Proximity to recreational bike path entrance (separated from the road) + length of path
Paths that lead no where are not that interesting to most folks. Some weighting needs to be given to whether separated bike paths are useful to get places or are just recreational trails to nowhere.
mag ellen commented
Length and path surface is very important. And connection to other long trails.
Commuter bike lanes are important. But off-street recreational biking opportunities are also important, especially for families. I would suggest two metrics: (1) Distance to nearest recreational bike path entrance (park, etc.) and (2) Total length of recreational bike path within a given radius (say, 5 miles).