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Intermodal Choices and Greenhouse Gases

Road Rail Air Water

Many websites provide online greenhouse gas emissions calculators. To use a typical emissions calculator, users specify the weight of a shipment, the number of miles it must travel, and the mode of transport (road, rail, water, or air). The calculator then provides numbers representing the greenhouse gas emissions associated with the shipment.

The TERC Intermodal Emissions Calculator is specifically designed to help users compare different routings and transportation modes. The format makes it convenient to enter data for multi-stage routings between transfer points, and provides graphical comparisons of alternative routes. Otherwise, the results it provides are similar to those which would be provided by most typical calulators using the same input data.

How are the results calculated? Can they provide a fair comparison between modes? This page is intended to help users understand what the numbers are based on, and what they can actually tell us.

How do emissions calculators work?

Greenhouse gas emissions are directly related to fuel consumption. The dominant contribution is from the carbon dioxide released when the fuel is burned, and it is almost always released directly into the air. So if you know how much fuel you used moving from A to B, you've got all the input data you need for a good estimate of the associated greenhouse gas emission.

Typically, a shipper will know the number of miles between transfer facilities and the weight of the shipment, but will have no information about actual fuel use. In order to provide a meaningful estimate, calculators generally assume that the amount of greenhouse gas emitted is proportional to the distance traveled and the weight of the load -- double either one, and you double the emission. The emission is equal to a proportionality constant times the distance times the weight.

The proportionality constant specifying the rate of emission per unit of weight and distance is called the "emission factor", and is expressed in units of grams of "carbon dioxide equivalent" per ton of freight per mile traveled (g CO2-eq/ton-mile). (The "equivalent" in carbon dioxide equivalent accounts for emissions of other gases besides carbon dioxide, like methane and nitrous oxide, that also have greenhouse warming potential.) Typical values for various transportation modes, using numbers based on an EPA publication, are 253 g CO2-eq/ton-mile for diesel trucks, 27 g CO2-eq/ton-mile for rail, and 41 g CO2-eq/ton-mile for domestic water transportation. (Other sources provide somewhat different numbers, but these values are reasonably representative of the general consensus.)

What can the numbers tell us?

The numbers generated by simple emissions calculators can only be estimates. Even two identical vehicles traveling the same route with the same load can use somewhat different amounts of fuel under different conditions. The effects of all the other variables that can affect fuel consumption are lumped into the emission factor, and are accounted for only in an average sense. The question is whether the estimates are accurate enough to be useful.

Many familiar environmental impact comparisons -- paper vs. plastic, for example -- are too close to call. But the emission factors for road and rail differ by a factor of ten. As far as greenhouse gas emissions are concerned, a shipper could decrease the carbon footprint of a shipment traveling a hundred miles by road by sending it instead by rail, even if the rail route were less direct. How much less direct? Based on emission factors alone, the shipper would conclude that the load could be sent by rail up to nine hundred miles out of the way and still create less environmental impact (by that one measure) than if sent one hundred miles directly by truck. Refining the calculation might cut that ratio down somewhat, but it will remain a safe conclusion that shipping by rail over a route several times longer will create less greenhouse gas emission than shipping by road.

But this is not the end of the story. Even if strictly true, are mile-by-mile comparisons a fair measure of environmental performance? There may be environmentally sound reasons to prefer modes with a higher greenhouse gas emission if all the circumstances are taken into account. For example, if perishable goods are delayed in transit, the greenhouse gas emissions associated with replacing them could significantly outweigh the emissions from a faster transport mode. For a closer look at these issues, see the TERC page Best Speed for Fuel Economy.

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