways are also designed for use in military and civil defense operations within the United States, particularly troop movements.
One potential civil defense use of the Interstate Highway System is for the emergency evacuation of cities in the event of a potential nuclear war. The Interstate Highway System has been used to facilitate evacuations in the face of hurricanes and other natural disasters.
An option for maximizing traffic throughput on a highway is to reverse the flow of traffic on one side of a divider so that all lanes become outbound lanes. This procedure, known as contraflow, was first employed in the 1998 evacuation of New Orleans, Louisiana in preparation for Hurricane Georges. Contraflow was also employed in the 2004 evacuation of the Tampa, Florida, area ahead of Hurricane Charley and again 2005 during the evacuations of New Orleans, Louisiana, and Houston, Texas, prior to hurricanes Katrina and Rita, respectively.
Contrary to popular lore, Interstate highways are not designed to serve as airstrips.

Dual-purpose design
While the name implies that Interstate highways cross state lines, many do not (For details, see List of intrastate Interstates). Rather, they are funded federally with money shared among the 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 except "on paper".
The numbering system can be summarized in these general rules. One- and two-digit highways are national routes:
I-5 runs up and down the West Coast, with I-95 on the East Coast.
I-10 runs along the Southern states, and I-90 runs near the US-Canada border.
Three-digit highways, or Auxiliary Interstate Highways, run only within a single city.
I-110 travels into downtown Los Angeles.
I-215 travels the perimeter of Salt Lake City.
I-895 serves as a bypass of downtown Baltimore.
I-215 eventually returns to I-15, a primary interstate highway.
I-185 in Georgia leaves I-85 to go into Columbus.
I-370 in Maryland branches off I-270, and is thus a tertiary spur of I-70.

Odd numbered routes run north-south, and higher numbers are farther east.
Even numbered routes run east-west, and higher numbers are farther north.
An odd first digit signifies a spur route, which may begin at a large highway and terminate at a city center.
An even first digit signifies a loop route around a city or a bypass through a city.
The last two digits signify the highway's origin.
Sometimes, a three-digit highway branches off another three-digit highway; this is called a tertiary spur route of a two-digit highway. Tertiary spurs do not meet their parent highways, but are associated with them via the three-digit highways they do meet. Terminology
See also: List of Interstate Highways
Interstate highways are typically known as Interstate XX or I-XX, where "XX" is the one- or two-digit route number; sometimes Interstate Highway XX (IH XX) or Interstate Route XX (IR XX) is used. In some areas, the more generic Route XX or Highway XX is used, or in the case of southern California, Nevada, and New York, The XX.
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. Most Interstates have two digits; there are only three one-digit Interstates in the system: I-4, I-5 and I-8. Within this category, east-west highways are assigned even numbers, and north-south highways are assigned odd numbers. Odd route numbers increase from west to east, and even numbered routes increase from south to north, though there are exceptions to both principles in several locations. Numbers divisible by 5 are intended to be major among the primary routes, carrying traffic long distances. For example, I-5 runs from Canada to Mexico along the west coast (the only interstate to do so) while I-95 runs from Miami north to Canada along the east coast. In addition, I-10 runs from Santa Monica, California, to Jacksonville, Florida while I-90 runs from Seattle, to Boston. However, not all primary routes divisible by 5 traverse long distances. I-45 runs from Galveston, Texas, north to Dallas, a distance of only 284 miles (457 km). It is the only primary route divisible by 5 that does not cross state lines. See List of intrastate Interstate Highways for other primary routes that do not cross state lines.
I-50 and I-60 do not exist (and there are no even-numbered Interstates from 46 to 62), 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 having the same number within 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 intersect US 69 in Lufkin, Texas, I-74 will overlap US 74 in North Carolina, and I-41 will do the same with US 41 in Wisconsin.
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.
For the sake of efficiency, some Interstates double up for short or sometimes long distances. This is usually referred to as a concurrency. One example is where I-75 and I-85 combine just below downtown Atlanta to form the Downtown Connector, a major thoroughfare through the city. Another example is the merging and diverging of Interstates 90 and 94, which double and then separate several times across the upper Midwest and Great Plains. I-90 and I-94 even join with I-39 from Madison to Portage, Wisconsin, creating the longest triple concurrence in the Interstate system. Interstates 90 and 80 are concurrent for almost 280 miles in Indiana and Ohio. A recent addition is the I-73/I-74 duplex, which runs along U.S. Route 220 from Asheboro to Rockingham, North Carolina. I-73 will also share routes with I-40 and I-85 on the Greensboro Urban Loop, a new bypass route around Greensboro, North Carolina. Strict adherence to the directional nature of the system results in some amusing oddities. For a nine-mile (14 km) stretch east of Wytheville, Virginia, the driver can be traveling on both I-81 North and I-77 South at the same time (and vice versa) (see also Wrong-way concurrency).

Primary routes
See also: List of auxiliary Interstate Highways
Auxiliary Interstate Highways are given three-digit route numbers, which consist of a single digit prefixed to the two-digit number of a primary Interstate highway, to designate spur or loop routes branching from either the primary route or one of its other auxiliary routes. A spur route is one that deviates from its parent and does not end at another Interstate; it is given an odd first digit. A loop route is one that returns to its parent; it is given an even first digit. The number given to the first digit of a route that branches from the parent to end at another Interstate depends on the state; some consider these routes spurs and assign odd first digits, while others consider them loop connectors giving them even first digits.
When letter-suffixed two-digit Interstates (see above) were abundant, their auxiliary routes were given a number without a letter suffix (with the exception of I-180N in Boise, Idaho, which is now Interstate 184).
Due to the large number of these routes, auxiliary route (aka "spur route") numbers may be repeated in different states along the mainline; but no two three-digit Interstates in the same state can share a number. For instance, I-90 in New York alone 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.
Closed loops usually retain a single designation for the entire route, even when they enter other states. For example, Cincinnati, like many other cities, features a large loop around the city that intersects with the primary routes I-71, I-74, and I-75 and travels through Ohio, Kentucky, and Indiana. The entire 84 mile (135 km) loop is labeled I-275. (It is also the only three-digit interstate to ever go through three states.)
The loop highway around Washington, D.C., known as the Capital Beltway, carries both I-95 and I-495. Originally, the loop carried only I-495, in anticipation of I-95 being routed through Washington, but in 1977, I-95 was rerouted on the eastern half of the loop due to the cancellation of the segment that would have connected downtown Washington to College Park, Maryland, while I-495 remained on the western half. I-495 was added back to the eastern half of the loop in 1989, creating a rare concurrency of primary and auxiliary routes.
Unlike primary Interstates, three-digit Interstates are signed either east-west or north-south depending on the general orientation of the route, without any regard to the route number. For some looped Interstate routes, Inner and Outer banners are used as a directional labeling system as opposed to compass directions.
Of course, there are exceptions to these guidelines:
Interstate highway
A contiguous loop surrounds the entire Minneapolis-St. Paul Metro area. I-94 intersects the loop in two spots and runs directly through it separating it into southern and northern halves; the southern half of it is labeled I-494 while the northern half of it is labeled I-694. The northern half rejoins I-94 approximately five miles before the southern half does; this stretch is signed I-94/I-694.
I-270 and I-255 form a beltway around the greater St. Louis area. On its southwest corner, I-270 becomes I-255 as it crosses I-55 counter-clockwise only to terminate back at I-270 in the northeast. In the early 1980s, local residents stopped a plan to designate the entire closed loop as I-270 and renumber the stub of I-270 from Glen Carbon to Edwardsville to I-870.
New York City has numerous spur routes off from I-78 and I-95, but none of I-78's spur routes actually intersect with I-78.
An auxiliary route numbered I-238 connects San Leandro and Castro Valley, California, yet there is no I-38. Instead, I-238 carries the designation of the portion of State Route 238 that the Interstate designation replaced. I-238's parent route is I-80, and at the time of I-238's designation, all child routes of I-80 (I-180 through I-980) were already designated in California.
There are many three-digit state routes that carry an Interstate-compatible designation, such as Illinois State Route 394 near Chicago, which is technically a spur route of I-94. IL 394 is often mislabeled as I-394, despite having never been signed as such. Other examples include Virginia State Route 895 near Richmond, Maryland Route 295 between Baltimore and Washington, D.C., and California State Route 110 in Los Angeles, which acts as an extension of I-110. Three-digit Interstates
A major exception to the overall numbering system is Interstate 99 in Pennsylvania, which was written into law as I-99 by Pennsylvania Congressman Bud Shuster; I-99 (which is also U.S. Route 220) is west of several Interstates that are numerically less than 99, but 99 was the nearest odd 2-digit number available for the interstate.
A less-notable exception is I-82, which lies fully north of I-84; this is a relic from I-84 previously having the designation of I-80N.
Some Proposed Interstate Highways 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 U.S. 3rd Infantry Division.

The following two-digit Interstates change signed direction from their normal (even=east-west, odd=north-south) direction:
The I-69 segment is an extension of its original route; I-76 only runs for two miles (3 km) in Nebraska before ending at I-80.
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.
Business Loop and Business Spur Interstates are not subject to any of the Interstate Highway 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.

I-69 east of Lansing, Michigan
I-76 in Nebraska Other notable examples
About 56%. This has led to the proliferation of toll roads (turnpikes) as the new method of building limited-access highways in suburban areas. Also, some interstates are being privately maintained (e.g., VMS maintains I-35 in Texas) in order to cut rising costs of maintenance and allow state departments of transportation to focus on serving the fastest growing regions in their respective states. The future of the interstate system as we know it is in question. It is possible that parts of the system will have to be tolled in the future to meet maintenance and expansion demands, as has been done with adding toll HOV/HOT lanes in certain cities like San Diego, Salt Lake City, Minneapolis, Houston, Dallas, Atlanta and Washington, D.C.

The dominant role of the federal government in road finance has enabled it to achieve legislative goals that fall outside its power to regulate interstate commerce as enumerated in the federal Constitution. By threatening to withhold highway funds, the federal government has been able to stimulate state legislatures to pass a variety of laws. Although some object on the ground that this infringes on states' rights, the Supreme Court has upheld the practice as a permissible use of the Constitution's Commerce Clause.
The first major example was the introduction of the 55 mph (90 km/h) national speed limit in 1974. While its purpose was to save fuel in the wake of the 1973 energy crisis, federal speed controls stayed in effect for 21 years. The initial acceptance of the national speed limit emboldened various presidents and Congresses to enact additional pieces of legislation, some of which have little to do with highways or transportation. Examples include:
States must also meet minimum enforcement standards for all federally-mandated legislation (for example, minimum penalties for violation of these laws and a minimum number of per capita underage drinking convictions or a compelling explanation regarding why this number is not met). This has proved to be controversial. Supporters hold that it is a way to provide an impetus to states to pass uniform legislation. Critics maintain that using highway dollars in this fashion upsets the balance between federal and states rights in favor of the federal government, and effectively holds funds as ransom in order to coerce state governments into passing laws that would not have otherwise been introduced. Some have even argued that the current arrangement is unconstitutional. Law enforcement agencies in some states argue that efforts to meet quotas for underage drinking convictions have distracted them from other matters and strained relations with those under 21. Any state that was to lose federal highway funding would quickly face deteriorating infrastructure, fiscal impoverishment, or both.
Of course, a state that lost federal highway funding could theoretically threaten to stop maintaining its highways, if that were politically palatable to its residents.

Increasing the legal drinking age to 21.
Megan's Law legislation, requiring states to disclose identities of sex offenders.
Lowering the legal intoxication level to 0.08%.
Requiring the use of carpool (HOV) lanes. The federal role in financing
In addition to Interstate highways financed with federal funds (Chargeable Interstate routes), federal laws allow other highways to be signed as Interstates, if they meet the Interstate Highway standards and are logical additions or connections to the System.
Called Non-Chargeable Interstate routes, these additions fall under two categories:

Routes that already meet Interstate standards. These may immediately be signed as Interstates once their proposed number is approved, or may be retained with a non-Interstate designation.
Routes not yet upgraded to Interstate standards. These cannot be signed as Interstates until they have been fully upgraded. Non-chargeable Interstate routes
Interstate Highways are signed by a number on a trademarked red, white and blue sign as shown to the right. In the original design, the state was listed above the highway number, but in many states, this area is now left blank. The sign itself usually measures 36 inches (91 cm) high , and is 36 inches wide for two-digit interstates or 45 inches (114 cm) for three-digit interstates.
Business Loop and Business Spur Interstates use a special shield where the red and blue are replaced with green; the word BUSINESS appears instead of INTERSTATE, and the word SPUR or LOOP usually appears above the number.
The majority of Interstates have exit numbers. All traffic signs and lane markings on the Interstates are supposed to be designed in compliance with the Manual on Uniform Traffic Control Devices (MUTCD). However, there are many local and regional variations in signage.
For many years, California was the only state that did not use an exit numbering system. It was granted an exemption in the 1950s due to having an already largely completed and signed highway system; at the time, placing exit number signage on the signs across the state was deemed too expensive. Since 2002, however, California has begun to incorporate exit numbers on all of its freeways - interstate, U.S., and state routes alike. To mitigate costs, a common occurrence is for Caltrans to install exit number signage only when a freeway or interchange is built, reconstructed, retrofitted, or repaired. The majority of the exits along Interstates 5, 10, and 80 now have exit number signage, particularly in rural areas.
In most states, the exit numbers correspond to the mileage markers on the Interstates (with an exception being I-19 in Arizona, whose length is measured in kilometers instead of miles). On even-numbered Interstates, mileage count increases to the east and decreases to the west (except on the I-90 portion of the New York State Thruway, I-90 between Chicago and Rockford, IL, the I-190 spur into O'Hare International Airport in Chicago, and the portion of I-76 in New Jersey, all of which count up going west); and on odd-numbered Interstates, mileage count increases to the north and decreases to the south and the exit numbers increase and decrease accordingly.
Many northeastern states label exit numbers sequentially, regardless of how many miles have passed between exits. States in which Interstate exits are still numbered sequentially are: Connecticut, Delaware, Massachusetts, New Hampshire, New York, Rhode Island and Vermont. Maine, Pennsylvania, Virginia, Georgia, and Florida followed this system for a number of years, but recently converted to having the exit numbers correspond to mileage markers. The Pennsylvania Turnpike uses both the mile marker number and the sequential number. The mile marker number is used for signage, while the sequential number is used for numbering interchanges internally. The New Jersey Turnpike also has sequential numbering, but other interstates within New Jersey generally use mile markers.

Further information: List of structures built on top of interstates

Alaska and Puerto Rico have roads designated as Interstates for funding purposes but which are neither planned for nor currently built to Interstate Highway standards. The public controlled-access highways of Puerto Rico are the Autopistas (PR-22, PR-52, and PR-53).
A widespread urban legend states that one out of every five miles of the Interstate Highway System must be built straight and flat so as to be usable by aircraft during times of war.
Washington, D.C.: As the city and District are identical, the District maintains the portions of Interstate 66, Interstate 295, Interstate 395 and Interstate 695 inside the city.
The Interstate System has a few gaps that violate the Interstate Highway standards and AASHTO rules.
The most heavily traveled area of the Interstate Highway system is the 405 Freeway in Seal Beach, California, with a 2002 estimate of 377,000 vehicles a day. The least traveled section is Interstate 95 just north of Houlton, Maine (near the Canadian border), with 1,880 vehicles a day (2001 estimate).
The most extreme directional points of the Interstate Highway system are:

  • Northernmost-the northern termini of Interstates 5, 15, and 29, crossing the Canadian border at the 49th parallel near, respectively, Blaine, Washington, Sweetgrass, Montana, and Pembina, North Dakota.
    Southernmost-a bend on Interstate H-1 in the Kaimuki section of Honolulu, Hawaii, less than half a mile (0.8 km) before its eastern terminus (lat. 21.3 deg. N.). The southernmost point in the 48 contiguous states is the southern terminus of Interstate 95 in Miami, Florida (lat. 25.8 deg. N.).
    Easternmost-the northern terminus of Interstate 95 near Houlton, Maine, at the border of New Brunswick in Canada (long. 67.8 deg. W.).
    Westernmost-the western terminus of Interstate H-1 in Kapolei, Hawaii (long. 158.06 deg. W.). The westernmost point in the contiguous states is a curve on Interstate 5 near Wolf Creek, Oregon (long. 123.23 deg. W.).
    The highest point on the Interstate Highway System is at the Eisenhower Tunnel on Interstate 70 in the Rocky Mountains of Colorado, at the Continental Divide (elev. 11,158 feet (3401 m)).
    The lowest point is on Interstate 8 at the New River near Seeley, California (elev. -52 feet (-16 m)).
    The longest Interstate highway is Interstate 90, which runs 3099 miles (4 987 km) between Boston, Massachusetts and Seattle, Washington. The longest north-south Interstate highway is generally cited as Interstate 95; when completed, it will run 1927 mi (3 101 km) between Miami, Florida and the Canadian border (there is a gap in Pennsylvania/New Jersey).
    The shortest Interstate is Interstate 878, a 0.7-mile (1.1 km) portion of New York State Highway 878 adjacent to John F. Kennedy International Airport in Queens, New York. However, it is not officially signed. The shortest signed Interstate is Interstate 375 in downtown Detroit, Michigan, at 1.06 miles (1.71 km). Criticism

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