How much do 20mph limits slow Cambridge traffic?

20mph speed limits have recently been introduced in a number of parts of Cambridge which previously had 30mph limits. The council have been monitoring the changes in traffic speed, and the results have been decidedly mixed. As a number of news reports have highlighted, in many places traffic speed hasn’t changed much, and in some cases speeds have actually increased.

On Monday next week, members of the Cambridge Traffic Management Area Joint Committee, which includes both City and County councillors, will be considering a report on traffic speeds before and after the changes. Here’s the data from the report:

Pity the poor councillors who have to digest that. Yes, all the information is there, but its presentation doesn’t exactly make it easy to take in at a glance. What it needs is a bit of data visualisation. Even a simple bar graph makes it so much easier to understand what’s going on in the data – your visual cortex can compare the length of lines much more quickly than you can read a table of numbers.

So let’s see if we can make the data a bit more vivid. Here are the average traffic speeds before and after the 20mph limit was introduced. The previous speed limit was 30mph:

Certainly nothing like 10mph slower, and in some places speeds have clearly increased. We also have data for the 85th percentile speed. This is the speed that 85% of the traffic was slower than; 15% was faster. This has changed noticeably in one place:

Mostly it’s not very different, but look at those Regent Street changes. Did that jump out of the data table for you? Me neither.

There’s a different set of data for the Wulfstan Way area, where the speed limit change seems to have had no effect at all on traffic speed:

The traffic speeds before the change were monitored in February 2010, and afterwards in September 2010. Of course the speed limit isn’t the only thing that was different between those two periods:

  • Some of the February measurements were taken during half term week, which was 15th to 19th February in 2010, whereas the September measurements were taken during termtime. As Cambridge drivers well know, they can often expect to travel faster during school holidays.
  • There are around 10 hours of daylight in mid-February in Cambridge, versus 12 towards the end of September. More daylight might mean higher speeds.
  • There was a bit more rain in the September period, but it was much colder in February, frequently below freezing.

There may also have been particular local factors affecting individual roads. Overall, though, the picture is pretty disappointing for those hoping that the 20mph limits would slow traffic down significantly. In most places, the traffic speeds were little changed, and in some places they increased. The only significant effect of the new speed limits seems to be that the faster drivers in Regent Street have slowed down quite a lot. And of course, it means that most drivers are now breaking the law, whereas previously they weren’t.

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5 Responses to How much do 20mph limits slow Cambridge traffic?

  1. I observed two area committees at which reports on the impact of the 20mph areas were presented. The West/Central were told their data was based on recording speeds of all vehicles over a period; whereas the South Area was told their data was on samples of a couple of hundred vehicles.

    The benefit of the South Area method was that the officers knew details behind the information, for example being able to say that only one vehicle was observed going at a speed which would be sufficient to justify prosecution and that was a bus.

    Cllr Brooks Gordon requested the number of observations the West Central data was based on; to enable her to carry out a Student’s t test to see if any of the changes were statistically significant. (Quite why councillors didn’t instruct their officers to do the calculation, or why Cllr Brooks Gordon didn’t ask for the data to be published rather than asking for it to be sent privately to her I don’t know).

    Depending on exactly where/when the data was collected on Regent Street it is possible the roadworks associated with Gas works has had an effect there.

  2. Andrew Bower says:

    The 20mph scheme was an ideologically driven and bonkers idea – a triumph of emotion over reason. The average speeds beforehand give plenty of indication that normal people are more than capable of realising that a 30mph speed limit is not a target. 85th Percentile speeds were established for determining speed limits for good reason.

    • The Queen Edith’s scheme was proposed to us by the Conservative County Council. They do have some weird ideas, but this wasn’t one of them.

  3. Pingback: 20 mph speed limits – Portsmouth’s experience | Phil Rodgers

  4. The before and after conditions are clearly very different, different amount of light, weather, temperature and term time or half term. This comparison is worthless and a complete waste of time. I can’t believe it’s even being considered.
    20mph is a safer speed in built up areas, in busy areas clearly it’s unlikely people go much faster anyway but it’s at quieter times it matters most as these are statistically when more accidents happen I’m led to believe.
    Where speeds in both tests are under 25mph anyway this is fine – the differences are so tiny. It’s roads where the limit is 30mph and people do 40mph or generally where 30mph plus on roads where pedestrians have to cross without crossings that this change to 20mph is most relevant. I lived in Holland for 10 years where this is the norm and it’s fine, it’s really normal and they have a much better safety record. They take road deaths seriously unlike here where it might as well be an occupational hazard of modern living. Walking around and crossing roads with kids is the most dangerous thing I do and I live in fear that my kids will do something stupid resulting in their death under a car or lorry.
    20mph in built up areas may be an emotional response but that’s because I see motor vehicles wizzing around where I live as a flipping nuisance, roads weren’t designed for cars and I would frankly like to see them used alot less than they are.

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