The bicycle population of the planet is about one billion, two bikes for every car. It's easy to see why they're so popular: They're cheap, human-powered, low-maintenance, easy to repair,and super-efficient. In terms of energy expenditure, if humans could drink gasoline for fuel (admittedly an apples-to-oranges comparison), a cyclist would get 1,000 miles per gallon. No surprise, considering that a typical car-to-driver weight ratio is about 25 to 1, or 1,500 times less efficient than a bike with a bicycle-to-rider weight ratio of 1 to 6.

More realistically, comparing oranges to oranges, the efficiency of a cyclist is at least twice that of a walker. On firm, flat ground, a 160 lb. person expends about 30 watts to walk at 3 miles per hour, while the same energy expenditure on a bike will speed them along at an average 10 m.p.h. Indeed, bikes are the most efficient self-powered means of transportation in terms of energy expenditure per mile traveled.

One reason is that they have a minimal number of moving parts, and the ones that do move, move efficiently. The chain drive between the pedals and rear wheel delivers an astonishing 98 percent of leg power to the back wheel. That's with large sprockets, but even with smaller sprockets (higher gears), at least 85 percent of the energy expended by the rider gets delivered to the back wheel.

The chain drive system of propulsion used on most bikes today was developed in the 1880s, without modern gearing, of course -- back then, folks rode a one-speed with no rear-wheel freewheel. The invention of the chain-drive, diamond-frame "safety bicycle" (in contrast to the earlier high and unstable penny-farthing) transformed society, giving freedom of travel to the vast proportion of people who couldn't afford to travel by horse. By the turn of the 20th century, bicycles had reduced crowding in inner-city tenements in the U.S. and Europe by allowing workers to commute from more spacious dwellings in the suburbs. They also played a major role in the emancipation of women. Susan B. Anthony told an interviewer in 1896, just eight years after John Dunlop had introduced the pneumatic (air-filled) tire, that the bicycle "has done more to emancipate women than anything else in the world. It gives women a feeling of freedom and self-reliance."

I believe we'll be seeing a lot more folding bikes in the future, given their utility for commuting and increasing acceptance on public transport. Not just buses and trains: airplanes, too. Louisa and I spent the month of May bicycle-touring in southern France on our Dahon folders. These days, airlines charge up to $100 extra for any baggage whose "height + breadth + depth" dimension exceeds 62 inches, so we appreciate the fact that our 20-inch wheel bikes just fit into a 62-inch box. They're great for touring, too.

Efficient, cheap, versatile, pollution-free, healthful, commuter-friendly: Bikes are good for body, soul, pocketbook and planet.

Barry Evans (barryevans9@yahoo.com) knows that what the facts don't say is that bikes are just plain fun to ride. A book of Barry's first 80 Field Notes columns (with additional material) is now available at Eureka Books.