For me it's the usual...work, kids, pretty girls, other personal stuff...I don't sing while cycling, 'cause everyone could hear me. I used to sing when I would ride my motorcycle, though. Oddly enough my two favorites to sing while on the motorcycle were Janis' "Piece of My Heart" and the Vanilla Fudge version of "You Keep Me Hangin' On".
Oh yeh...pardon my pedantry, but Steve Goodman wrote "City of New Orleans" and it was popularized by Arlo Guthrie, Woody's son.