Source: Federal Transit Administration

I spent the morning geeking out on a new report from the Federal Transit Administration that answers a question I’ve had and I’ll bet many of you have also pondered: what’s the impact of taking mass transit versus driving in terms of greenhouse gas emissions?

It’s a natural question because buses, of course, burn gas (natural gas in the case of Metro buses) and Metro’s subway and light rail fleet are powered by electricity that is created in a variety of ways, ranging the burning of fossil fuels such as coal (bad for climate change) to nuclear (good for climate change although there are other issues).

Carpooling helps makes driving cars be more competitive with transit when it comes to emissions of greenhouse gases. Click above to see a larger image

The good news for Metro is that the agency’s heavy rail (the subway) ranked fourth best out of 14 systems reviewed by the FTA and Metro’s light rail system was third best out of 29 systems. Metro’s bus system, which runs mostly on natural gas, was middle-of-the-pack but bested the national average. Metrolink commuter rail (which is partially funded by Metro) was sixth best in its category in the nation and also was better than the national average. All four charts are posted after the jump.

As the chart at right shows, carpooling greatly helps lower the greenhouse gas emissions per passenger mile of driving a private car, but still can’t beat a nice, crowded subway train.

The report also provides a good overview of some other research that has been done in this area. Two big points stood out:

1. Studies have found that greenhouse gas emissions from transit are significantly higher when construction of transit is factored into the equation. That’s not good and a lot of hay has been made of that both in academic circles and, in particular, among anti-rail advocates. That said, even with construction as a factor, transit still produces fewer greenhouse gas emissions than private cars — on the condition that transit passenger loads reach certain levels. A subway, for example, needs 19% of its seats filled to be competitive with private cars in terms of lower emissions.

2. Mass transit tends to promote more compact development and that has a carryover impact even to those not taking transit. “In other words, in areas served by public transportation, even non-transit users drive less because destinations are closer together,” according to the report.

Of course, there are some caveats here. The report is based on 2008, a year when transit ridership peaked; the fewer the riders, the less efficient transit becomes. It’s probably also fair to say that there have been subtle shifts in the makeup of the nation’s private fleet of autos and that they’re slightly more fuel efficient now than two years ago — although if I’m doing the math correctly (and I wouldn’t bet on it) using data from www.fueleconomy.gov, a Toyota Prius driven solo is competitive with some transit systems but still has a hard time beating a subway or light rail with decent numbers of passengers.

On the other hand, the FTA report also points out that mass transit powered by electricity should also be getting a deeper shade of green as long as more renewable sources of electricity come online. That’s excellent news. Excerpt:

As the electric power industry shifts to more renewable sources of energy, as being mandated in several states, electric public transportation systems provide even more emissions reduction benefits. When the electricity is generated from a zero emissions source, such as wind, hydroelectric, nuclear, or solar, the public transportation systems that use these power sources are also zero emission.

Several transit agencies are installing on-site renewable energy generation to power parts of their systems. Boston’s transit agency is installing wind turbines, New York City Transit plans to harvest power from the tides by installing turbines in tidal waters, and Los Angeles Metro is installing solar panels on its properties.

Below are the charts showing the greenhouse gas emissions for heavy rail, light rail and buses — the charts are taken from the report. Click on each to see a larger version of them.