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The Atlantic Cities has a good piece today debunking the fashionably counterintuitive notion that cars are in fact greener than public transit. But buried in there are some figures from a 2009 Federal Transit Administration report that paint D.C. in a not-so-favorable light.

Take a look at this chart from the report:

Of the 10 largest “directly operated” bus systems in the country (as opposed to privately operated systems, which account for only 14 percent of bus passenger miles nationwide), the D.C. area’s Metrobus comes in dead last in terms of pounds of carbon dioxide emitted per passenger mile. In fact, Metrobuses emit more carbon dioxide per passenger mile (.782 pounds), on average, than the general car trip emissions rate of .59 pounds (though considerably less than the average single-occupancy car trip rate of .96 pounds).

What accounts for this? A large part of the answer is ridership: Metrobus is in a three-way tie for last place in average percentage of seats full, at just 23 percent. When buses are full, they’re much greener than cars. But as the Atlantic Cities piece points out, buses don’t just aim for eco-friendliness; they also provide a service to poor, underserved areas, where ridership may not always be high but the need is greatest.

Still, Metrobus is among the worst performers in terms of vehicle carbon efficiency. Its buses emit .182 pounds of carbon dioxide per seat mile regardless of ridership, higher than all of the other top 10 bus networks except for Chicago, Philadelphia, and Miami, and much higher than King County, Wash.’s .118 and Minnesota Metro Transit’s .122.

And it’s not limited to buses: According to the FTA report, WMATA is also a lower-than-average performer in the heavy rail department:

A WMATA spokesperson responds to the below-average emissions performance numbers:

On bus, the main difference is ridership. To calculate the CO2 per passenger mile, the denominator is ridership. Lower ridership = higher lbs. We’re at 0.7 lbs CO2/passenger-mile, and many of our peers are 0.5. Our average load is 11-12 passengers, our peers are 13-18. Our deadheading, average speed, and fuel efficiency are basically normal.

On rail, a lot of the difference has to do with our local utilities, and is not a reflection on Metro. Pepco and Dominion, like the rest of this region, rely on a lot more coal than utilities in other parts of the county.

But before you despair, take comfort: When it comes to traffic safety, D.C. is tops in the country. Our 3.97 traffic deaths per 100,000 people are not only fewer than any state; they’re one-seventh of the deaths in the most dangerous state, Wyoming.

Update: The response from WMATA was added after this post was published.