We have discussed airspace when flying South along the front range at club meetings, but you may be a new member or may have missed the meeting where this was discussed. Here is information to help avoid airspace when flying South from Lookout, or Boulder!
The issue is that many commercial flights into Denver (DIA) are given the Larks Four Arrival procedure. This heads along the path of the V160 airway towards the Falcon VOR (South of DIA, East of Buckley AFB). In the lower left hand corner of the chart you can see the LARKS waypoint. Commercial jets arrive there at 14,000ft and start descending to approach altitude. Those pilots are briefing their approach at that time and since they are being guided by ATC they expect a clear path with no other traffic (i.e. they aren't watching to avoid HGs or PGs). In the image you can see that the Class B airspace near Chatfield is between 10k and 12k. Technically you can fly above 12k and avoid airspace, but that could put you right in the path of a large, fast, airplane on the Larks Four Arrival. The airspace extends along the Western edge of Chatfield park and continues South past Roxborough Park before arcing to the SE over Castle Rock. The Airspace floor is 10k.