With the implementation of traveled format graphs (thanks again @yakra!), and in response to an email I received from a user, I put together a small program that reports the least- and most-traveled segments in a graph. Running on the full tm-master-traveled.tmg graph, which has 481,044 edges, there are 190,498 not traveled by anyone.

Before I report the most-traveled edges, I'll give a chance for guesses. There are 2 of them tied, traveled by 138 of our 241 users, and they're consecutive edges along a single route. What do you think?