Number of bicyclists
Ummm.... what other more direct indicator is there? Algorithms are helpful, but sampling of actual cycling is key.
I like this a lot. Safer biking is correlated to number of cyclists on the road. The more cyclists on the road the better the coexistance between drivers and cyclists will be. For example there are huge shifts in driver behavior between Copenhagen, Portland, Chicago, and Dallas. More specifically different parts of town have different intensities of cyclists. For example even though downtowns often have major streets, almost any street in any downtown is moderately bikeable as opposed to a street in the suburbs.
Ville M commented
Even better if you get sampling of type of cyclist, 1 kid or women is worth 2 points vs men 1 point each or something like that. This data wont be available in most towns, but where you have it, data availability alone means thats its a bikeable place.
More evidence that for bicyclists, there's safety in numbers, via Wash Cycle http://bit.ly/bgndvz
I agree. Maybe look at number of local bike clubs & bike races.