photo: Richard Masoner via flickr.
We all know that the amount of greenhouse gas emissions varies widely whether you're taking a bus, a train, or flying; by the fuel efficiency of your car and by how many people are in the vehicle. But one thing which gets overlooked is the environmental impact of all the infrastructure, vehicle maintenance, fuel production, and all the other things that encompass the full life-cycle of each mode of transit. A new piece in the journal Environmental Research Letters attempts to rectify that for transit in the United States. The results knock train travel down a peg, but it still comes out a solid green travel choice:Infrastructure a Huge Part of Train's Impact
In their article Environmental assessment of passenger transportation should include infrastructure and supply chains, authors Mikhail Chester and Arpad Horvath of UC Berkeley find that when,
...total life-cycle energy inputs and greenhouse gas emissions contribute an additional 63% for onroad, 155% for rail, and 31% for air systems over vehicle tailpipe operation. Inventorying criteria air pollutants shows that vehicle non-operational components often dominate total emissions.
What's more, the report finds that active operation of vehicles varies widely: For buses and cars active operation is 65-74% of emissions and for flying 69-79%. However, for rail travel that drops to 24-39%.
Energy Source Another Large Factor
Part of the other issue with train travel that the report points out is similar to an ongoing issue with switching over to all-electric cars: They may be more efficient than other forms of transit, but unless the electricity that powers them is from non-carbon emitting sources then much of the environmental benefit is lost.
The comparison made is between Boston's Green Line and San Francisco's Muni, with former having nearly double the operational greenhouse gas emissions of the latter—even though the energy consumption per passenger is slightly higher in San Francisco. The reason? Massachusett's electricity mix contains about 82% fossil fuels, while San Francisco only uses about 49%.
How Does This Change Things?
There's a bigger version of the chart above in the journal article, but you get the picture:
For road travel, off-peak urban diesel buses are the worst, going down to conventional gasoline sedans—they used a Toyota Camry; using a Prius would've increased the differences between SUVs and pickups still more—with travel during peak hours on a diesel bus coming out well below these. In fact this last one is the most efficient form of travel per passenger mile of any form of transit analyzed here.
For train travel, long-distance rail beats local rail travel, though not by much. All are better than flying however.
Flying is pretty obvious, and consistent with what we've been recommending for a long time: If you have to fly, fly direct on the largest plane possible on a given route.
In total then, what that means is that though in general rail is still better than flying, the gap between the two closes a bit. Keep in mind that this is talking about passenger travel, not freight; and it's talking about specific conditions in the United States. Your mileage may vary under different assumptions.