Ah yep, I got distracted halfway through, and when I got back it no longer made sense :o  I presume there are much fancier rules used in practice, which include distributions of source travels, preference for comfortable seats, certain psychological preferences for travel times (people treat trips under half an hour quite differently to trips over half an hour).  I suspect there are similar steps for long distance trips: Once I am committed to a long trip, I would rather do it slower and in comfort than at top speed, so I choose the 10 hour train journey to my grandmas rather than the 1 hour plane flight (requiring me to leave home 3 hours before hand, at 4am).  I expect reliable and cheap internet access will tilt this even more.