Seattle Bus flickr user dkjdThe typical Seattle-area bus has 42 seats.  One of the big, articulated buses has as many as 64 seats.  But on a commute the other day, I counted at least 92 passengers in my bus, including folks who were standing in the aisles.  There were so many folks on the bus that the driver had to leave a few would-be riders at the bus stop.

And then there’s rail.  Seattle’s soon-to-be-opened Link Light Rail will have 74 seats per car.  But including standing passengers, each car can hold as many as 200 passengers.  If transit ridership stays high, and commuters take to rail as much as some people hope, I imagine that many of the rail cars will be close to full during peak hours.

I mention all this because of the ongoing debate over the climate impact of various forms of transit.  In particular, I’ve gotten some very reasonable pushback on the chart to the right, which we published last year.  The chart compares the climate impacts of different transportation modes: the blue lines represent CO2 emissions, measured per passenger mile traveled. 

But as you may notice, I don’t estimate the impacts of buses or trains that are full to capacity.  That may be an oversight, since quite a few buses are over-full at this point! If I were to bump the ridership figures up to actual peak levels that we’re experiencing now, the climate impacts of rush-hour transit would look even better.

Now, new research out of UC Berkeley confirms these findings:  when a transit vehicle is reasonably full, it’s a fantastic deal for the climate.  And that’s true even when you consider the entire “life cycle” impacts of travel—including the CO2 released from manufacturing vehicles, and building roads and rail lines.

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  • The Berkeley Institute for Transportation Studies recently took a look at comprehensive life-cycle emissions from various forms of transportation, focusing on the Bay Area. Take a look:Berkeley ITS life cycle transportation emissions

    As you can see, a well-utilized transit system—a peak bus, or a BART train—can cut CO2 emissions by well over two-thirds, compared with driving.

    The ITS has done a real service here. By including the impacts of fuel production, infrastructure, and vehicle manufacture, they’ve made the comparisons of different travel modes much clearer.  So we can see, for example, that building a rail line (the red segments) has quite significant  greenhouse impacts.  Still, the overall emissions of trains are far better than for cars.

    To me, the most interesting comparison here is the buses.  At rush hour, buses do great!  Off-Peak, buses do lousy!  The only real difference is the number of seats that are filled.  A near-empty bus trundling along at 3 to 4 miles per gallon is no great deal for the climate. 

    We could probably do similar peak-vs-off-peak comparisons for cars and trains as well.  During rush hour, cars do worse than they do at other times: people commuting to work are particularly likely to drive alone.  But a rush-hour BART train is likely do even better than the lines above indicate, since that’s when the trains are fullest. 

    I love charts like these.  But unfortunately, we have to interpret them with care; it’s easy to misunderstand their messages.  To me, the clearest lesson in all of this is that  the best way to reduce the impact of travel is to fill seats.  Train, bus, vanpool, car—as long as there aren’t too many empty seats, they’re all fairly comparable.  If the seats are empty, they’re bad news.  So the bus vs. rail debate is mostly a distraction.  Neither mode is inherently better; full, they’re pretty close; and as long as they attract enough riders, both are much better than driving.

    Bus photo courtesy of Flickr user dkjd under a Creative Commons license.