This climb ranks #1 (by a long stretch) on the Sonoma-Napa-Lake County Top 25 list with a Fiets Index of 7.23 (well beyond its closest competition, Bald Mountain, Socrates Mine Road and Geysers Road). The first 6 miles are paved (3 miles on Old Lawley Toll Road and 3 miles on Hwy 29) and averages a respectable 6% which is fairly steady throughout, particularly on Hwy 29.
Old Lawley Toll Road Segment
The second half of the climb is on a fire road consisting of dirt and gravel which is easily manageable for the first couple of miles on a road bike, but becomes steeper and softer/looser as you climb and is particularly difficult over a .2 mile 13.9% stretch just before the summit. We used a Specialized Roubaix with 28mm tires, 30t cassette and compact crankset and barely made it up the final stretch. It is recommended that the gravel/rock portion of this climb only be attempted on a cross or mountain bike.
Traffic/Roadway surface: The road is good for the first 6 miles but does become dirt, gravel and rocky over the final 5.5 miles to the summit. There is minimal traffic on Old Lawley Toll Road, moderate traffic on Hwy 29 and no traffic once you turn onto the gravel fire road. Caveat - we are not sure if the first mile of the fire road (which we take in lieu of the hiking trail at the entrance to Robert Louis Stevenson State Park) is private property.
Steepest ¼ mile starts at the last ¼ mile (12.3%)