Boulder, Colorado is one of just three “platinum” bicycle friendly communities in the United States (along with Davis and Portland). And it also has the 2nd or 3rd highest bicycle mode share in the country, as well.
How did Boulder do it? The answer, perhaps surprisingly, can be summed up in a single word: “underpasses”. Or, to get fancier, “Grade-Separated Crossing Treatments“. The Pedestrian and Bicycle Information Center describes the history:
“In 1984, the city adopted the Boulder Creek Corridor Plan that recommended development of a continuous path along the entire length of Boulder Creek. This corridor would serve both as a flood hazard mitigation measure and as a continuous urban park for recreational and transportation use. It would also serve to restore and enhance wetlands along with fish and wildlife habitats.
“The construction of a continuous shared-use facility required separated grade crossings at each intersection throughout the corridor. Existing creek underpasses were converted to include shared-use path underpasses through fairly simple modifications. Upon its completion, the Boulder Creek Path was instantly popular and quickly became a much loved community amenity.
“The public acclaim of the Boulder Creek project led to an increase in public discussion about the desirability of extending and continuing the concept of the Boulder Creek project along Boulder Creek’s tributaries within the city. As a result, the city designated over 32.2 km (20 mi) of stream corridors along six tributaries of Boulder Creek for inclusion in the Greenways Program.