Climate Action — Driving

Driving

Cars, Cars, Cars

Thinking about emissions from driving is conceptually harder than any of the other actions we look at. The reason is that people buy cars, and making a car emits a truly prodigious amount of CO₂. Berners-Lee notes that people are particularly prone to underestimate this because they use lifecycle analysis rather than the (harder but better) input-output method. Here are some of the figures he gives for the UK:

Spending the Same Money Twice

There’s one other technical point I’ll note and then ignore for simplicity. When computing Little Planet warming, I’m assuming emissions reductions of 1.5% each year due to technological improvements. But one of those technological improvements is electric cars! If I tell you that

  • you can reduce emissions by switching to electric cars and
  • emissions from driving will go down by 1.5% a year because of better technologies like electric cars,

Manufacturing and Driving

There’s a useful fact from Berners-Lee that I’ll use several times below:

The UK

Miles per Gallon

Berners-Lee notes that the average UK car burns 36 miles per gallon (MPG) and emits 530g per mile or 0.33kg per km driven. This includes all the emissions needed to get the petrol into the cars, and the manufacture and maintenance of the cars.

Distance Driven

What about distance driven? Berners-Lee says that the ‘UK average annual distance’ is 7600 miles (12,200 km), but I’m not sure whether that’s per car, per driver or per household. The Department of Transport estimates car driving accounted for a total of 278.2 billion vehicle-miles in 2019. Dividing by the UK population of 66.8 million, we find 6,700 vehicle-km per person per year. To avoid double-counting, we will attribute that to the driver. (It’s arguably better to split it equally between all passengers, but I don’t want to complicate the main article by making people do arithmetic.) It works out at about 11 miles driven a day.

Emissions

6,700 km times 0.33kg per km gives average annual emissions of 2.2 tons per person per year, which warms the Little Planet by 0.23 °C in 2100.

  • 0.9 tons per person per year embedded in the new cars people buy, warming the Little Planet by 0.09 °C in 2100.
  • 1.3 tons per person per year from driving those cars, warming the Little Planet by 0.14 °C in 2100.

The US

Miles per Gallon

The latest edition of Berners-Lee only covers the UK, but the 2011 edition has old data for the US. It notes that the average US car drove 22.4 miles per gallon and emitted 850 grams per mile. As with the UK figure, that includes manufacture and maintenance and so is higher than figures you’ll see elsewhere.

Distance Driven

The Bureau of Transportation Statistics estimates that in 2018 2.23 trillion miles were driven in ‘light duty vehicles with short wheel base’, which includes ‘passenger cars, light trucks, vans and SUCs’. (A small proportion of that may be driving for business, which should be counted via embedded emissions in goods and services, but I can’t find the data to make that correction.)

Emissions

10,900 km times 0.42 kg per km gives average annual emissions of 4.6 tons per person per year, which warms the Little Planet by 0.48 °C in 2100.

  • 1.8 tons per person per year embedded in the new cars people buy, warming the Little Planet by 0.19 °C in 2100.
  • 2.7 tons per person per year from driving those cars, warming the Little Planet by 0.29 °C in 2100.

Reducing Emissions

Driving Less

One way to emit less is simply to drive less; if you drive 20% less, you’ll reduce your emissions by 20% or so. (It’s not quite exact, because you might eventually have to replace your car because it’s old, but I think that’s a minor effect.) So driving 20% less would cool the Little Planet in 2100 by 0.05°C (average UK driver) or 0.10°C (average US driver).

Switching Car

Another way is to switch to a more efficient car. Smaller cars can be made with fewer emissions and emit less while driving. The most efficient car is, of course, an electric car. For the UK, Berners-Lee gives the following emissions figures for driving a mile, inclusive of vehicle manufacturing emissions:

  • 180g in a mid-sized five-door electric car.
  • 530g in an average UK car at 36 miles per gallon.

Driving Differently

Driving more slowly and avoiding stops and starts can also reduce emissions. Berners-Lee notes that driving on a congested road can triple your emissions, and that driving at 60mph saves 15% compared to driving at 70mph. And finally, emissions per mile driven are barely affected by the number of people in the vehicle, so you can save a lot by driving with more passengers. For four people, Berners-Lee notes that taking a train emits very slightly more than taking a small efficient petrol car.

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criticalscience

criticalscience

Key science, with sources. Minus bad statistics. Minus shaky methodology. Minus politicisation, left or right.