I've been noodling on some of these "eat local" ideas while doing some mindless tasks around the house, I just googled a few things to tie my thoughts together...
Obviously there's much more that goes into the price of a head of lettuce than just cost of diesel to ship it from the left coast to the right coast.
... That lettuce was so old by the time I got it, it didn't have much shelf life left and I ended up throwing half of it out. So, how "cheap" was my lettuce, really?
When I go to my farmer's market and buy directly from the small grower who lives 15 miles away...
OK, there are plenty of reasons why someone might choose to buy local - added variety, possible higher quality/freshness, the chance to talk directly to the grower, etc. Or they just want to. I do some of it for those very reasons. However, I'm just not buying that it is a slam dunk that the "15 mile meal" is vastly superior energy-wise, to the proverbial "1500 mile meal". And that seems to be the main reason I hear around the "buy local" fans - they focus on those shipping distances. Some personal background, and then some numbers...
So my local Farmer's Market is a short 3-4 mile drive, but it becomes a "destination" trip because I don't do much shopping in that town, and at one day a week it's unlikely for me to combine the trip with something else there. So 7 miles to pick up a few veggies - 1/3 gallon of gas. Small market, not so much choice, sometimes I buy nothing, other times what I buy doesn't fit so well with our meal plans, so some goes to waste. IME, no big difference from shopping at the big stores in that regard.
OTOH, a COSTCO, three big chains and a Trader Joes are all near each other ~ 5 miles away in the other neighboring town where we do most of our shopping. And we often combine that trip with other things, plus we get our paper goods, toothpaste, etc, that we can't get at a Farmer's Market.
Now, there's a larger FM in the county that I've heard good things about (though DW went once and was not impressed). But that's a 25 mile round trip for me - a gallon of gas.
So let's extrapolate a bit....
From this link,
Is local food really miles better? - Salon.com
the writer says he observed at his local FM, the farmers brought 200 to 2,000# of produce in their truck. He mentions that a semi carries 40,000#. So it takes 20 trucks/farmers to provide that semi worth of produce (assuming they all packed out at 2,000#). If we use your 15 mile number (30 mile round trip), that is already 600 "food miles" accounted for, or 6,000 worst case. Hmmmm.
OK, so a small truck *might* get 3x the mpg of a semi, so that would be ~ 200 to 2,000 "food mile" equivalent (see where "food miles" breaks down?). That also assumes the farmer doesn't have another carload of staff driving along. But it gets worse...
Let's say that 1 out of 20 visitors to the FM come from 10 miles away, and the other 19 don't go a single inch out of their way (I think that is being very generous). That is an average one mile per customer. And lets assume that each customer on average buys 20# of produce (that also seems very generous to me). At 1 mile per 20#, that takes 2,000 extra miles for those customers to by one semi-load of food. So we are way over the 1500 "food mile" mark with what I would say are some very generous assumptions. I'd bet reality is far worse, with the average miles being higher, and the average purchase being much smaller.
One could pick numbers all over the map. I'm not trying to (and can't) "prove" that one is more fuel efficient than the other - just trying to point out that what might seem obvious on the surface isn't necessarily so.
I'll definitely go out of my way when the quality and value warrants it. I've never bought the sick looking sweet corn I've seen at Costco, but I can swing by a nearby farmstand any day of the week during the season, and get some great tasting corn (I know, I spent some years on a farm - to us fresh corn meant picked with an hour of dinner time). But I'm not claiming that it saves fossil fuel, I just like that sweet corn. And I like pineapple and bananas and mangoes, too. And raspberries and herbs from my yard. IOW, I think if we just choose the value that fits our needs, we'll be OK. The other stuff will fall in line. If energy prices affect produce prices so much, adjustments will be made.
Well, that got long - so to reiterate my intro, in case anyone got the wrong idea along the way - I'm not casting dispersions on "buy local" or anyone who chooses to do so (as I do myself when it fits my needs), but I don't think they can automatically justify it on the fossil fuel used just by comparing a "1500 mile meal" to a "15 mile meal".
-ERD50