Typically climbs are up a single road (like Old La Honda Road), or up a relatively unambiguous path to a summit (like Diablo North Gate Road to Summit Road). But in the San Francisco Bay area, like many other places, there are often ridge roads into which roads up the hillside intersect. As Baird Webel pointed out to me when I was at Stanford, these side roads will typically seek out relatively low points in the ridge, so by turning when hitting the ridge road, the climb can be extended. He wasn't claiming this as an opportunity, but rather as a fate: don't take the ridge road for granted as the top of the climb. Another Baird quite, "We need to descend into that?!?!", when looking down into the smog-shrouded Silicon Valley was similarly memorable, but that's a digression.
Anyway, I typically ignore these sorts of options as "climbs". From the top of Old La Honda Road, for example,the climb can be extended without much traffic interference by turning right on Skyline Boulevard, climbing further to the peak before the descent to the store (and nearby Alice's Restaurant). I've never made a continuous targeted effort all the way to the summit. Old La Honda is what it is and that's the climb: the longer climb isn't "better" due to being longer. Rather it's a dilution.
But one section of Skyline is particularly challenging for Skyline, and that's the climb from Black Road to Castle Rock State Park. The thing about Skyline climbs is they are so easily underestimated. Skyline is a ridge road, after all, and as Baird was referencing, once you've reached the ridge road there's a tendency to consider the hard work over. A climb on the ridge road, even if it's substantially less challenging than the climb taken to reach it, nevertheless may be the coup de grace on tired legs.
This is what happened to me during the Sequoia Century 200 km ride in 1994. We'd climbed Mountain Charlie Road to Summit, which is a southern extension of the ridge road, then ridden north through a hilly narrow section of Skyline before arriving at Black. There the road begins an uphill slog to Castle Rock, gaining approximately 192 vertical meters (630 feet). With fresh legs and expecting a climb that's a nice little challenge, but finishing up the climbing of a 200 km ride with 3000 meters of total climbing, it can be a long, slow grind.
So I crawled up the hill until finally the grade relents and one reaches the rapid descent to the Highway 9 junction. Soon after we reached the ride's final rest stop at the fire station, where to my amazement there was an electric blender spinning up delicious smoothies. It was one of the most delicious smoothies I've ever had, given the context, and with it my fatigue disappeared for the final rollers to Page Mill which we descended in conjunction with Moody Road to reach the finish.
Anyway, that section of Skyline is a good climb. And since it begins at the top of Black Road, the most challenging single road to Skyline Boulevard, it makes for a nice combination. Black gains close to 11% sustained over two nice chunks, with some respite at the Lakeside school in between. A nice alternative to upper Black is heavily switchbacked Gist Road which connects to Skyline in between, but Black by itself is a tough climb on its own.
Here's the profile:
I ran these climbs through my rating code and I get the following. Note the rating formula is not linear: two climbs together need not have the same rating as the sum of the component climbs (this nonlinearity is by design, as I think it matches perception: a gradual climb situated between steep climb segments may actually subtract from the total rating, even if the gradual climb itself has a positive rating):
Black Road : 150% Old La Honda Skyline to Castle Rock : 31% Old La Honda total climb : 170% Old La Honda
The Strava KOM is presently held by Adrien Costa, the 2012 Low-Key Hillclimbs solo male rider champion.