A non-hybrid bus, Seattle

Today, the Seattle Post-Intelligencer breaks what it seems to think is an important story: the diesel-electric hybrid buses King County Metro operates are burning a lot more fuel than anticipated. In fact, their fuel economy is no better than that of the older buses they are replacing.

This news is a little disappointing but ultimately doesn’t matter much. Here’s why, in a series of points that begin with the vehicle and zoom out to the cityscape.

1. Metro’s hybrid buses are performing well in other ways. In particular, they emit far fewer local air pollutants. In fact, a last-minute switch from Cummins to Caterpillar engines to get cleaner emissions probably explains the increased fuel use. Diesel buses contribute a far larger share of health-threatening local air pollutants (such as deadly fine particulates discussed in today’s Sacramento Bee) than of local fuel use (and the directly proportional emissions of climate-changing greenhouses gas carbon dioxide). For buses, burning cleaner matters more than burning less.

  • 2. The hybrid buses are not serving routes on which they would be expected to save much energy: hybrids’ fuel economy is best in stop-and-go traffic but Metro is mostly running the hybrid buses on express routes through the downtown bus tunnel. Regular engines operate most efficiently when they’re somewhere near highway—or, at least, arterial—speeds: in the transit world, that’s an "express." Hybrid technology saves the most energy at slower and variable speeds, where it can compensate for the tremendous inefficiency of the regular engine. In the transit world, that’s a "local." Ideally, Metro would put hybrids on local routes, to save more energy. But Metro is unlikely to do that. The hybrids were bought to run in the downtown bus tunnel, where low particulate emissions and noise are important. And the bus tunnel is designed to serve expresses, not locals. So the hybrids are destined to remain ill-adapted to their main duties, at least from a fuel-saving perspective. (On the other hand, they’re perfectly adapted to keeping the air clean in the tunnel.)

    3. The technical energy efficiency of a transit vehicle is a wee-small consideration compared with its systems efficiency—which depends on how many people are on board. The best-case scenario for hybrid buses was that they might go an extra mile per gallon: in very rough numbers, maybe 4.5 mpg instead of 3.5 mpg. But getting one additional pickup- or SUV-driver off the road and on the bus saves almost exactly the same amount of fuel. And, of course, the power of transit is not that it can get one driver off the road but that it can get 40 per bus. So the main thing to worry about isn’t what’s in the bus’s engine compartment but who’s in its passenger compartment. A full-but-inefficient (3.5-mpg) bus uses a tenth as much fuel to move its passengers as would their own (16-mpg) SUVs if they drove alone. Hybrids don’t make any appreciable difference in attracting new riders.

    4. The "dirty little secret" of public transit—which transit advocates and environmentalists hate to admit in public—is that average transit trips in the United States are not much more fuel efficient than average trips in private cars, measured in fuel use per person per mile. (You can see this in tables 2.11, 2.12 and the related figures in Oak Ridge National Laboratory’s Transportation Energy Data Book. An interesting discussion of the same issue, from independent researcher David Lawyer is here. And here’s a chart we developed that compares the related variable of per-person CO2 emissions of transit at different capacities with other options: our chart corrects some of the flaws in Oak Ridge’s approach but still shows regular transit not doing as well as you might expect.)

    The reason is that transit is poorly utilized in the United States. Outside of rush hour and outside of central cities, the buses mostly run near empty. A typical bus needs to have at least five and typically more like seven passengers on board to start saving fuel, compared with those passengers driving alone.

    And the reason for empty buses is that transit service is distributed politically, not through a market. Suburban and even rural political leaders want their share of transit service. So they bargain for bus routes that are almost guaranteed to run below the break-even level for fuel savings. This decision may be thoroughly justified because transit is partly a social service, rather than a public utility. But transit’s fuel efficiency would be larger if transit service were allocated strictly to conserve energy.

    5. Let me add, quickly: for individuals confronting the choice of whether to drive or take transit, taking transit almost always saves fuel, because you can assume that transit will operate whether or not you’re on it.

    6. The key to getting seven-plus passengers per trip is density. Other things—transit prices and passes, speed and reliability, safety and comfort—matter, too. But density matters the most, by far. Bad transit in dense neighborhoods attracts more riders than good transit in sprawling neighborhoods. Where enough people live or work close to a stop, transit saves money, fuel, and emissions. (The great debate on transit, among leading experts, is not whether density leads to transit use but whether transit investment, especially light rail lines, leads to density. The debate last appeared on this blog here.)

    To close the loop, then: hybrid buses help reduce air pollution but they will never play a starring role in fuel economy. The land-use decisions that shape our cities are what ultimately determine the energy efficiency of Cascadia’s transit systems, and ultimately, of Cascadia.