A taste of Velo-efficiency

I once read that a human being on a bicycle is the most efficient means by which any animal has ever moved through space. Indeed, it is a claim often made. Forty years ago Ivan Illich wrote that:

“The bicycle is the perfect transducer to match man’s metabolic energy to the impedance of locomotion. Equipped with this tool, man outstrips the efficiency of not only all machines but all other animals as well.” (Energy and Equity, 1974)

Although the efficiency of cycling is well accepted, efficiency can mean many things. In the physical sciences it is often narrowly defined as a “measure of effective work” and expressed as the percentage of total energy expended that produces external work. Based on that criterion, conveying oneself via bicycle is no more efficient than via car – both processes “waste” three quarters of the energy expended. This energy does no “effective work” and is lost as heat. But this dimension of efficiency is not particularly helpful in the present context. More relevant is the amount of energy required to travel a certain distance.

The following chart provides a comparison across a range of different transport types. Unsurprisingly cycling comes out pretty well.

Although not a direct confirmation of Ivan Illich’s thesis, it does nod in the direction of the cyclist as hyper-efficient. Illich was inspired by an earlier article in Scientific American that found a human on a bicycle to be more efficient than any animal – even the condor (in terms of energy expended per gram of propelled matter).

Also inspired by that article was Steve Jobs. In a 2001 speech he said: “That’s what a computer is to me: the computer is the most remarkable tool that we’ve ever come up with. It’s the equivalent of a bicycle for our minds”. If only bicycles were as integrated into our lives as computers!

I did a (very rough) back of the envelope energy calculation based on my personal experience. It supports the figure quoted above for cycling (.06 Mj per km) and was based on the fact that during long haul bicycle trips I consume, around 6000 kj extra per day and average 700 km per week. That works out as .06 Mj per km.

The findings presented in the graph[1] above come from David Banister – professor of transport studies at Oxford – and were not done on the back of an envelope. They measure efficiency in terms of primary energy expended so they (unfortunately) do not account for full product life cycles nor the infrastructures that make such transport modes viable.

Another LCA (transportation options for commuters) found similarly, that of all the transport options, cycling is most environmentally efficient. But it failed to account for the cyclist’s energy consumption. Ongoing energy expenditure was not accounted for because the study was interested in short commuter journeys (a transport domain where cycling rules). These were assumed to not require supplementary energy. In terms of commuting (not particularly long distances) this is not such a bad assumption.

But if we really want to subject the idea of velo-human to fair scrutiny and to explore its potential as a mode of longer distance transport then not only does food energy need to be factored in but the system boundary needs to be expanded. The 6000 kj that is consumed to cycle 100 km comes laden with embedded energy. This does naturally depend on what you eat and where you eat it. (Locally grown potatoes are excellent, air freighted beef less so.) The diet of affluent Western societies comes with about ten times the food value as embedded energy – fossil fuel energy used to grow the food and bring it to the opening of the human digestive tract.

In an LCA that sought to quantify the benefits of more cycling, the European Cycling Federation couldn’t decide whether cycling incurred a food energy cost. Although this LCA neglected infrastructure, it found that a km cycled either resulted in 5 or 21g CO2e depending on whether or not supplementary dietary intake was required.

So using a life cycle approach, Banister’s cyclist’s energy consumption just shot up to .6 Mj per km – but is still well ahead. The next edition will explore the interaction of velo-efficiency and urban planning. (Hint: Velib)

[1] This visual representation of Banister’s study is courtesy of The Conversation.