Flagstaff Mountain is a Boulder icon. Right on the edge of town, it is extremely popular with hikers and tourists, as well as cyclists looking to make their lungs explode. It is possible to drive up to the summit, so it is unlikely that you will have the summit to yourself. But aside from that, Flagstaff is a wonderful hike.
It is relatively steep the whole way up, criss-crossing the switchbacking road as you climb. The hiking trail crosses the road five times, and the crossings are well marked. But do use caution. The cyclists often descend at great speeds here and auto traffic can also be heavy. While not the most isolated hike in the area, do not let the road stop you from doing this hike at least once. The trail does get away from the road for large portions of the hike and it really is worth it.
With the potential for crowds in this area, it is recommended that you get an early start, especially on weekends. Parking is available on Baseline Road or at the Chautauqua Trailhead parking area. (The most convenient lot, at Gregory Canyon, is currently closed due to the flooding of 2013)
The Flagstaff trail starts at the entrance to the Gregory Canyon trailhead, and immediately starts climbing. The switchbacks are gentle and there aren’t too many ‘stairmaster’ sections. It’s just a nice, steady, strenuous climb without too many technical obstacles. The hike will take you in and out of the forest of big pines, opening up frequently for spectacular views of Boulder and the Chautauqua area. A unique side view of the Flatirons is available on the lower stretches of the hike. After about 2.5 miles and 1300 feet of elevation, you will reach the Flagstaff Mountain summit area. The summit has many amenities, including restrooms and many picnic tables. It is quite popular to get married on Flagstaff Mountain at the Flagstaff Amphitheater, which is worth checking out. (if you’re getting married in Boulder, you might want to add this site to your wedding website for your visiting guests!!) There are great views of Boulder from the amphitheater. Before turning around and heading back down, be sure to go the far west side of the summit area to Writer’s Point where there are spectacular views west to the Indian Peaks and Rocky Mountain National Park!
Below is the route and cross section of the hike from my Strava recording.