I just came accross this great short essay entitled, “In Praise of the Bicycle”.
…Man on his feet is thermodynamically more efficient than any motorized vehicle and most animals. For his weight, he performs more work in locomotion than rats or oxen, less than horses or sturgeon.
…The bicycle is the perfect transducer to match man's metabolic energy to the impedance of locomotion. Equipped with this tool, man outstrips the efficiency of not only all machines but all other animals as well.
Its a nice read if you like cycling, commuting via bike, or are stuffed in your car in rush-hour traffic.