“Trail access to the Skyline Boulevard region from Portola Valley.
— Brendan Busch
Alpine Road (trail) is one of few trails that allows mountain bikes to climb from the valley to the Skyline region. This dirt road was open to cars until the 1960s, and was maintained as truck-capable fire road until the 1990s when repeated mudslides washed lower parts away.
Now the trail is rapidly turning singletrack and only the top section resembles the graded road of the past.
To access, take the Alpine Road exit from Hwy 280 and follow Alpine west until it ends. Note that as soon as Alpine becomes narrow and wooded, it is good cycling with little traffic, so you can park much earlier on Alpine Road and warm up on the lower paved section.
At the end of paved Alpine there is a private road (Ciervos St) and a dirt turnout with a gate sporting a "Do Not Block" sign. Park here or retreat down Alpine for parking if there is no room.
Ride uphill on the old road, and then the clearly marked "bypass" section. The bypass section has steep sections, but they can all be ridden up by the determined. The bypass ends and the old road continues but all the steep parts are over.
Watch out for descending cyclists. The trail has sprouted a lot of sporty kickers recently and you may encounter downhill traffic having too much fun.
There are three trails that exit right (west) through Coal Creek Open Space to Skyline Blvd along the way. All are very steep climbs--the most reasonable being the third option ("Meadow" trail).
Eventually, you'll achieve Page Mill Road and access to Montebello Open Space (across the street) or Russian Ridge Open Space (right on Page Mill for about .6 mile).