We've all heard it — "this city" or "that city" has more bicyclists because it has warmer weather. Looks like it's time to retire that statement.
If you look at the graph above, you can see that there's essentially no correlation between number of days below freezing and biking or walking to work. Furthermore, as Angie Schmitt of Streetsblog USA notes, "Alaska, the coldest state in the U.S., has the highest rate of active commuting. About 8 percent of workers there commute by foot and another 1 percent by bike."
However, aside from it being only a weak correlation, there's something very important to note: population in the US south boomed with the proliferation of air conditioning, which also happened to coincide with a big boom in automobile ownership and an auto-oriented planning paradigm. Probably much more influential than the heat, when it comes to levels of bicycling and walking, is the auto-oriented layout of the US South. Indeed, the Alliance for Biking & Walking has a chart on the correlation between auto ownership and bicycling/walking to work that supports this idea:
Boom! There's a match.
Of course, correlation doesn't equal causation. However, a bit of common sense helps with that. In very spread out cities with poor bicycle and pedestrian infrastructure, a bunch of speeding cars, and gigantic highways, people are less interested in biking or walking for transportation.
I grew up in Southwest Florida. I still ended up discovering (after going through my teenage car phase) that bicycling was more enjoyable than driving there. However, it was much less enjoyable and more difficult to bicycle there than in the 6 cities where I've lived car-free. The problem wasn't the weather — it was the highways and lack of off-road bicycle paths.