Are walking and cycling as green as we imagine?

Recent research concludes carbon emissions from walking are very large – around the same as taking the bus and nearly half those from driving – and it’s mainly down to what we eat. As usual, it’s not that simple.

Life cycle GHG emissions by transport mode for the UK

According to recent research, some of the popular ideas about the relative carbon emissions from different transport modes are a far cry from reality.

The exhibit shows that when assessed on a lifecycle basis, the emissions per passenger kilometre of travel by bus in the UK are 86% of those for travel by car. That’s mainly because of the low occupancy of buses e.g. off-peak, counter flow.

Probably more surprising is the finding that emissions from cycling and walking are around 40% of those for driving.

These numbers are from a recent paper, The effects of physical activity on greenhouse gas emissions for common transport modes in European countries, by Aidan Duffy from the Dublin Institute of Technology and Robert Crawford, from the University of Melbourne. It’s published in a respected, peer-reviewed journal, Transportation Research Part D.

The finding that worries me most is the contention that emissions from food eaten to sustain walking exceed those from the manufacture and disposal of cars!

The authors are right to point out that industrialised food production is a major and often overlooked generator of emissions. I’ve no reason to think there are serious flaws in their accounting methodology either; and I note they seem to have avoided the usual mistake of assuming all energy for walking comes from carbon-intensive beef.

While Duffy and Crawford don’t make anything like the extreme claims of some, I think  the way their findings are interpreted and the significance they’re given nevertheless needs to take account of a couple of issues.

One is that apart from athletes, most people don’t add extra food to their diet to compensate for an extra kilometre of walking. We eat for other reasons like gratification or because we’re bored.

The growing problem of obesity attests to the psychological disconnect for many people between calories in and calories out. And of course those who eat nil or very little meat will generate many fewer grams of carbon than walkers and cyclists who consume the average European diet.