"Planners and researchers use transit catchment areas—the land around stations—as geographic units for predicting ridership, assessing the impacts of transit investments and, recently, for designing transit-oriented developments (TODs)," explain Erick Guerra and Robert Cervero. "In the US, a half-mile-radius circle has become the de facto standard for rail-transit catchment areas."
However, they note, "[t]here is surprisingly little evidence to justify any particular catchment area. Why a half mile? Why not a quarter mile or two-fifths of a mile? Is there anything special about a half mile or is this simply a convenient figure that has become an industry standard?"
In an article published in Access, the magazine of the University of California Transportation Center, the authors report the results of tests "to see whether the half-mile circle explains transit use better than other boundaries do."