In search of the biggest fuel saver? Try smoother roads

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There are dozens of ways to improve the fuel economy of vehicles with petrol and diesel engines and, thereby, reduce both the quantity and toxicity of their exhaust fumes.

The most high profile and heavily invested efforts have been directed at the vehicles themselves, and certainly there have been design advances to make engines more fuel efficient and clean up the gases that come out of their tailpipes. Improvements of up to 50 per cent have been achieved since the millennium.

Poor maintenance practices and clumsy driving have been rightly blamed for largely negating these gains. But the focus on vehicles and drivers tends to obscure what is overwhelmingly the most crucial factor in how much pollution (of any and every kind) that motor vehicles produce. The biggest influence — by far — is the road they drive on.

Take a specific journey, for example a 15km trip from the suburbs into city centre; a typical commuter routine. Under optimum conditions of engine tuning, driving style and traffic flow, this journey would take about 15 minutes and the average saloon car would use about 1 litre of fuel. In congested conditions, this same journey takes between 30 and 90 minutes, and uses about 2 litres of fuel.

Maintaining steady speed requires relatively little energy, using substantially less fuel than if you are repeatedly slowing down, stopping, idling, accelerating again … and so on … whether because of traffic or bumps and holes.

So, by having smooth roads, with enough capacity to allow a steady flow of traffic, and motorists obeying traffic codes to minimise disruption of that flow, the volume of fuel consumed by commuter vehicles (hundreds of thousands of them) can be halved. Twice a day. A massive economic saving in fuel costs, not to mention the saving in time.


As well as halving fuel consumption, the quality of exhaust emissions would also change. Steady speed helps achieve optimum combustion of fuel and emissions are predominantly carbon dioxide (relatively harmless), nitrogen oxide (inert) and water vapour.

The nastier emissions (hydrocarbons, nitrous oxide, carbon monoxide etc) increase when combustion of the fuel is poor, and poor combustion is caused not only by worn-out or ill-tuned engines, but also (even with engines in excellent condition) by acceleration, braking, idling …

That is why no single measure would have a greater effect on reducing exhaust pollution in Kenya than steady traffic flow on smooth roads. The savings in fuel, wear-and-tear and time would far, far outweigh the cost of constructing and administering the necessary road system and traffic behaviour. The added consequence of purer air would be a free benefit.

Only when that has been done, or at least the process has started, will there be any margin in looking at the finer high-tech details.

And when the road is smooth and clear, riddling it with speed bumps (or very slow vehicles) is ludicrous.

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