The Electric Car vs. Mass Transit
Decatur Metro | December 13, 2010 | 3:50 pmBelieve it or not, this isn’t an easy “versus” to get supporting data for. Even on the interwebs.
Most conversations concerning both the electric car and mass transit seem to devolve into more general discussions about other benefits of each – mainly personal freedom vs. land use – and have trouble staying focused on the issue at hand: energy use.
So, I’m going to assume folks who frequent this site are familiar with the general pro v. con arguments surrounding autos vs. transit. Let’s leave all that stuff on the sidelines and concentrate solely on energy use, which for decades has been an important element of the pro-mass transit argument.
What does the reemergence of the electric car in America do to the energy-consumption playing-field between mass transit and the auto?
As I opened with, even Google has trouble providing an easy click-thru answer to this particular question. While comparisons between cars and mass transit and gas and electric vehicles abound, little if anything popped up comparing energy consumption between electric vehicles and mass transit. So we must make due with what we have.
But what do we have? We have BTUs per passenger mile courtesy of the Transportation Energy Data Book -2010…
- Transit Buses – 4,348 BTUs
- Cars – 3,437 BTUs
- Personal Trucks – 3,641 BTUs
- Certified Air Route – 2,995 BTUs
- Rail – 2,541 BTUs
- Motorcycles – 1,875 BTUs
So yes, holistically, rail transit is currently more energy efficient than a car.
But what happens when the electric car knocks the energy consumption of the car through the floor? This post on “The Straight Dope”, citing many of these same stats, notes that just by achieving an average 35 mph fuel efficiency would bring the auto down to 2,300 BTUs. That’s lower than the current rail projection.
So is that it? Have we closed the debate once and for all? Hardly. Improvements and extensions in rail service, along with denser urban development, can quickly translate into higher ridership levels, which will drop rail’s BTUs per passenger. Conversely, an potentially greater reliance on autos could produce even larger expanses of lower density suburbs, creating greater levels of energy-sucking gridlock on our streets and highways. (I guess it’s not THAT easy to separate out the environmental argument from the land use argument).
So we haven’t really solved or determined anything. Mission Not Accomplished!
However, one thing is for sure. If the electric car is going to be introducing “100 mpg” equivalents into our ongoing energy conversation, mass transit advocates may one day soon need to find other talking points to hang their hats on.
Great post, DM! It’s particularly interesting to me that air travel is more energy efficient than personal vehicles (cars & trucks). Even more fascinating that buses are so inefficient!
It seems to me that part of the calculus here needs to be the marginal (energy) cost considerations. For a Nissan Leaf, up to four passengers ride with little marginal energy use. After that, you need to put another car on the road to handle the passengers. A train can accommodate a much higher number of users (by increasing # of riders per car & adding cars) before its marginal energy cost goes up much. So, basically, it looks like the overall efficiency depends heavily on mode split…
Aren’t you forgetting that the BTU’s for a car is based on mpg of GAS not KWH of electricity so it is irrelevant if the electric car gets 100mpg? You must convert Kilo Watt Hour into BTU before you can compare apples to apples.
You make a good point. But irrelevant?
How do I covert kilowatt hour into BTU?
Use the kwh to btu conversion calculator, of course. I found it on google. http://www.unitconversion.org/energy/kilowatt-hours-to-btus-it-conversion.html
So how do I do this? I found this info on the Nissan Leaf…
The energy content of 1 gallon of gasoline is 33.7 kilowatt-hours. The Nissan Leaf uses 34 kilowatt hours per 100 miles per this sticker.
So assuming that the average fuel power vehicle gets 25 miles per gallon, it would use 4 gallons of gas – or 134.8 kilowatt hours to drive 100 miles. Again the Leaf would use 34 kilowatt hours.
So, if a Leaf uses a quarter of the kilowatt hours to go 100 miles, I assume I can just apply that to the “car” BTU number above? So 3,437 divided by 4 is 859.
PS. I would also argue you have to consider the sources of the power source. As oil, coal comes at a very high price to the environment both in emissions and extraction of it (mountain top removal, water pollution, etc.). I guess the question is if it more palatable than oil?
I think we need a better metric for transportation energy efficiency than just “passenger miles traveled”. We need something along the lines of “population and service traversal,” if you will. For example, if I am in a car-oriented area and traverse one mile, I have likely not passed that many people’s homes, that many businesses, etc. However, if I take the Subway for one mile in Manhattan, I have likely passed through the equivalent, with respect to homes, businesses, cultural attractions, services, etc., as passing through ten miles in a less dense area. Sure, the “energy per mile traversed” may come out similar between the car and the Subway, but the “population density traversed”, or whatever you want to call it, is going to be much, much greater.
I figured the following graph might be useful to the discussion (it plots modes of transport with respect to average speed and energy consumption):
http://www.inference.phy.cam.ac.uk/withouthotair/c20/page_128.shtml
It is taking from a great book comparing energy usages of different things in sensible numbers.
I think one should be careful about numbers for transit (buses and trains) because their energy consumption depends heavily on the people using it: There are few modes of transport more inefficient than a bus which runs empty half the trip and with 1 passenger for the other half. On the other hand, you can’t do much better than full trains.
Lifecycle and infrastructure costs are important too, when accounting for energy usage. A large cost of cars at the moment is the redundancy involved in private ownership: most cars are idle most of the time. If this was factored in, I expect cars would sit far worse than they currently do in energy usage measures.
If/when cars become self-driving, schemes for collective ownership will become far more feasible. Less cars produced combined with more efficient cars may tip the energy usage scale back in their favour. Still, that’s just speculating.