Adding bicycling directions wasn't an easy task because Google had to gather information about bike trails, bike lanes, uphill or downhill slopes. Google's algorithms try to find the best route and they take into account a lot of interesting factors.
"Our biking directions are based on a physical model of the amount of power your body has to exert given the slope of the road you're biking on. Assuming typical values for mass and for wind resistance, we compute the effort you'll require and the speed you'll achieve while going uphill. We take this speed into account when determining the time estimate for your journey, and we also try hard to avoid routes that will require an unreasonable degree of exertion."