I saw something the other day that mentioned that if the working population of the US pedaled generators 8 hours a day, it would only produce about .1% of all the US energy requirements.

But if everyone pedaled a bike to work just one day a week, it could cut our foreign oil imports by half.

The bottom line is that while humans may not be able to make much energy, automobiles are terribly inefficient, and waste a LOT of energy.

Edit: I found the quote and the numbers were even more divergent than I remembered.

"If every adult in the world rode a stationary bike for 8 hours a day to generate electricity, they would crank out only 80 gigawatts, still only half of 1% of our energy needs. But if each American driver cycled to work just one day a week, we could cut our Persian Gulf oil imports in half." -- Popular Science p 61, March 2009.