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The streets and highways of Washington, D.C. are based on a plan that provides not only for vehicular transportation in the capital, but also for a distinctive city layout and addressing scheme.

L'Enfant's plan for Washington

Washington, D.C. is divided into four quadrants: Northwest, Northeast, Southeast, and Southwest. The axes bounding the quadrants radiate from the U.S. Capitol building.

Washington, D.C., was created to serve as the national capital from its inception. The original street layout was designed by Pierre Charles L'Enfant at the time of the city's founding. L'Enfant incorporated a basic grid system, inter-cut with broad diagonal radiating avenues to provide vistas.

At the center of the design is the United States Capitol Building, from which four quadrants radiate along the four compass directions: Northwest, Southwest, Northeast, and Southeast. The quadrants are separated by North Capitol Street, South Capitol Street, and East Capitol Street, with the centerline of the National Mall taking the place of what would be the western demarcation line. Two avenues, Constitution Avenue and Independence Avenue, line the sides of the Mall. The boundary of the District with Maryland (which forms part of a square) are marked by Western Avenue (the northwest edge), Eastern Avenue (the northeast edge), and Southern Avenue (the southeast edge).

Streets that are oriented north/south are designated by numbers and count upwards from east to west in the Northwest and Southwest quadrants of the city, starting at the Capitol (1st St NW, 2nd St NW, 3rd St NW, etc.); these streets repeat in the Northeast and Southeast quadrants, counting upwards from west to the east, going away from the Capitol.

Streets that are oriented east/west use a single letter of the alphabet in the central city. Thus, east-to-west lettered streets "count" upwards from south to north in NW and NE, and likewise repeat in the opposite direction in SW and SE. Street numbers count upwards traveling outward from the dividing lines of the quadrants. "A" Street, as named, is only found in NE and SE. In NW and SW, the roads that would be known as "A" Street are named Madison Drive NW and Jefferson Drive SW. These one-way roads run parallel within the National Mall. There is no J Street in any quadrant, which was omitted due to similarities in how the letters I and J were written, and confusion that could cause.^[1] (In a nod to this, the food court at The George Washington University is named J Street.) There is a legend that J Street was omitted as a deliberate slight to John Jay, but is not the actual reason.^[2] North of the Mall, the road that would be B Street is now known as Constitution Avenue; south of the Mall, the second east-west street is called Independence Avenue. There are also no X, Y, or Z streets.

"I," "Q," and "U" streets properly use a single letter of the alphabet for their names. "Eye Street," "Que Street", "Queue Street" and "You Street" have also appeared in some historical and contemporary instances for clarity; however, their use is informal.

Because both the numbers and the letters repeat for each quadrant, each street name is appended by the quadrant in which it is located (NE, NW, SE and SW). Use of these directional designations is required in giving directions and in the addressing of mail. Informally, however, the NW is often omitted from addresses in the northwest quadrant, as it is substantially larger than the other three.

Once single letters are exhausted as east-west street names, the "alphabet" system continues anew with two-syllable names beginning with the first letter of the alphabet; streets in this set are commonly referred to as being in the "second alphabet" (for example, Calvert St. NW would be the third street of this second alphabet). As in the previous series of names, the names beginning with X, Y, or Z are not used. In Northwest and Northeast D.C., which are the largest quadrants, a "third alphabet" is started using three-syllable words, e.g., Brandywine St. NW. In only the Northwest Quadrant is a "fourth alphabet" necessary. This fourth alphabet uses botanical names without regard to the number of syllables: Aspen, Butternut, Cedar, etc. Verbena Street NW is the last in this series before the Maryland border.

Diagonal avenues, generally named for states, are exempt from this syllabic rule, as are streets designated as "Place" or "Way" and streets laid out after the alpha-name was established for that area (for example, between E and F Streets in Southeast, one will find Duddington Place).

The original city plan for Washington included only the original city of Washington, i.e., the part between the waterfront and what is today Florida Avenue. It has since been extended throughout the District. Georgetown, which was founded before the District of Columbia, had its own street naming system until 1895, when the streets were renamed to conform to the street names in use in Washington.

The city's addressing system is best understood in terms of a Cartesian coordinate system with its origin at the Capitol. While the system may appear complex, once learned it allows one to pinpoint not only where one is but also where and how far one may need to travel. For example, if one needed to find 633 A Street SE, the address informs that the address is southeast of the Capitol, with A Street one block south of East Capitol Street, and that the location on that street is on the 600 block, which is between 6th Street and 7th Street SE. Another more familiar example would be the White House, located at 1600 Pennsylvania Avenue NW, is located at 16th Street NW (Lafayette Square) and Pennsylvania Avenue NW. This works both ways; an address at 514 19th St NW would be on 19th St west of the Capitol, and since E is the fifth letter of the alphabet, the 514 address would be between E and F Streets NW. Note, however, that because J is omitted from the scheme, it is not counted in numbering. Thus Between I and K streets is the 900 block of a numbered street, while the block from K to L is numbered in the 1000s.

A large system of freeways was planned in the District. Many, especially in the northern half, were cancelled after freeway revolts. (Cancelled freeways are shown in light blue.)

Washington, DC Metropolitan Area Road Map

Interstate 495, also known as the Capital Beltway, creates an artificial boundary for the inner suburbs of Washington and is the root of the phrase "inside the Beltway". Almost completely circling Washington, D.C., it crosses a tiny portion of the District at its southernmost point which is at the Woodrow Wilson Bridge. I-66 runs from the eastern edge of Georgetown, connects with the Beltway, and continues through northern Virginia to I-81. I-295 comes up from the south starting at the eastern edge of the Woodrow Wilson Bridge on the Beltway and crosses the Anacostia River into downtown, linking up with I-395 (also called the Southwest-Southeast Freeway), a major commuter route extending from New York Avenue to the Beltway and Interstate 95 in Springfield, Virginia. The Inner Loop was a proposed network of freeways in the city center; however, only portions of it were ever built.

The Anacostia Freeway (DC-295) splits from I-295 on the south side of the Anacostia and links with the Baltimore-Washington Parkway, which eventually becomes Maryland State Highway 295, via a short section of Maryland State Highway 201. The Suitland Parkway connects the city with the southeastern suburbs in Prince George's County, Maryland. The Whitehurst Freeway, an elevated freeway over K Street in Georgetown, allows U.S. Highway 29 traffic to bypass congested Georgetown between the Key Bridge and K Street downtown. The E Street Expressway connects I-66 with the city's Foggy Bottom area and the areas immediately to the west of the White House. The Rock Creek and Potomac Parkway provides access to downtown from the northern and western ends of the city.

Among the major roads in the city are MacArthur Boulevard NW, 14th Street NW, 16th Street NW, 18th Street NW, 7th Street NW, Connecticut Avenue NW, K Street NW, H Street NW, Wisconsin Avenue, M Street NW, H Street NE, Pennsylvania Avenue, Constitution Avenue, Independence Avenue, Massachusetts Avenue, U Street NW, South Dakota Ave NE, North Capitol Street, South Capitol Street, East Capitol Street, Georgia Avenue, Minnesota Avenue, Nannie Helen Burroughs Avenue, Benning Road, Martin Luther King Jr. Avenue SE, New York Avenue, and Rhode Island Avenue.

While city streets generally do not have route numbers, U.S. Routes 1, 1 Alternate, 29, and 50 are routed predominantly or entirely over city surface streets.

• List of Circles in Washington, D.C.
• List of Washington, D.C. numbered highways

1. ^ Hagner, Alexander B. (1904). "Street Nomenclature of Washington City". Records of the Columbia Historical Society: p. 240. 
2. ^ http://www.snopes.com/history/american/jstreet.htm