Can Mass Transit Save the Environment?

Friday, November 23rd, 2012

Can mass transit save the environment? It’s not likely:

At any given time, the average auto has somewhere around 1.6 passengers, and the average (typically 40-seat) bus has only about 10. Rail vehicles typically have more passengers (on average about 25), but then again they are also typically much larger. Thus their average load factor (percentage of seats filled) is also not high, at about 46 percent for heavy rail systems (think subways in major cities) and about 24 percent for light rail (think systems that mostly run at street level).

It is not clear that moving around large and largely empty vehicles is much of an improvement over moving around smaller ones. In fact, it may be worse. According to the Department of Energy’s Transportation Energy Data Book, in 2010 transporting each passenger one mile by car required 3447 BTUs of energy. Transporting each passenger a mile by bus required 4118 BTUs, surprisingly making bus transit less green by this metric. Rail transit admittedly fares better, at 2520 BTUs per passenger mile, but even this is not the kind of slam-dunk advantage over the auto that transit advocates might hope for.


For the most part, any new transit service has to go to relatively low-density cities and low-density areas within cities, meaning that new investment would drag transit’s overall efficiency down, not up.

To give an idea of how this phenomenon works, the heavily used New York subway system (58 percent of seats are typically filled) produces .171 pounds of CO2 per passenger mile, less than 1/3 the average for cars nationwide. However, the much more lightly used Cleveland, Pittsburgh, and Memphis light rail systems actually produce considerably more CO2 per passenger mile than cars do.

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