Also called a "ring road," a beltway is a highway circling an urban center. In (slightly outdated
) urban planning
theory, a beltway is useful for several reasons, most of them having to do with moving traffic outside the city core.
First, a highway is inherently easier to construct outside a city than within. Land is cheaper and easier to acquire without fuss. Road designers can thus make them many, many lanes wide without disrupting existing neighborhoods and traffic patterns.
Second, it's an efficient way to deal with the intersection of two (or more) major highways. In the United States, the interstate system is a web of asphalt linking each city to several others; the intuitive spot for a land-gobbling interchange would be in the heart of downtown. Beltways not only move interchanges outside of the city, but also divide one interchange into several smaller ones, which are easier to engineer and are less vulnerable to blockages.
Third, they're two-route bypasses. They keep just-passing-through traffic out of the middle of the city, taking pressure off highways there and making local traffic run more smoothly. And if there's a crash or other delay, you can just go the other direction.
In practice, beltways have problems. Chiefly, they're predicated on the idea that the cities will never grow out to engulf them -- if they do, beltways start carrying local traffic, get clogged, serve as walls between adjacent districts that should be linked, and generally create all the problems they were supposed to avoid. In fact, beltways built with a lot of exits encourage the suburban growth that ruins them.
Beltways are now out of fashion.
The best-known beltway in the world is in Washington, D.C. It was built between 1961 and 1964 for about $175 million US, and was designated Interstate 495. From an urban-planning perspective, the "Capital Beltway" illustrates all the problems from which a beltway can suffer.
Washington's beltway is also a metaphor, the notional boundary between the crazy land of national politics in the United States and the "real world." People who have spent too much time "inside the Beltway" are often perceived to have trouble understanding what matters to people outside it, whose lives they are theoretically trying to improve.
Authors of pulp spy novels and cheap political thrillers generally include at least one scene in an automobile on the beltway, to demonstrate their deep familiarity with Washington. If they really are that familiar with the city, the scene takes place in a traffic jam.