The recent popularity of hybrid cars has obscured an even more impressive surge in North American demand for diesel-electric city buses.
Daimler Chrysler, whose Orion brand has close to a 60 per cent market share, estimates that, based on existing orders, the number of hybrid buses on US and Canadian streets will grow by three-quarters over the next year from 1,200 to 2,100.
"They're selling very well," says Brian Macleod, senior vice-president at Gillig, a Californian bus manufacturer.
Since Gillig began commercial production of hybrid buses in 2005, these models have grown to a fifth of its output. By contrast, hybrids make up less than 1.5 per cent of US car and light-truck sales.
Politicians are under pressure to cut smog and adopt environmentally friendly policies. Hybrid technology typically delivers bigger fuel savings for a city bus than a car because of the bus's low speed and frequent stops.
The batteries are barely used at speeds over 20mph. Buses in New York, which has the world's biggest hybrid fleet, travel at an average speed of just 7mph. Much of the battery power required for acceleration comes from energy stored whenever the brakes are applied.
Gillig has experimented with various alternative fuels, including methanol, ethanol, propane and natural gas. All were "somewhat successful" in cutting emissions" Mr Macleod says, but some – such as methanol – produced other toxic substances. Furthermore, he adds, "none of them was as efficient as a diesel engine". However, with the help of ultra-low sulphur diesel fuel that went on sale in the US last month, "you've got a very clean vehicle".
Gillig's hybrid technology comes from a General Motors subsidiary, Allison Transmission. GM also supplies New Flyer Industries, another bus builder based in Winnipeg. Orion uses BAE Systems' technology.
GM, in partnership with DaimlerChrysler and BMW, is adapting Allison's bus technology to cars and light trucks. GM plans to launch a hybrid sport-utility vehicle followed by a pick-up truck.
An evaluation by the US National Renewable Energy Laboratory concluded last month that the fuel economy of New York's hybrid fleet was a third higher than its diesel buses and 60-120 per cent better than vehicles powered by natural gas.
In addition, the hybrid vehicles' reliability was well above minimum standards, and drivers liked their acceleration. New York is in the throes of expanding its Orion hybrid fleet from 500 to 825 vehicles. In Canada, hybrids are set to make up a third of Toronto's buses.
"We believe that over time, the bulk of transit bus procurements will be hybrids," says Pat Scully, chief commercial officer in DaimlerChrysler's North American bus division.
Hybrids' political appeal has helped offset their price. A hybrid bus costs about $530,000 – 60 per cent more than a diesel model. In the US, the federal government foots 80 per cent of the bill for new municipal buses.
The price premium is a barrier in some markets, including Europe. According to Mr Scully, "the hybrid phenomenon is, for the time being anyway, a US-Canada event".