Some people have pointed out that most transportation will be going, whether or not you take them, and use it to suggest that it doesn’t matter if you take them, since the vehicle would go anyway. I disagree with this perspective.
Whenever you buy a bus/plane/train ticket, you are increasing the demand for such services. Sure, your specific presence isn’t enough to make a bus travel when it otherwise would not have, but think of it like voting. It’s your small contribution to the continuation of the service. You are telling the world that this is a good way to travel, and that the environmental outputs are acceptable to you. If you validate an industry you have issue with, you aren’t really being true to your beliefs. Buying a ticket gives that industry more money to continue doing what it is doing and expand their services.
As I see it, the only zero-emission methods of getting across the country are either using your own feet (or bike wheels) or catching a free ride. By this, I mean any method of transport that really would be doing what it was doing, even if not a single person offered money to ride on it. This includes hitchhiking, hopping a freight train, or driving someone else’s car to get it somewhere they need it (search around! People really do do this, and you might just get lucky.)
Of course, whenever you take a motorized transport, your weight alone reduces fuel economy, but if you want the absolute minimum, those are the best ways to go, and if you do your research and talk to others who’ve done it, you’ll realize they aren’t all as crazy as they might sound. I’m off hitchhiking tomorrow, actually! It’s a great, cheap way to see the country and get to know the people you share a national identity with.