Is Driving Greener Than Walking?

By at least one measure, cars are tremendously inefficient: Only 15 percent of the energy in a gallon of gasoline ultimately goes to making the car move. The rest is wasted as sound and heat. The primary culprit is the engine: According to the EPA, it wastes 62.4 percent of the car's energy all by itself.

But how does a car compare to a person? You'd think 4 billion years of evolution would have led to a biological system that could handily beat the automobile, but by one measure, that simply isn't the case: If you do the math* it turns out that a Honda Civic expends less than half of the energy a person does to move one pound of itself one mile.

So why aren't we all drinking gallons of oil every morning before we walk out the door? Easy: A person weighs much less than a car. That's why, if you walk a mile to work every morning, you'll expend around 531,000 joules (or 126 calories) of energy, whereas if you drive alone in your relatively fuel-efficient Honda Civic, which is 20 times heavier than you are but more than twice as efficient per pound per mile, you'll expend about 4,332,750 joules (or 1036 calories) of energy.

On the other hand, it's been estimated that, because our industrialized food system is so dependent on fossil fuels to power our tractors, make our fertilizer, ship things across the country, etc., it can take up to 10 calories of oil to make a single calorie of food in the United States. Which means that a one mile walk to work could ultimately cost up to 1260 calories of fossil fuel energy — more than driving the same mile.

Of course, this strange fact is an indictment of the vanishingly small percentage of our food that is local, organic and seasonal more than it is of human physiology, much less a nod to the tarnished ingenuity automotive engineers.

Walking has other benefits, of course, but it isn't an option for the majority of Americans — and, mechanically efficient or not, driving isn't great for our health, our quality of life, or the breathability of our air.

So what's a commuter to do? Buy an electric bike. Really. They're already wildly popular in China. J.L. Radtke has written an incredible exegesis of the economic, energy and climate logic of commuting via eBike, and the takeaway is this:

• 90 percent of all trips taken in the U.S. are within an electric bike's range (and that of an electric car)
• 40 percent of the trips Americans take are already completed more slowly than could be completed by e-bicycle (because of traffic, parking, waiting for trains etc.) by some other means of transport
• E-bicycles are arguably the single most cost effective means available to reduce CO2 emissions

Of course, Radtke is also the president of an e-bicycle company, so he would say that. Regardless, in the name of transparency he makes all his calculations available on his Google Knol on the subject, and it's well worth a read.

* And by the way, here's my math:

There are 130 million joules in a gallon of gasoline, and a Civic is going to get you something like 30 miles to the gallon. 130 million joules / 30mpg = 4,333,333 joules to drive one mile in a Civic.

A 145 pound person uses about 531,083 joules to walk one mile.

So for a person walking, 531,083 joules / 145 pounds = 3663 joules / mile / pound

For the same person driving in a Civic, 4,333,333 joules / 2650 pounds = 1635 joules / mile / pound

Photo credit: motoyen