Brandon Martin-Anderson has produced a series of interesting maps from several US cities depicting the shortest path tree within its transportation networks.
The shortest path tree is produced by loading street and transit information into a piece of software that computes shortest routes, called Graphserver, and then exporting the resulting tree to a custom-format text file. That text file is read by a program written in Processing, which calculates the width of each branch by recursively summing the length of every branch upstream from the given branch. The Processing program then spits the output to screen.
Seen here are the shortest path trees of San Francisco Bay Area (first image) and Portland. Red lines represent transit, black lines indicate walking. You can read more about the process here.