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Why do only certain bus line numbers exist?

The current bus line numbering system is somewhat complicated and inconsistent, but it does follow a general framework. Routes that only have numbers in them (like the 42 or the 70) were adopted from the original streetcar lines that operated in D.C. until 1962. The streetcar started with a line that ran from downtown to Georgetown. From there, the lines—which are generally numbered in series of tens—ran clockwise around the city. Today, for example, those lower number bus lines still go to Georgetown. Bus lines with letters in them have always been for buses. That numbering system was first introduced in Anacostia and moves counter-clockwise around the city, according to James Hamre, the city’s director of bus planning. And while D.C. and Maryland bus lines typically place the letter before the number (like S2), Virginia lines do the reverse. This is the identification system the companies that originally operated the Virginia bus lines used. When these companies merged with Washington Metropolitan Area Transit Authority in 1973, the numbering system was left the same.

In general, odd-numbered routes mean the line only runs during rush hour or other special times. Even-numbered routes always run (and a nine, as in the S9, generally indicates an express bus). So if a line offers more full-time routes than rush hour ones, it would skip some odd numbers. And if it has more than five full-time routes, the remaining full-time routes would be assigned an odd number.