Some time back the Straight Dope column carried a story comparing the efficiency of public transit with automobiles on the basis of passenger miles per gallon (or per BTU when the transit used electricity). They discovered that any advantage was slight on that comparison. Such comparisons neglect two critical points:
1) When there is more than one person in the car, the analyses are incredibly generous about their definition of "passenger mile" when it applies to automobiles.
2) (And much more important) The comparisons assume that there is an already-existent route and an already-existent housing pattern to be compared. This is incredibly silly when you are discussing automobiles.
Details after the jump.
1) Consider two kids going to soccer practice. Joey takes a bus for five miles -- 5 passenger miles. Sammy's mom drives him five miles, drops him off, and drives back home until it is time to pick him up -- 15 passenger miles. You might answer that most automobile trips are with only one person in the car; that is true, but the passenger miles per gallon figures are arrived at by assuming that a significant fraction of the trips include more than one person.
And this doesn't even consider the trips that the driver makes to care for the car.
2) Much more vital, though, is the common observation that areas without public transit are grossly spread out. You need more lanes in the road, parking spaces for every adult in each household, other parking spaces for each shopping destination. And, when actual needs are met, the sprawl is so great that you might as well have some more lawn as well.
If you don't have enough people in walking distance of your small store to support it, then the small store can't compete with the big store even farther from the customers. All the stores are larger with significantly longer drives to go there.
There was a study made in El Paso decades ago. Their streets were crowded, and they asked what the total effects of widening them to accommodate traffic would be. The computation was that this would result in longer trips, but no less crowded streets. There are population densities which are great enough that people walk most places and are satisfied with public transit for most of their long trips. There are much lower population densities in which people can live comfortably with only automobile transportation. Intermediate population densities simply are unlivable.
And, just as high densities encourage and support public transit, public transit encourages and supports high densities. Most EL stations on the CTA are surrounded (often occupied) by retail stores. They sell to people getting on and off the trains, but they also sell to people living nearby. The availability of retail services attracts residents. For that matter, the closeness of the nearest bus stop or EL line is a frequent matter mentioned in apartment ads. Since public transit attracts tenants, the next time that a lot in the neighborhood is built on, it tends to be for a larger number of apartments.
So miles is a ridiculous measure of travel efficiency between cars and busses. People might be a more reasonable measure. How many people can you reach on a gallon of gas per passenger?
Well, one starting comparison is between Manhattan and Montana. It's a little unfair, not only does Manhattan have a greater resident population, but almost a million people come into Manhattan every week day and have to travel. Almost nobody from outside has a destination in Montana, and hardly any more people from outside cross the state on any given day. Still, ignoring that many more people have to reach destinations in Manhattan than have to reach destinations in Montana, how long does the average trip take in either location? A healthy person can cross Manhattan -- lengthwise -- on foot from one tip to the other in a few hours. You couldn't get across the state of Montana in a car in that length of time. If you wanted to go from one average location in Montana to another by private plane, your time from home or office to new home or office -- if not from runway to runway -- would take as long as a walk in Manhattan.
But we aren't comparing time consumed; we're comparing energy consumed. Here, the distinction is even more impressive. The gasoline consumed per mile is slightly lower for a bus trip in Manhattan than for a car trip in Montana. The gasoline consumed per representative trip is something like 1/100th as much.
Now, these are extremes. Most people -- even most US drivers -- live in an area more navigable than Montana. And there are no other areas as densely populated and well-served by transit as Manhattan. Still, the extremes tell you something. Once we reach the density that makes mass transit efficient, further density, further mass transit, decreases the average citizen's energy footprint.