Brad Plumer reports on the simple math reported in a new study [PDF] published in the journal Transport Policy, that proves the single most important factor in encouraging cyclists is the creation of bike lanes and bike paths. Ralph Buehler and John Pucher, authors of the study, "found that the presence of off-road bike paths and on-street bike lanes were, by far, the biggest determinant of cycling rates in cities. And that's true even after you control for a variety of other factors like how hot or cold a city is, how much rain falls, how dense the city is, how high gas prices are, the type of people that live there, or how safe it is to cycle," reports Plumer.
By the way, you're not the only one who thought this equation seemed pretty obvious. However, as Plumer writes, "the authors do note that previous research was somewhat scattered on this question. A few studies had found that more bike lanes in a city were associated with more cycling, though it was unclear which was causing which. Perhaps cities were building bike lanes because they already had a group of devoted cyclists."
Buehler and Pucher seem to have finally settled the question.