Thursday, November 7, 2013

Driving versus walking

We are late to this party but consider:
When it comes to energy use and greenhouse gases emitted, appearances can be grossly deceiving. Granted, people who drive everywhere are energy users and polluters. But walkers also use fossil fuels through the food they eat to replace the calories burned while walking. Of course, driving can be more polluting under some circumstances, such as when large SUVs are the preferred vehicles or when drivers insist on doing wheelies at every stoplight. Bicycling the distance can also be less polluting than driving. Dunn-Rankin sums up the central, largely counterintuitive, point of this commentary: "Driving a small [or moderate-size] car and not having to replace burned calories saves more energy (and greenhouse gases) than walking when the extra calories expended are replaced."
Tim Stuhldreher jumps on the biggest issues:

First, if you’re going to look at the entire food chain to determine the energy cost of human walking, it’s only fair to do the same thing for the car. That means you have to factor in the energy costs of producing it. It’s not clear how much that adds, but it’s significant: Estimates range from 10 percent to 100 percent and everywhere in between. I’m not going to speculate on the exact number, but if it’s 50 percent to 100 percent, then we’ve just put walking right back on par with driving, mile for mile.
Second, Dunn-Rankin’s result depends on the use of a high-mpg car, around 40 miles per gallon. If you drive a pickup truck or an SUV, your mileage is worse, and you have to adjust the figures accordingly.
 Third, doing this calculation on a per-mile basis ignores the obvious and important point that people typically drive much farther than they walk. No one buys a car to go half a mile here, a quarter-mile there. Moreover, land use patterns change as cars become more prevalent in society – you get less density and more suburban sprawl. To see the real impact of driving vs. walking, you have to take that into account.
I think the last point is the most salient.  The argument also has other issues -- like if we used bicycles instead of cars for the commute then the bikes would be a lot safer and people would be a lot more physically fit.  It may be implausible to walk twenty miles to work but I have met people who actually do it by bike.  Heck, it is also possible that people would lose weight, become more efficient as a result, and actually generate less emissions themselves. 

Furthermore, this assumes that the same fossil fuel use would remain in agriculture while being removed from personal use.  And that more efficient ways of agricultural transport could not be found (or that we couldn't alter our diet by season).  

It's a point mostly of interest to see how far people will accept the counter-intuitive as true, just because it is so at odds with conventional wisdom.  Sometimes the conventional wisdom exists for a reason. 


  1. "[N]ot having to replace burned calories "

    This seems to assume a strong, simple relationship between exercise and food consumption -- every five and a half miles the walker stops for a Big Mac -- but we know the relationship between exercise and eating is more complicated than that.

    For example,

    1. Richard H. Serlin spells out this point in detail in a comment on Stuhldreher's post.