Re-reading "The Next American Metropolis"...

...it strikes me that the use of the half-mile boundary of a TOD is actually a bad one. Calthorpe specifies not a distance, but a 10 minute walk. The half-mile standard comes from assuming that people walk 3 miles per hour, hence a half mile in 10 minutes. However, that represents an unimpeded distance, without having to wait to cross roads.

The reality is more prosaic: an impeded 10 minute minute walk might only take you 2/3 as far. And indeed, when Calthorpe gives a number, he suggests 3/8ths of a mile (600m/1980').

And of course, distance from the transit station varies by rather much by the street network. Jarrett Walker of Human Transit describes it well, so I won't belabor the point.

Cities delineating a TOD might do better to use a '10 minute walk' standard, measuring linear distance along public right of ways, and demanding frontage on such a right of way.