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Transit: A Full Ride

Buses and trains can be a good deal for the climate -- especially if they're full!

bus.jpgThe 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.climate-CO2byMode-Sm.jpg

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.

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:


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.

This article originally appeared on The Sightline Institute's blog, The Daily Score.

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Well put comments about interpreting the charts with care, thanks!

I'm surprised that the vehicles chosen for comparison don't include the Prius. Apparently the Camry was chosen instead as the "top selling model for the year." I'm not clear why the sales rank of one model should be informing an environmental analysis.

The study, credited in the publication to a "Volvo Center of Excellence," seems to use a complex hybrid of process-based life-cycle analysis, and econometric surrogated life-cycle analysis. Given the tremendous disparities in externalization of various costs and subsidies through the US economy, that might be a bit problematic.

Some factors in this study are derived from complex software models, rather than direct data, while others come from empirical surveys of actual practices, and still others are inferred from stated industry standards, assuming the standards to present facts. Life-cycle environmental cost of production for the bus example - a major element - was not available, so it appears it was based on heavy truck manufacturing, simply adjusted by dollars of vehicle cost.

Life cycle analysis, while deeply important, is challenging and easily gets squishy. In this case, admittedly from a quick read, the squishiness seems to be too great to put much weight in the comparisons as charted. It is very interesting work, in need of much critical review, adjustment, and refinement before it's solid to base conclusions on.

Since we tend to want to use this information for planning purposes, looking forward in the face of climate change, I'd like to see life cycle costs which based on best practices, displayed alongside these which seem to add up (via lots of estimating) common practice, a lot of which is just wasteful.

Posted by: Kevin Matthews on 6 Nov 08

What if transport policy was planned around this model? Putting on more buses at peak times would mean the roads were less congested *and* it would be more climate friendly.

The art is making sure there are the right number of buses or trains, so that they're full but not overcrowded. In the UK, where I live, public transport at peak times in so full it's not a pleasant experience, even if you do manage to get a seat. So I think it's key to make public transport a better option to have any hope of getting people out of their cars.

Posted by: Debra on 7 Nov 08



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