The effect of 20mph speed limits in north Cambridge

Update: See the end of this article for the latest data.

It’s now several months since the speed limit in most of north Cambridge was reduced form 30mph to 20mph. The City Council has recently been measuring traffic speeds to see just how much difference the change has made. They’ve kindly sent me the results of their speed survey, so I can exclusively reveal that the average change in traffic speeds in north Cambridge since the 20mph limit was introduced is…0.7 mph.

The before and after figures vary quite a lot from street to street. Here’s a graph showing the change in average traffic speed at each measuring point. Green represents a speed decrease; red a speed increase. Ready for some scrolling?

A note on the names – a small letter in brackets refers to the direction of traffic, whereas a capital letter refers to the part of the street. So “Stretten Avenue (S) (n)” refers to northbound traffic in the southern part of Stretten Avenue.

As you can see, more streets have experienced a speed decrease than an increase, but the typical speed decrease is a great deal less than the 10mph change in the speed limit. And some roads have experienced an increase in traffic speed, despite the reduction in the speed limit. The average change in the average speed, across the whole of the area, is just 0.7 mph. Whether this represents a good return on the many tens of thousands of pounds spent on the scheme will no doubt be hotly debated. We’ll have a better idea about this when before-and-after accident figures become available, but with such a small reduction in average speeds, it’s hard to imagine a very dramatic change.

Update: Here’s the raw data: north_area_speeds

Further update: The City Council have now sent me some slightly revised data, which you can find here. This separates the roads into those where the average speed was above or below 20mph before the limits were changed. You might expect the faster roads to be more affected by introduction of the 20mph limit, and indeed this is what the data shows. However, the drop in average speeds on the faster roads is still only 1.1 mph, compared to 0.4 mph on the slower roads.

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10 Responses to The effect of 20mph speed limits in north Cambridge

  1. Sal says:

    A lot of people said it wouldn’t work without better policing, and it looks as though they were right. It is awful that there has been an increase in some roads, though in Stretten Avenue, I think it’s because of the parking and the oncoming cars. People hurry up because other drivers are waiting!

  2. Pingback: @CherryHintonBlu on Twitter » Blog Archive » Disappointing results from the substantial investm…

  3. Most of the streets where there has been an increase in speed already had averages below 20mph. Would actually be consistent with argument made by some that 20mph improves traffic flow.

    But I’d be more interested in what’s happened to the top speeds. Average doesn’t tell you much as includes sitting in traffic, or rushing between speedbumps and slamming on brakes. Where safety is the concern, everything below 20mph is okay. What proportion of drivers are above?

    • Phil Rodgers says:

      The email I got from @camcitco said “The speeds shown are mean (average) speeds, as the 85 percentile speeds are no longer deemed relevant under the speed assessment guidelines.” – which I’d be interested to know more about

  4. Joe V says:

    So how does this data reflect the imbeciles wheel spinning past a school in Carlton Way?
    Or past the One Stop Shop with elderly people crossing to the bus stop?
    Since the introduction I have heard speeding vehicles possibly exceeding 60. The people behind the wheel seem intent on sticking two fingers up at the limit.
    Without police enforcement this whole exercise is just a box ticked for some ineffective politicians, and a very expensive one at that.

  5. What’s the average drop if street segments that were already below (say) 18mph average are excluded?

    • Phil Rodgers says:

      I’ve had a further update from the council (see bottom of article) which shows that the average speed drop in streets which were previously above 20mph was 1.1mph.

  6. I’m really interested in this issue as I think getting speeds on residential streets down is an important thing to do (and its about more than reducing deaths, liveability improves too) but blanket 20MPH limits appear to have little or no impact. For me a couple of things jump out.

    The Edinburgh Road N and S results are wildly different – I wonder if something else changed between the before and after?

    The variation in streets is quite high. I wonder what the differentiating factors are? Do “rat runs” see less of a speed drop? By the nature of the way they are used this would appear likely (if you’re rat running to get ahead is an unenforced limit change going to slow you down?). Is there a difference in speed change on streets mostly used bye people who live on that street (or an adjoining one)? Is there a difference between roads that people perceive as through roads and those they perceive as residential? If we could answer some of these questions we could perhaps create a more useful approach to applying lower limits.

    BTW I don’t live in/near Cambridge, just noticed your tweet about someone else’s 20MPH post, so please excuse me if local knowledge makes some of these obvious!

  7. Rachael says:

    Interesting analysis, but it’s confusing to include roads like Kings Hedges Road where the limit is still 30.

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