Welch Creek is perhaps one of the two toughest public paved climbs in the San Francisco Bay Area, with Bohlman-On Orbit-Bohlman.
Welch Creek was first on the Low-Key Hillclimb schedule in 2007, and is on the 2010 "menu of pain".
It starts out innocently enough from Calaveras Road, quickly crossing a cattleguard. Indeed, it appears to be simply a short ranch access road: nothing much of interest there. But from its humble beginnings it quickly turns upwards, with extended portions at over 12%. The pavement (as of 2007) is strangely excellent for such a remote road, helping with the steepness. The effort is not surprisingly rewarded with wonderful views of the Sunol Regional Wilderness from the top.
This is a must-do for Bay area riders.