Today I had the "pleasure" of crossing 101 4 times on my bike between San Mateo and Santa Clara. 3rd Ave, Whipple, Maple, and Steven's Creek Trail. Whipple deserves a separate post so I'll start with the other three. Feel free to add in the comments and I'll integrate them.
Let's start with 3rd Avenue. 3rd has a separated bike path that goes down the middle of the overpass, a similar sensation to the bike path on the Dumbarton Bridge. You are protected while ON the overpass, which is good. The problem is that you have to get onto and off of the overpass itself.
Looks pretty straightforward, right? Wrong.
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The approach to the bike path is actually on 4th St (3rd is one way the other direction) and if you are riding on the right side of the road, you must cross three lanes of traffic, one of which could be going either way. A block back at the intersection of Humboldt and 4th, I decided to cross to the left side of 4th. There is a white line that creates a "chunk of separated space" there - it's not a bike lane, who knows what it is. I guess it's where San Mateo hopes you will ride to approach the bike path. I ended up jumping onto the sidewalk, with more experience I might take the "chunk of separated space". You are allowed to ride on the left side of the road on one way streets, but this thing is just sort of ugly.
Once you get on, you are onto the "onramp" of the path, there is an "offramp" going the other direction towards 3rd. Both "ramps" are bidirectional, I actually saw a cyclist approaching the main pathway from 3rd. I assume they rode on the sidewalk down 3rd since 3rd is one way the other direction.
At the East End, more strangeness.
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The path ends in the middle of the street, at an intersection. You are in the middle of traffic going either direction around you. My thinking is that the official position is that at this point you are supposed to walk your bike to the appropriate side of 3rd Ave based on where you are headed, when you get a crosswalk signal to cross 3rd from the middle. This is what a pedestrian would do, and it's what the cyclist in front of me did, I followed. I then got back into regular traffic on "J Hart Clinton" which was no fun quite frankly, then left into Ryder Court Park to get on the bike path to join my friends. Looking at Google Maps, it appears I made a mistake. The left into Ryder Court cannot be made by cars, as such there is no real provision to make a left hand turn there (turn pockets) but there is a light. Turns out the pro move is to go South on Norwalk after exiting the bike path, then make (a somewhat difficult) left onto a bike path, which then ends in the crosswalk into Ryder Park.
From the reverse direction, the best approach to the 3rd St Path is from Norwalk, in the left turn pocket (taking the lane) and then turning left directly into the bike path instead of the travel lane (or right from Norwalk the other way, and god help you if you are on J Hart Clinton).
Right then. Next - Maple. Maple is a piece of cake relatively speaking. From the Peninsula side, you turn right from Veterans NB or left from Veterans SB, or if you are lucky you are on Maple already, perhaps coming from Caltrain via Winslow/Middlefield. There is very little auto traffic on Maple, and no on/offramps. Coming from the other direction you are coming from either the Marina or Blomquist Road, both low traffic areas. This sort of makes Maple an "overpass to nowhere", I used it today in fact to bypass the nearby "Bridge to Nowhere" which is in a field that is currently muddy.
Next - Steven's Creek Trail.
Self Explanatory. The only rub is knowing where the access point is. On the Bay Side, take Pear SB from Shoreline, then right onto Inigo, left onto La Avenida, which dead ends into the trail entrance. The closest entrance on the Peninsula side is Moffett. This is a pretty nice trail, observe reasonable speed as there are lots of pedestrians.