I've always been frustrated with Google Maps' biking instructions. There are some major trails by where I live that are attached to communities by little 50 foot extensions, but the extensions aren't documented. As such, when you search directions from the neighborhood to another part of town, it doesn't use the obvious MUP that's right there. If anybody is looking to move into the area and is interested in biking or walking anywhere, the connection of the trail to the neighborhood is hidden.

I don't know if I'm late to the game on this, but I had never seen anything about it online anywhere… you can edit the public maps yourself. If you open up Google Maps, in the bottom right-hand corner there's 'edit in google map maker.' You can select 'Add New -> Roads Rivers and Parkways -> Trail/Path and just lay down where the bike path is that isn't documented. Like Wikipedia, there's a peer-review process from other users. Typically within a few hours edits are added, and within a few days, the trails are included in the direction finding algorithms.

As an aside, you can go in and review changes that other people have made. These range from addresses to building locations to points of interest.

In the past couple of days I've added 2 miles of trails just to my little town. I think it helps document the presence of bikeable areas, which is important in getting people to go out and try it sometimes… plus seeing Google Maps updated with stuff that YOU put in there, is pretty sweet.