Speed Limits

Speeding picture

Cars travelling quickly. Courtesy BBC

Visiting the USA last week I was reminded that driving in the USA is a doddle compared to the UK. The roads in Cleveland, Ohio:

  • were wider,
  • less crowded,
  • had a lower normal speed limit (25 m.p.h),
  • and a lower motorway speed limit (typically 65 m.p.h.)

And so once I had got used to driving on the ‘wrong’ side of the road, it all made driving very simple and relaxing. On returning from the US, it was immediately noticeable how much narrower and crowded were the UK’s roads, and how much faster people drove. So I was surprised to learn of the government plans to ‘consult’ on the proposal to increase motorway speed limits in the UK to 80 m.p.h.

Considering safety as paramount, this is a bad idea. To the extent that it will cause a general increase in speed it is likely to increase the number and seriousness of accidents. The number of people dying on our roads has been in steady decline, but is still nearly 3000 every year – 8 people a day. This is a tragedy, and if they all died together we would consider this a national tragedy. However, although the number of fatalities has declined, the number of accidents has not. Cars are now much safer than they were and so fewer people are dying in the same number of accidents. However, people are still suffering crippling injuries and it costs us all a fortune to cope with this ongoing carnage. Surely we ought to be lowering speeds, perhaps to US levels.

Ecologically speaking, the suggestion is madness. Fuel consumption depends a lot upon driving style – constantly speeding up and braking is wasteful.  But at a constant speed, cars use much the same amount of fuel to travel a given distance up to some speed in the region of 55 m.p.h. Above this, fuel economy falls off rapidly. All cars are different but geusstimating numbers from this US site, it appears that travelling at 80 m.p.h. rather than 60 m.p.h. uses around 25% more fuel. Nationally this would result in a measurable increase in imported oil and emitted carbon dioxide.

Socially speaking, however, this makes perfect sense. The 70 m.ph. limit is widely disregarded and it makes sense to have a law which reflects what citizens feel comfortable with.

My suggestion would be to retain the 70 .m.p.h. as a recommended maximum speed. However drivers would only be prosecuted for travelling faster than 80 m.p.h. but in order to stop a drift to ever higher speeds we need to make travelling at 81 m.p.h. prosecutable in practice.

I also think that if people knew the costs of travelling faster they would not do it quite so much. For a 60 mile journey:

  • If a car’s optimum fuel efficiency is 40 miles per gallon, travelling at 80 m.p.h. will save roughly 20 minutes on a journey time of 1 hour 5 minutes, but the journey will cost an additional £3.26 or roughly 16 pence for every minute saved.
  • If a car’s optimum fuel efficiency is 60 miles per gallon, the additional cost will be only £2.19 or roughly 11 pence for every minute saved.
  • If people travel 2000 motorway miles per year, travelling at 80 m.p.h. rather than 55 m.p.h. will cost an additional £100 per year.
Fuel Efficiency

Timing and cost for a 30 mile car journey. Left-hand axis (blue) shows journey time as a function of speed. Right-hand axis (red) shows cost for the same journey

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