I've written before of the lowest-carbon way to get from here to there, and summarised it in this graphic:
The people at 1BOG (which stands for One Block Off the Grid) have made a much larger infographic which is more complete and breaks things down by the distance traveled. I don't usually like large infographics, but this one is worthwhile because it shows that for transportation, not one size fits all.
A few takeaway points:
For travel inside or near town, nothing beats a bike. (And it makes you feel happy. And it's good for you!).
For regional travel, a bus is best right now, at least in the US where trains have a far larger carbon footprint than elsewhere. Certainly buses are cheaper. But where high-speed trains are installed and running frequently, they are obviously the preferred way to travel: they're always full.
For long-distance trips, buses (and shared rides in efficient cars) are the very low-carbon way to go. But if you factor in the travel time, you can be forgiven for opting for flying.
On this infographic, travel by car assumes that it's only the driver in the car, which is lonely and tiring. If you share the car, a chore becomes a social outing and, as my graphic above shows,the per-passenger emissions go way down.
And I can't help mentioning: calling a car that does 32 mpg "efficient", as this infographic does for the benefit of American readers, is sort of embarrassing. In reality, 32 mpg is not very efficient. To give an example: my diesel Golf (38mpg) is on the gas-guzzler side of the average new European car, even the ones sold back in 2011. In a few years, my car will be a fossil-fuel relic, as the European emissions standards are progressively tightened.