Here's one semi-nerdy link that gets into it a bit.
http://www.sportsscientists.com/2012...iscussion.html
People are climbing Alpe d'Huez a few full minutes minutes slower than during the 90's and early 2000's. A few minutes is an eternity in cycling, and that is just one climb.
Its now thought that the times that were turned in climbing that mountain during Armstrong's time are just not physiologically possible for a human being, even a top world-class athlete, without doping.