20 mph speed limits – Portsmouth’s experience

In a bit of a departure for this blog, today I turn my attention to the city of Portsmouth. This is not because I’m bored of Cambridge (perish the thought), but rather because Portsmouth was the first place in the UK to introduce a city-wide 20 mph speed limit, of the sort that some people would like to see introduced here.

In Cambridge, the  Liberal Democrats are currently distributing a survey about 20 mph speed limits to selected households. Although presented as a questionnaire, the accompanying leaflet and letter from Julian Huppert argue strongly for a blanket 20 mph limit, and some of the questions seem designed to get the answers that the Lib Dems want to hear. Question 4, for example, reads “Do you think we should have a 20mph limit for all roads in the city, or just in residential areas?” We should be cautious about any claims the Lib Dems make about Cambridge opinion on the basis of this survey. I hope they will publish the data they collect, so it can be analysed independently.

Anyway, what has Portsmouth’s experience been with 20 mph limits? The Portsmouth speed limits were introduced in 2007-08, at a cost of about £570,000, and cover 94% of Portsmouth’s road network. The Department for Transport published a detailed report last year on the results so far. Here are some highlights from it:

Firstly, what difference has it made to traffic speeds? Well, not a huge amount. Across the city, average speeds are 1.3 mph slower than they were before.

This is similar to Cambridge’s experience with our own more limited 20mph zones, which on the whole have made little difference to traffic speeds. It’s worth noting that, unlike Cambridge, average traffic speeds in Portsmouth were already mostly below 20 mph before the reduced limit was introduced.

Secondly, has it made the roads safer? Well, yes and no. Here are the figures for the number of people killed or seriously injured before and after the change:

As you can see, the number of people killed or seriously injured actually went up after the change. This was entirely due to more pedestrian casualties – there were 8.7 per year after the change, compared to 6.3 before, a rise of 38%. However, the numbers involved are very small, so random fluctuation may have played a significant role. The story was better for slight injuries:

Here the figures fell by 18.6%, with all categories of road user seeing improvements. However, it’s worth noting that traffic accident figures have also been falling elsewhere in recent years, so it’s debatable how much of this drop was due to the speed limit change.

Thirdly, and of particular interest to politicians, do people like the change? On the whole, they do. Here are the satisfaction figures from a survey of around 1,400 Portsmouth residents:

Unfortunately the report doesn’t give the raw figures, so I’ve just taken the graph directly (recolouring it for clarity). Also, although the survey asked people whether they were “fairly” or “very” satisfied or dissatisfied with the change, the report doesn’t give these figures either. So it’s possible that there are stronger opinions at one end of the spectrum than the other. But the overall picture is clearly that more people favour the change than oppose it.

Like Cambridge, Portsmouth has a Lib Dem majority on its City Council. It’s likely that the 20mph debate will be a feature of Cambridge local politics for some time to come. Let’s hope it will be influenced by the data.

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One Response to 20 mph speed limits – Portsmouth’s experience

  1. Alex Hosking says:

    It really shouldn’t be a political decision what speed limit to set. You ask the people who live on the streets most will demand it be a low as possible.
    Also if you set all roads at 20 there’s still non uniformity in that many roads of a much higher standard will have the same limit as ordinary residential streets, which will make people take them less seriously.

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