Interstate 495 (Capital Beltway) is a 64-mile Interstate Highway that encircles Washington, DC, that was finished in 1964. The terms “iinside the Beltway” and “outside the Beltway” were used literally, but by about 1975, “inside the Beltway” meant a political insider in Washington’s federal government. The term “Beltway insider” has been cited in print since at least 1981.
 
A “Beltway bandit” is a highly paid consultant with offices near the Beltway and a “Beltwaytarian” is someone who believes in a powerful federal government.
 
   
Wikipedia: Inside the Beltway
“Inside the Beltway” is an American idiom used to characterize matters that are, or seem to be, important primarily to officials of the U.S. federal government, to its contractors and lobbyists, and to the corporate media which cover them—as opposed to the interests and priorities of the general U.S. population.
 
The Beltway refers to Interstate 495, the Capital Beltway, a circumferential highway (beltway) that has encircled Washington, D.C. (the capital of the United States) since 1964. Some speakers of American English now employ the word as a metonym for federal government insiders (cf. Beltway bandits).
 
Geographically, Inside the Beltway describes Washington, D.C. and those sections of Maryland and Virginia that lie within the perimeter of the Capital Beltway. Specifically, this includes Arlington County, Virginia; the city of Falls Church, Virginia; most of the city of Alexandria, Virginia; and portions of Montgomery County, Maryland, Prince George’s County, Maryland, and Fairfax County, Virginia.
 
Wiktionary: inside-the-Beltway
Etymology
Derives from the Beltway, a highway that encircles Washington, D.C. Transferred to New Zealand
Adjective
inside-the-Beltway
1.(US) Of or relating to things that occur in Washington, D.C., usually in relation to United States national politics.
2.(US) Out of touch with sentiments of persons living outside of Washington, D.C.
 
(Oxford English Dictionary)
beltway n. U.S. a highway circling all or part of a city or other metropolitan area, esp. that around Washington, D.C.; a ring road; also transf. (freq. attrib.) Washington, D.C., as the seat of the U.S. government.
1952 Sun (Baltimore) 22 Jan. 7/2   Preliminary construction on the proposed beltway circling the city through Baltimore county is expected to get under way.
1973 Times 13 Aug. 10/7   The beltway built round Washington to relieve traffic jams was jammed with traffic.
 
Google News Archive
14 November 1970, Washington (DC) Afro-American, pg. 1, col. 7:
Inside the Beltway
By ART CARTER
(Political news column—ed.)
 
12 October 1975, New York (NY) Times, pg. 230:
In the White House of Richard M. Nixon, it was said that Watergate would become serious only if it “got outside the Washington Beltway.” (...) It can be said that the myriad of doubts about the Warren Commission’s findings in the death of President Kennedy represent the reverse situation. The doubts would never be taken seriously until they were inside the Beltway.
   
27 April 1979, The Times-Picayune (New Orleans, LA), “Reagan’s Smart ‘Nixon Strategy’” by Patrick J. Buchanan, sec. 1, pg. 10, col. 1:
If cocktail party chatter inside the Beltway were decisive, Goldwater, Nixon, McGovern and Carter would never have contested their party’s nomination.
 
20 January 1981, USA Today, “Inaugural galas aren’t always a ball”:
Letitia Baldrige, social secretary in the Kennedy White House and another Beltway insider, elaborates: “Out-of-towners get all glassy-eyed and teary-eyed and patriotic and romantic when they hear the word ‘ball.’”
 
OCLC WorldCat record
Washington inside the beltway/heart of our nation’s capital.
Author: National Geographic Society.
Publisher: Washington : National Geographic Society, 1983.
Edition/Format:  Book : English
 
27 March 1984, Lawrence (KS) Journal-World, “The U.S. second-in-command: Bush like no vice president before him” by Seth Kantor (Cox News Service), pg. 4, cols. 5-6:
Bush says life is very different out there for a candidate “once he gets outside the Washington beltway. A while back there was all this fascination in Washington with Jim Watt (the former interior secretary). But outside the beltway I was never asked about him.
 
“Inside the beltway reporters shout questions at President Reagan all the time. Outside, though, I notice when questions are asked, they listen for the answers.”
 
20 March 1987, Orange County Register (Santa Ana, CA), “A short-term agenda for Howard Baker” by Gregory A. Fossedal, pg. B7, col. 2:
Beltway insiders say that Chief of Staff Howard Baker, anxious to re-establish his president’s primacy, is shopping hard for several short-term accomplishments that can be rammed through to impress the public that Ronald Reagan is no lame duck.
 
OCLC WorldCat record
Getting Inside the Beltway: Perceptions of Presidential Skill and Success in Congress
Author: Brad Lockerbie; Stephen A Borrelli
Edition/Format:  Article : English
Publication: British Journal of Political Science, v19 n01 (19890127)
Database: CrossRef
Other Databases: JSTOR
 
OCLC WorldCat record
Inside the beltway : a guide to Washington reporting
Author: Don Campbell
Publisher: Ames : Iowa State University Press, 1991.
Edition/Format:  Book : State or province government publication : English : 1st ed
 
OCLC WorldCat record
A view from outside the beltway : winning policy themes for the 1990s
Author: Thomas H Kean
Publisher: Washington, D.C. : Heritage Foundation, [©1991]
Series: Heritage lectures, 357. 
Edition/Format:  Book : English
 
OCLC WorldCat record
Murder inside the Beltway : a Capital crimes novel
Author: Margaret Truman
Publisher: New York : Ballantine Books, 2008.
Edition/Format:  Book : Fiction : English : 1st ed
 
OCLC WorldCat record
Homeland security and federalism : protecting America from outside the Beltway
Author: Matt A Mayer; Edwin Meese
Publisher: Santa Barbara, Calif. : Praeger Security International, ©2009.
Edition/Format:  Book : English