Practically speaking, legs beat wheels only in certain domains. Humans have spent thousands of years building roads because wheels are so cost-effective. "Legs will never be as energy efficient as wheels," Kuffner says, "and I doubt that they'll be as high-speed." That's one reason Kuffner's current research focus is on smart cars with sensors. But in rough terrain, like a dense rain forest or uncharted wilderness, legs could have the edge.

Kuffner is leading one of six teams of robotics researchers to teach a four-legged robot called LittleDog (built by Boston Dynamics) how to walk over unpredictable terrain. LittleDog's elder sibling, BigDog, is a gas-powered test mule already hauling 340 pounds up and down snow-covered hills. Make these robots a bit bigger, add space for passengers and onboard controls, and you could have an ambulating automobile.