From belay at bottom of second slab, climb down left for 10 feet under first bolt and follow line of 7 bolts straight up center of slab. Crux about 1/3 of the way up. Excellent edging.
Location
Park 0.6 miles past siphon tube. 4th class up road side and climb any of 6 routes to base of slab two. Anchor chains at belays top and bottom. May need two ropes.
These climbs were great in the day. I have a copy of Allen Simmons Big Thompson guide, and wanted to say thanks, but I have approached a lot of these slabs including the Double O Slab, and I'll tell you, very dangerous approaches for you and cars below. I'd hate to kill some pedestrian from my failed foot hold, and since the Big Thompson is very adventurous when it comes to free climbing. I bet these routes have been unclimbed for years.
That would be the "Al Simons Bootleg guide". I'll second what you said about the loose approach and the possibility of kicking rocks off on cars. A couple of suggestions. Most often when I climb up there I hike the gully at 0.4 miles west of the tube and hit the ridge from that approach then rap into what ever climb im going to do. A lot less likley to kick rocks off. When rapping to the road from the loose gully at the 0.5 mi I wait till there is no traffic then move or rappel. You're right though, climbers in this area should take extra caution not to kick rocks into the highway. By the way, I'm working on a more detailed guide with photos and such. Allen