Typically, I ride the bus during rush hour, when it’s so crowded that it’s sometimes hard to nab a seat. I’ve done a quick passenger count a number of times, and the bus almost always has at least 35 passengers. Sometimes, especially on the big articulated buses on a busy afternoon, it’s more than 50.

But apparently it’s pretty unusual for a transit bus to be so full. According to nationwide statistics (see table 2.10, here), transit buses carry fewer than 9 passengers, on average. That average includes both the busy routes and the sparsely-ridden ones; apparently there are lots of buses that have just a few riders, especially in spacious suburbs or during late-night runs.

The average transit bus gets a little around 3.5 miles per gallon in regular service. When I do the math, I find that buses average about 30 passenger-miles per gallon of fuel, or thereabouts.

Curiously, that makes transit buses—at least, at current levels of occupancy—just about the least efficient form of personal travel around!

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• Take a look at this graph, derived from the US Department of Energy’s Transportation Energy Data Book.

Intercity buses—which are typically pretty full—are just about the most efficient form of transportation in use today, as measured by energy consumption per mile of travel. But transit buses—which are typically pretty empty—are the least efficient.

The only difference is the number of vacant seats.

The lesson here is not—I repeat, not — that transit buses are inherently inefficient. It’s that all too often they’re underutilized. A reasonably full bus can get something like 120 passenger-miles per gallon. But a reasonably empty bus is less efficient than a Hummer. It all depends on how many seats are filled.

The same goes for just about any other form of transportation: the easiest way to ramp up efficiency is to fill the empty seats. Vanpools are particularly efficient because they average 6 riders per van; most of the seats are full. Just so, we could substantially increase the net efficiency of car travel by ramping up carpooling back to what it was in the early 1970s. On net, a full, 4-passenger car that gets 30 mpg does about as well, in terms of fuel consumed per passenger mile, as a full 40-passenger bus that gets 3 mpg.

And note another curiosity: rail transit, and transit buses, cars, and even SUVs are all pretty darn close in terms of the energy that’s currently used to transport a passenger one mile, at current levels of ridership. (SUVs & other light trucks do a bit better than would have thought, because their average ridership is higher than for cars. Go figure.)

But what’s true for an individual vehicle might not be true for a transportation system. For example, places with high rates of transit ridership tend to consume less energy overall for transportation. But that’s not really because of an inherent efficiency of any particular transportation mode (buses vs. trains vs. cars). Rather, transit-intensive cities also tend to be dense places—since it’s in denser urban areas where transit can attract enough riders to be convenient and cost efficient. And dense cities tend to be places where a) the seats on the buses and trains are likely to be filled, and b) people don’t have to go very far to get to work, stores, and services.

In other words, it’s the land use patterns that make a transportation system efficient or inefficient; to a large degree, transportation mode is an afterthought.