Smarter Cambridge Transport

Why an 80mph speed limit turns out to be bad for us all

It’s not uncommon for ‘common sense’ intuitions about transport to be wrong. That’s why I was so annoyed to hear Liz Truss, a government minister with an entire department of experts on hand to advise her, bringing up the popular fallacy that increasing motorway speed limits would help drivers get to their destination more quickly. Let’s put this one to bed, shall we?

We all know that the real issue with journey times is vehicles being slowed down by other vehicles, not speed limits. This is called congestion. It turns out that allowing cars to travel faster actually makes this worse. Slower traffic naturally results in smaller gaps between vehicles, which means more vehicles fit in a given space.

Even more of a problem occurs when vehicles are travelling too fast in comparison to others. Sudden braking gets amplified down the line, producing a ‘phantom’ traffic jam with no cause obvious to those in it. If you increase maximum speeds, you increase the difference between fast and slow vehicles, resulting in more phantom traffic jams.

The advantage of lower, more consistent speeds is demonstrated by ‘Smart Motorways’, which have variable speed limits controlled by computers to achieve the maximum flow of vehicles. These often end up reducing maximum speed as low as 40mph in order to get everyone through that section of motorway in less time.

But let’s ignore the science and assume that a higher speed limit did allow us to travel faster. Sadly, there’s unarguable evidence that another result would be more crashes. Perhaps those who want to drive faster can live with that on their conscience. I couldn’t. But even so, more crashes means… even more congestion.

If we’re serious about reducing fuel consumption and pollutant emissions, we also need to be aiming for slower traffic travelling at more consistent speeds. An increased speed limit takes us in the wrong direction for both.

We shouldn’t get carried away with the time saving argument either. The M11 motorway is 55 miles long. In the unlikely event that we had the road to ourselves, and could drive its entire length at 80mph instead of 70mph, we’d get to the other end less than 6 minutes earlier. Is that really going to make a difference to our lives?


This article was first published in the Cambridge Independent on 17 October 2018.

Chris Rand

Chris Rand is a blogger and campaigner from Queen Edith’s ward, with a keen interest in improving the communication between local government and residents. He believes that a key factor in making the City Deal successful will be generating ideas from the people who live in and around the city, and not leaving things to commercial interests, which will inevitably try to hijack this fantastic opportunity.

3 comments

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  • Hi Chris,

    Your article says “the science” is against higher motorway speed limits, so I was slightly disappointed that you didn’t include any links to peer-reviewed evidence to back this up. Do you have any such links?

  • Hi Phil. These articles are from the newspaper, where it’s impossible to include references. The ‘science’ I proposed we ignore, to make a point, has been used to justify the (expensive) introduction of Smart Motorways. This and this are articles I noted when writing the piece. This shows how higher speed causes more fatalities. I’m sure there’s a lot more out there. Do let us know if you find anything interesting yourself.