Between August 13th and October 24th 2009 I cycled coast2coast in the USA.
Day 30 - Trail Ridge Road, highway 34 which is the highest continuous highway in the USA. As an added bonus it goes through the Rockie Mountains National park.
This was probably the most beautiful days cycling I've ever done. Whether that is down to the scenery or an oxygen shortage after climbing from 8000ft to 12,200ft(3700m) (40km in 5 hours) who know but at the time I was very impressed (and quite emotional). I was of course very lucky that the weather was nice, it really would of been a downer to cycle 5 hours only to see mist at 12000ft.
Trail ridge road though tough because of the altitude is a nice climb because the road is generally on 6% gradient. Just beware of those damn RV drives who leave their steps out or doors open, the road is very narrow, I almost got wiped out by a swing door on an RV.
For the full story see shanecycles.com/coast2coast/