I've never really thought cost per mile is very useful, because it favours commuting by car and long distance travel. The more you drive, the lower the cost. I probably spend more per mile on transit, but I can't really compare it because the expenditures are quite heterogeneous. I could factor in the cost of shoes, but car drivers also have shoes, they just don't use them for walking as much. They might also own a bike, but that's rarely factored into transportation methods for car owners. Likewise I only use the train once a week or so; per mile it might be higher, but yearly it's nothing.
