Heh heh, I only did about the first 2 miles before it got stupid-steep. But even that first part had lots of 15%+. 'twas a Personal Growth Experience.
The Stanford cycling page (which I found out about after the climb, naturally) says this about Welch Creek:
"...Sunol Regional Park stretches from 0.3 to 2.8 mile markers. Within the park, a stretch of road between 2.0 and 2.5 mile markers is bloody steep, in excess of 15%. Above 2.8 mark, the road emerges from the canyon and becomes steeper again, bloody steeper. By the finish the gradient is truly bloody relentless."
Someday I'll make it all the way up that sucker...