A gallon of gas has 30,000 Calories. An SUV weighs (technically masses) 2000kg and gets 20 MPG. Thus the kilogram miles per Calorie -- the efficiency -- for an SUV is 1.33.<p>A peanut butter sandwich is 600 Calories (with bananas and maple syrup on whole wheat, but corn syrup on Wonder bread is similar). A bicycle plus messenger bag and rider weighs 80kg and gets 10 miles to the peanut butter sandwich. Thus the efficiency of the bicycle is 1.33 kg.mi/C.<p>So far the bicycle and SUV seem exactly equal in efficiency and have equal environmental impact.<p>But that doesn't take into account cost. The peanut butter sandwich costs $5, or $3 if you make it at home. Call it $4. The gallon of gasoline also costs $4. But the gasoline has 50x the energy. The total cost to society is 50x higher to operate the bicycle, taking all process costs into account through the price system.<p>So in fact, biking is terrible for the environment. We'd all be better off banning bicycles and buying every cyclist an SUV and then having a mass lycra bonfire. Plus, think of all the cash we would save by not blowing it on extra bicycle fuel. It could save the economy.