As transportation energy use and greenhouse gas (GHG) emissions rise higher on the policy agenda, all sorts of claims are being thrown around as to which modes of transportation are "greener" or more energy-efficient. Since the consequences of bad choices could have huge implications for both personal mobility and goods movement, we really need good information on which to base policy decisions. Everything under the sun is being promoted as smart transportation by those who favor a particular mode, whether it's bicycles, short-sea shipping, or light rail transit. I've recently come across two studies that offer some useful data to help us sort these things out.
The first begins with a detailed spreadsheet on energy use and CO2 emissions for 15 urban transit modes. Most of the data come from the 2006 National Transit Database, with energy use (BTUs) and CO2 emissions calculated by the analyst from reported fuel use data. The data are for all transit agencies that report to the Federal Transit Administration. Analyst Randal O'Toole has drawn on this database for a recent Cato Institute paper (.pdf) titled "Does Rail Transit Save Energy or Reduce Greenhouse Gas Emissions?" In the paper, O'Toole compares BTUs and CO2 emissions per passenger mile for various transit modes, as well as current passenger cars.
By far the worst performer is ferry boats (1.73 pounds CO2/passenger mile) and the best is the Toyota Prius (0.26). The current automobile fleet averages 0.54, light trucks (SUVs, pickups, etc.) average 0.69, and motor buses average 0.71. By contrast, light rail systems average 0.36, commuter rail 0.29, and heavy rail 0.25.
But before you jump to conclusions, you also need to do what O'Toole does in the paper and look at trends in modal emissions. New rail systems being planned today won't begin running till about 10 years from now, and will have about the same energy consumption as current ones. But fuel economy for autos and light trucks will be steadily improving over that time period, and much progress is also possible with hybrid-electric buses and biodiesel buses. We really need to do such calculations looking forward, not backward.
Another recent work takes a broader view of the subject, estimating full lifecycle energy and GHG impacts (though of a more limited number of modes). The authors are Mikhail Chester and Arpad Horvath of UC Berkeley. Their analysis includes five modes of transportation and 12 sample vehicles, including a Toyota Camry and a Boeing 747. By "lifecycle," the analysts mean the energy and GHG impacts of producing the vehicles and their infrastructure, as well as their operating and maintenance costs over their useful lives. Among the results:
Urban buses during peak periods have the best energy and GHG performance, but are the worst performers during non-peaks, when they operate with very low passenger loads;
Air travel is environmentally competitive with rail travel, and outperforms rail on these measures when aircraft load factors exceed 80% (as they have recently);
Urban diesel buses are "greener" than rail systems (such as the Bay Area's BART) once infrastructure and other lifecycle effects are included.
This lifecycle analysis did not include future automobiles such as plug-in hybrids, or even conventionally fueled vehicles after the fleet turns over in response to the new federal CAFÉ requirements. So it's hard to tell what these comparisons would look life for, say, 2020 or 2030. This appears to be an ongoing project, so I'm looking forward to further analysis from these researchers.