Which burns more calories: walking or riding a bike?

So here it is. Which one gives you more exercise? Casually riding a bike through a generally flat area, or walking at a slightly brisk pace.

I’m really kinda on the fence about this one. Walking seems to require much more energy to propel the 120–180 pounds across the ground, while on a bike, you seem to just be rolling with the wheels momentum, occasionally pedaling.