Attributive noun, not a true adjective. DAVilla 15:56, 7 July 2011 (UTC)
Do you think go (“become”) is an acceptable substitute for "become" in one of the CGEL adjectivity tests? I think it is, and desirable for including current usage. I've added one cite as an example so far. DCDuringTALK 17:42, 7 July 2011 (UTC)
I'm not sure that any one adjective test is met by three cites, but one can find instances of meeting each. DCDuringTALK 18:01, 7 July 2011 (UTC)
One of the posted cites is "Nothing more Beltway than that" which could well be using it as a proper noun.—msh210℠ (talk) 18:30, 7 July 2011 (UTC)
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RFV-sense "Washington D.C." Tagged but not listed. - -sche(discuss) 03:41, 12 October 2012 (UTC)
Also I'm dubious that the adjective exists in a comparative sense. "Most Beltway" doesn't sound like something anyone would say. Happerslaffer (talk)