Never. Denver is road rage central.

Bicyclists in/around Denver are mostly posers and weekenders, not the type that will be on a bike to commute or run errands. Boulder is full of the "weekend athlete" sort that puts lots of miles on their AWD Subaru during the week, and pumps up their egos with ultramarathons on the weekend. I didn't encounter many people serious about utility cycling or bike commuting in either Denver or Boulder, taken as a fraction of the overall population. Sorry, but I gotta be brutally honest.

Nevertheless the old streetcar grid of East Denver around Park Hill was a great place to live if you want to bike everywhere. Boulder is a very mixed bag - the traffic jams and fast traffic make cycling around rush hour times a major headache if you aren't going somewhere that's on the established bike paths.