To Market, To Market

I spent some time today trying to work out how energy efficient local food is. Turns out it’s kinda complicated. I tried to do a simple example and got bogged down in the numbers. Here’s how far I got…


The Question
Is it more efficient to get 20lb of produce from Spain via the grocery store or from the farm via the Farmers Market? I figured 20lb sounded about right for a grocery trip. Why Spain? A lot of the produce in the grocery store says “Product of Spain”.

The Distances

• My grocery store is 6 miles away.
• The Farmers Market is 12 miles away.
• The airport is 25 miles from the grocery store.
• Spain is 4000 miles away.
• The nearest sea port is 120 miles away.

The Vehicles

• My car: 35 mpg, 20lb capacity
• Half-ton pickup: 14mpg, 1000lb capacity
• Tractor trailor: 6mpg, 48000lb capacity
• Air freight: 1.5mpg, 200000lb capacity
• Cargo ship: 0.05mpg, 10000000lb capacity

Assumptions

• All gallons are roughly the same. Not actually true. Some are gasoline, some diesel, some jet fuel, and whatever a cargo ship uses.
• All trips are one-way. This is a shortcut to avoid second-guessing how often various vehicles travel empty.
• The source in Spain is a large-capacity farm that can load up a tractor trailer directly.
• A farmer loads up a half-ton pickup to capacity and drives about 50 miles to the Farmers Market.

OK, let’s start with the Farmers Market. I drive 12 miles to the market while the farmer drives 50. The pickup is getting 14mpg but it’s also carrying 1000lb of food. So the fuel to get 20lb of food to the market is 50mi/14mpg*20lb/1000lb = 0.07g. Now, my car can carry more than 20lb of food but, unless I’m picking up stuff for my neighbors, I’m just carrying the 20lb. So the fuel to get 20lb of food to my house is 12mi/35mpg = 0.34 gal. Total fuel spent 0.41 gal.

Now, what about flying in by air? Food gets shipped about 50 miles to the airport by tractor trailer (0.003 gal), then flies 4000 miles to my airport (0.27 gal), ships 25 miles by tractor trailer (0.002g) and finally 6 miles to my place by car (0.17 gal). Total: 0.445 gal.

Let’s try cargo ship. Assuming Spain’s seaport is a similar distance from the farm, we get: 120 miles by tractor trailer to sea port (0.008 gal), 4000 miles by ship (0.08 gal), 120 miles by tractor trailer from sea port (0.008 gal) and 6 miles to my place by car (0.17 gal). Total: 0.266 gal.

Problems With This

• I have to make a round trip to the Farmers Market because they’re not on my way home from work and anyway, they’re usually only open on weekends.
• The grocery store is on my way home, so it doesn’t actually add to my fuel consumption. So, that makes the Farmers Market even less attractive for me.
• I have no idea what the pickup does but I’m assuming it does a round trip too, taking home anything they don’t sell.
• Tractor trailers? Ships? Airplanes? I doubt they return empty, or even go back to their starting point.

Lessons Learned

• It’s all very complicated. There are far too many variables and unknowns to get an accurate picture of food distribution efficiency.
• That said, shipping by cargo ship – or even by air – isn’t nearly as bad as I thought.
• Just eyeballing the figures, it looks like the most efficient means of transporting cargo are ship and train. But both are limited to their media – water and tracks.
• Tractor trailers and aircraft are less efficient but are more flexible, which is probably why they’re used at all.
• A pickup truck is even worse and should probably be used only for low-volume specialty items.
• And by far the worst of them all is my car. I guess that makes sense, since I’m carrying only what I’m going to use, instead of filling the car to capacity. I could mitigate that if I were willing to buy and store 700lb of food at a time but, yeah, that’s unlikely to happen any time soon.

Conclusion
What does all this mean? I have no idea. It looks like buying local doesn’t really buy you that much. On the other hand, it doesn’t really hurt either. Dunno, I’ll probably keep shopping at the grocery store, but feel slightly less bad about doing it.