How Far Will People Walk?
Jarrett Walker explains why it is so hard to determine how far people are willing to walk, and the common problem of measuring these distances by drawing a circle around a transit stop:
"Are we talking about true walking distance, or just air distance? Over and over, in transit studies, you'll see circles around bus stops being used to indicate the potential market area, as though everyone within 400m air distance is within 400m walk distance."
"Obviously, the market area around each stop should really be defined by the walkable area, which requires a knowledge of the local pedestrian network."