Is green U.S. mass transit a big myth?

weslinder · Aug 6, 2008

This is a really long essay, and it's worth reading the whole thing. I'm just going to post the first bit. Basically the conclusion is that while it's a greener choice to ride a bus or train that's already going somewhere, mass transit is slightly less efficient on average than cars. And those of us who ride motorcycles are the real Captain Planets.

http://www.templetons.com/brad/transit-myth.html

As part of my research for an article on robotic cars and how they change so much of the world, I've been looking into the energy use of various forms of transportation.

What I learned about public transit in the USA shocked me. I've been a fan of public transit, taking it where it's practical for me, and feeling green about it. That transit is a significantly greener way to get around than private car travel almost goes without saying in our thoughts and discussions.

Disturbingly, this simply isn't true. I started by pulling out various numbers on the energy used per passenger mile of various forms of transportation. These numbers can be found in places like the U.S. government bureau of transportation statistics figures and the Dept. of Energy Transportation Energy Data Book (Especially table 2-12). I've also found tables broken down per city.

These studies express transit energy efficiency in terms of BTUs per passenger-mile. The BTU is the English system unit of energy, and it's equal to 1055 joules. On pure conversion, there are 3413 BTUs in a kw/h. To turn BTUs/mile into miles per gallon, you divide into 125,000, the number of BTUs you get from burning a gallon of gas. Here's a useful table.

A "passenger mile" is taking one passenger a mile. If it takes 10,000 BTUs to take a vehicle with 10 passengers for one mile, that's 10 passenger miles, and 1,000 BTUs/passenger-mile. For solo vehicles, passenger miles are just miles. The figures below are for the DoE's average passenger loads over the entire USA, unless noted as solo or for a specific district.

(If you prefer to see this as MPG equivalents, check out the bottom of this page.)

The DoE figures describe the average car as using 5500 BTUs/mile (23mpg) or 3,500 BTUs/passenger mile with an average load of 1.57 passengers. This is a "fuel to wheels" number based on burning the gasoline.

Putting the car at 3,500 I was disturbed to learn that city diesel buses and electric trolley buses are both mildly worse than the car in energy efficiency. Light rail systems are also slightly worse, on average, though it varies a lot from city to city. Commuter rail and subway (heavy rail) trains tend to be a bit better, but not a lot better. (Non-hybrid cars are also better at long haul than they are short haul.)

I've put a lot of the figures on this chart. I've thrown in some other interesting forms of transit, including electric cars, scooters, hybrid cars, jet airplanes and more. (See a note below on the fossil fuel to wheels number for the cyclist.)

Particularly disturbing were the numbers for some of the worst transit systems, including the light rail in San Jose, which I sometimes ride. That system takes twice as much energy per passenger than private cars do. It's not even the worst.

It should be noted that the electric vehicles are much more efficient on a "tank to wheels" basis than shown here. However, while 1 kWh of electricity is only 3400 BTUs on a perfect conversion basis, most electricity is generated from heat, and 70% is generated by burning fossil fuels, mostly coal. So in fact each kWh of electricity consumes 10,339 BTUs using the DoE methodology. That's about a 2/3 loss mostly for generation inefficiency and some transmission loss. See also EIA numbers.

But yes, by and large, even the electric vehicles will generate a modestly smaller amount of greenhouse gasses per BTU as the fuel-burning vehicles, though this varies based on the electricity source.

What's not in these numbers

True "well to wheels" analysis includes more factors:

Energy to make and recycle cars and transit vehicles. For typical cars that's 120 million BTUs, or about 15% extra over a 150,000 mile life-cycle. I don't yet have figures for transit vehicles.

Energy to build and maintain roads (for cars and buses) and tracks (for trains) or both for street cars.

Energy to extract, refine and ship fuel, both to cars and diesel transit, and to power plants making electricity for electric transit. For gasoline, this is about a 22% surcharge. I don't yet have figures for the energy cost of mining and shipping coal, or extracting and piping natural gas for electricity.

Variations in the average passenger load of all the vehicles, including the cars, which make some systems highly efficient and others terrible. Likewise, some cities have higher passenger/car figures and others lower. These are, except where noted, national averages.

The fact that sometimes transit trips require more miles (changing lines) and sometimes fewer miles (private right-of-way).

These add-ons apply both to transit and cars, though in different ways. The comparison between purely urban cars and purely urban transit is likely somewhat more favourable for transit -- but I would like to get authoritative numbers, and my instincts suggest it does not change the basic conclusion. Likewise the figures for suburban (highway) cars vs. suburban transit will be less favourable for transit.

It's been asserted that the 1.5 passengers per car average provided by the DoE is biased slightly, in that highway driving has higher loads and urban driving has lighter, but I would like to see real numbers on this. A drop to 1.2 urban would make the car's number 25% worse.

How can this be?

A full bus or trainload of people is more efficient than private cars, sometimes quite a bit more so. But transit systems never consist of nothing but full vehicles. They run most of their day with light loads. The above calculations came from figures citing the average city bus holding 9 passengers, and the average train (light or heavy) holds 22. If that seems low, remember that every packed train at rush hour tends to mean a near empty train returning down the track.

Transit vehicles also tend to stop and start a lot, which eats a lot of energy, even with regenerative braking. And most transit vehicles are just plain heavy, and not very aerodynamic. Indeed, you'll see tables in the DoE reports that show that over the past 30 years, private cars have gotten 30% more efficient, while buses have gotten 60% less efficient and trains about 25% worse. The market and government regulations have driven efforts to make cars more efficient, while transit vehicles have actually worsened.

In order to get people to ride transit, you must offer frequent service, all day long. They want to know they have the freedom to leave at different times. But that means emptier vehicles outside of rush hour. You've all seen those huge empty vehicles go by, you just haven't thought of how anti-green they were. It would be better if off-hours transit was done by much smaller vehicles, but that implies too much capital cost -- no transit agency will buy enough equipment for peak times and then buy a second set of equipment for light demand periods.

Transit planning is also driven by different economies. Often transit infrastructure (including vehicles) is paid for by state or federal money, while drivers (but also fuel) are paid from local city budgets. This seems to push local city transit agencies to get bigger vehicles and fewer drivers where they can, since drivers tend to be hired full-time and can't be kept idling in low-demand periods.

Click to expand...

pgabriel · Aug 6, 2008

okay, so in summary? it takes a bus more energy to carry people per person, but that also considers how many people actually use transit which is only 9 people per bus. but if more people used transit it would be more efficient to use transit, but as is usage, our transit systems are slightly less efficient than cars?

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Is green U.S. mass transit a big myth?

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