I'z gonna say ... to the best of my knowledge, ATL, ORD, and LAX are by far the busiest (not necessarily in that order), and all three have McDonald's. What'd be even more interesting than knowing how many airports do and don't have McD's would be knowing total $$ sales of McD's at airports versus other chains. Not, I admit, highly interesting, but I wonder if it qualifies as a "relative scarcity" of McDonald's.