US Road bike sales were pretty low in the 90s. Lance helped road bike sales for a while in the aughts. Looks like they bested MTB sales in 2011, but then dropped off again. The free version of this info doesn't say whether it's by units sold or dollars sold.
Year/road/mtb
2005 16% 29%
2006 18% 24%
2007 21% 25%
2008 20% 26%
2009 20% 24%
2010 23% 22%
2011 24% 23%
2012 20% 25%