The mountains are already at capacity.  Whether that is skiing (inbounds or out), camping, climbing, or biking, trails, parking lots, roads and watersheds are capped.  At this point, I-70 is the only thing keeping some areas from being completely overrun.  Add more capacity, and more people will travel creating the same traffic.  Anybody who lived in Denver through T-Rex and now drives that same stretch of I-25 will remember that it was just as bad then as it is now.  More capacity just creates more demand and people will come until the traffic reaches a level that turns people off.  Humans seem to have a certain tolerance for traffic, and will live further away from where they work/play if the traffic is improved.

Leave I-70 alone, use the money to maintain it better and allow the traffic to self regulate the crowds in the mountains and on the front range