Saturday, 28 February 2015

Will fuel efficiency targets really reduce carbon emissions from driving?

On Monday I will be learning more about carbon pricing and what is the most practical way to reduce greenhouse gas emissions to limit the threat of climate change (The Social Cost of Carbon). The EU carbon emissions cap and trade system currently only covers industry and power generators - it excludes other sectors such as transport and home energy. The policy for those sectors is to use efficiency regulations. For example they have set targets for reducing the fuel consumption of new cars in 2015 and 2021 [1]. You'll be pleased to know we had pretty much hit the 2015 target already in 2013 [2]. But is this effective in reducing carbon emissions?


I have been analysing fuel economy and total fuel consumption by cars and taxis in Great Britain. The latest figures I have are for 2012. Relative to 2000:

  • Fuel economy for new cars is improved by 25% (weighted average of diesel and petrol)
  • Total fuel consumed by cars on the road is down by 16%
  • Distance travelled is almost the same (has changed by less than 1%)

If the distance travelled is the same, why isn't improving fuel economy fully reflected in fuel consumed? In fact you would expect a bit of a delay between improving fuel economy for new cars and total fuel consumption because we don't all have new cars every year. The average age of cars on the road is 7 years. However, looking at the trends in the chart, if there is a delay it is only a year or two, probably because most drivers who do a lot of driving tend to have new cars very often.

Fuel economy for petrol and diesel cars (average new car litres/100km) (table ENV0103) and total fuel use by cars and taxis on the road (ENV0101) adjusted to tonnes of oil equivalent and normalised to year 2000. Note the Y axis has a false origin. All data from Transport Statistics Great Britain 2014

In fact the reason for the discrepancy is that the fuel economy figures for new cars do not accurately reflect real driving conditions (see Why your car does not achieve the fuel economy it is supposed to). The EU has part of an answer for that, as the testing procedures are going to be updated. Needless to say, the automobile manufacturers are fighting this hard but the EU plan should have it in place by 2017 [3].

Even if the new efficiency test really does match real world conditions, and manufacturers up their game to meet the new targets, there is no guarantee that carbon emissions will reduce at the same rate because we might change our habits and drive further. Distance travelled has been very stable since 2000 despite rapidly rising fuel prices and more cars on the road but that could easily change (see Will we drive more now fuel prices have gone down?).

The total carbon emissions from passenger transport depends on a variety of factors:
  • fuel efficiency
  • fuel carbon emissions
  • distance travelled
  • mode of travel (car, bus, bicycle, train etc).
Any policy that only tackles one of these factors will always be rather hit or miss. So, what additional policies do we need? Would an overarching carbon tax as proposed by Citizens Climate Lobby be the right way to go? That would force fuel prices to rise even further. Can improvements in public transport ever really persuade us to shift mode? Should we just abandon maintenance of the road network, until congestion forces people out of their cars? Will driverless cars help?  Its a complex problem and I don't expect we will solve it on Monday but I hope we get some insights.


[1] Reducing CO2 emissions from passenger cars (ec.europa.eu) Feb 2015
[2] Transport Statistics Great Britain 2014 (Dep. of Transport) Dec 2014
[3] WLTP is the new four-letter word for European CO2 emission regulations (AutoBlog) Jun 2014

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