I think you missed my point. Transporting the meat to the market is only a relatively small part of it. The majority of meat's carbon footprint comes from the energy associated with feeding and watering the animals (including the production of the petroleum-based fertilizers, pesticides, etc.). If you took the resources used to grow the grain for a cow and grew crops directly for human consumption instead, you could feed probably 10 times as many people with the same amount of resources. (And even that may even be a gross underestimation.)
...or we could just eat pastured animals.
* theoretically more healthy (though I am skeptical this is the panacea we are told it is)
* if done in combination with ending corn subsidies, it frees up lots of tax money and actually lets people see the actual costs of their meat. Subsidizing feed corn is subsidizing meat (and in turn subsidizing things like fast food). This also frees up land for growing crops for direct human consumption.
Ending corn subsidies would certainly help (mostly by raising the supermarket cost of meat, lowering consumption), but pasturing would have little effect. Even then, you still need 10x as many acres of pasture to produce the same amount of food as you could if you grew plants for humans instead.
Using land for pasturing animals only makes sense, environmentally speaking, when it's not fertile enough to be used for anything else.
Most people don't want to do the research. Strictly from a "carbon footprint" standpoint it's likely that the crap supermarket chicken comes from more locally than the farmers market chicken. Just saying. I would also add that at least in my local supermarkets the ones buying meat aren't buying whole chickens and chuck roasts and pork chops. The only fresh meat that's getting bought is ground beef and only if it's on sale. Otherwise, they're buying chicken nuggets, hot dogs, frozen entrees and stuff of that nature. Remember, Americans don't like to cook.
In the absence of market distortions caused by subsidies, unit price would be a good proxy for carbon footprint -- generally, stuff that takes more energy to produce costs more. The trouble is, because "conventionally"-raised (i.e., petrochemically, industrially raised) meat is so heavily subsidized, it's very hard to tell whether the supermarket chickens would "naturally" be cheaper than the local/humane/organic/self-actualized hippie chickens or not.
(By the way, lest people get the wrong impression, I'll state for the record that I do eat meat, and when I do I buy the cheap "conventional" stuff -- but I buy whole chickens and large pork or beef roasts (or even primals) and break it down myself (or more often, have my wife do it) because it's usually cheaper per pound that way. (The exception is that ground chuck is cheaper than chuck roast, for some reason -- probably because of "pink slime," I guess.) However, since I recently learned just how bad meat production is for the planet I've been trying to cut back such that I start using meat as a "seasoning" as opposed to the bulk of the meal.)