Posted by: bmeverett | June 9, 2009

The false god of public transportation

I attended an excellent program yesterday (June 8) at the Center for Strategic and International Studies (CSIS) in Washington entitled “Transforming the Transportation Sector: Energy security, Climate Change and Transportation.” CSIS, by the way, is one of the very few Washington organizations dedicated to understanding issues rather than just advocating for certain policy positions.
Amid an excellent discussion of the state of various transportation technologies and transportation options for the future I found a rather troubling consensus on the importance of increased public transit as a critical part of the solution to our energy problems. The idea that public transportation is superior to private vehicle seems obvious inside the Beltway. Surely it must be more efficient to move people around together in one rail car than in single-occupancy private vehicles? Public transportation clearly has its advantages, and I should note for the record that I took the bus to and from the conference. However, like most obvious assertions, this one requires a little bit of critical thought and some data. As a starting point, here are a couple of pertinent facts.
Public transportation is sometimes, but not always more energy-efficient than private vehicles. According to the US Department of Transportation, transit buses use slightly more energy per passenger-mile than private cars. How is that possible? The answer lies in how many people ride the bus. Let’s do the calculation for 100 vehicle-miles. Private cars have, on average, 1.6 occupants, so driving cars 100 miles will give us, on average, 160 passenger miles. The average US car gets about 19 miles per gallon, and a gallon of gasoline contains about 125,000 British Thermal Units (Btus) of energy. That’s about 5¼ gallons of gasoline for our 160 passenger-miles or 4,100 Btus per passenger-mile. Buses carry, on average 9 people. At rush hour, of course, they can be packed to the gills, but most city buses also run on off-peak hours and significant parts of their routes cover lightly-traveled areas. The average diesel bus gets about 3½ mpg, and a gallon of diesel contains 140,000 Btus, so the average energy use is about 4,400 Btus per passenger mile. Buses do a lot of good things, including reducing congestion and offering transportation options to the poor. Energy-efficiency, however, is not one of its benefits.
How about heavy rail? Data show that heavy rail systems (subways and commuter trains) are about 25% more energy-efficient than private cars. These systems, however, are only cost-effective in high-density areas where the trains run full most of the time. Roughly half of all intracity rail passenger miles in the US are in the New York metropolitan area. These systems are extremely expensive (about $100 M per mile). That’s why people don’t build them except in very large, very dense urban area. Los Angeles, for example, is very large, but not very dense. Hence, heavy rail would make no sense.
Light rail has been the fastest growing component of mass transit, but again, its viability depends on how many people will ride it. These systems cost about $50 million per mile and have, according to The American Dream Coalition, suffered an average cost overrun of 41%.
Fact number two is that public transportation is not particularly carbon-efficient either for those of you concerned about climate change. Let’s look at the numbers. Our 100 vehicle-miles/160 passenger-miles by private car require 5¼ gallons of gasoline. At 20 pounds of carbon-dioxide per gallon, that’s about 105 lbs of CO2. What would happen if we all abandoned our horrible, terrible, very bad private cars and took mass transit instead? According to published data, heavy rail requires about 275 Watt-hours of electricity per passenger mile or 44 kilowatt-hours (kWh) for our 160 passenger-miles. Since the US is building essentially all the nuclear, hydro and renewable electricity that we can, any additional mass transit electricity would come primarily from coal, and our 44 kWh would emit about 91 pounds of CO2, only about 15% less than travel by private car. Light rail actually requires more electricity than heavy rail per passenger mile, primarily because of lighter passenger loads.
The final inconvenient fact is that mass transit accounts for only about 1% of all passenger-miles traveled in the US, while private cars account for over 80%. Doubling the US mass transit system, which would be very expensive and difficult, would reduce US oil demand half a percent or less with an impact on greenhouse gases that would be barely noticeable.
Mass transit clearly has some value, but it isn’t a panacea. The convenience of the New York subway, the Boston T and the Washington Metro are undeniable for the residents of those cities, where population densities are high. In most of the US, however, mass transit would be frightfully expensive and carry little, if any energy or carbon savings. I wish more of the speakers at yesterday’s event had just taken a few minutes to do the math.



  1. Отлично написано! Буду много думать…

  2. Как всегда на высоте!

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