Olympic sprinter Oscar Pistorius has been disqualified from running for an interesting reason. His carbon fiber feet give him what the Olympic officials consider an unfair advantage. He only uses a quarter the energy that a biological-legged person would use, and therefore could run four times farther on the same effort.
Mr. Pistorius lost most of his legs as a young child due to a bone deformity, in which his fibula, the longer leg bone, was missing, and the remaining bone did not support his weight. Rather than be saddled with legs that could not function, doctors amputated everything below his knees. Nonetheless, prosthetics have advanced to the point where he was able to play rugby, and his running habit began to recover from a rugby injury, according to CBS news. His limbs give me an idea.
I see a strapped-on device, attached to the lower leg of an ordinary person, which effectively lengthens their leg by 300 centimeters (or so) and ends in a carbon-fiber "foot" like Mr. Pistorius's prosthetic leg. After a brief adjustment period, the user would be able to run further and faster than before, and at the end of the run, the device could be removed. What for? I see it as advantages for people to use more foot-power to travel to where they need to go, and less gasoline. They would be small enough to stow in a locker at the destination, unlike a bicycle, and wouldn't require any more safety equipment than a jog would. With it, I could easily run to the nearest city for work or shopping without using my car.
The main disadvantage of the device would be that it would look slightly silly.