Sunday, 27 November 2011

20mph limits in Nottingham and Portsmouth

Earlier this week, the Nottingham Post ran a series of articles on the Councils proposals to impose 20mph speed limits on some streets in Nottingham (Incidentally, the term “impose” is itself a loaded term (see here)).

The council has chosen Sherwood as the pilot area because it "includes everything throughout the city which could benefit from a 20mph limit, such as residential areas, steep streets, major bus routes, industrial areas and wide and narrow streets."

This is all well and good, but BFTF was interested in the evidence behind this proposed course of action. How have 20mph trials performed in the past? What criteria will the criteria use to decide whether the pilot study will be a success or not?

Let’s start with some terminology:
“20 mph Speed Limits” indicates the use of speed limit signs alone (without traffic calming measures)
“20 mph Zones” indicates indicates the use of signs and traffic calming measures.

Before moving on to what do the “pro” and “anti” groups say?
You can find out about the case against 20mpt limits at :

And the case in favour for 20mph limts at:

AAnd then looking at the evidence that 20mph limits work?
Much of the evidence for 20mph limits relates to the experience of Portsmouth, who implemented a 20mph speed limit (without additional traffic calming measures) in many of the city’s residential areas. A report on the effects of Portsmouth’s 20mph limits can be found here:
Interim Evaluation of the Implementation of 20 mph Speed Limits in Portsmouth
Final Report - September 2010

Before the scheme was implemented accidents stood at 183 per year, whereas afterwards they were at 142 per year, a 22%drop. During that period casualty numbers fell nationally by about 14% in comparable areas.

Vehicle speeds were also measured before and after the intorduction of the 20mph limits:

It has to be said that the report very carefully omits mentioning that the speeds of vehicles in the sites that had a speed “before” the trial of 20mph or lower actually INCREASED during the trial (BFTF estimates by perhaps 1-2mph).

Accident statistics were aslo presented "before" and "after":

It is important to note that some variability is to be expected when the number of annual accidents is low, so the increase in KSA figures is not necessarily significant. For a more extreme example of this, the report stated that the number of school children injured increased from 5 (3yrs prior to change) to 7 (2years after change). BFTF can see that Daily Mail headline now : “20mph speed limit results in 40% INCREASE in children being run over”.

If the Mail actually has someone on the staff who understands numbers then one can imagine them working it out on an annual basis “20mph speed limit results in 110% INCREASE in children being run over”. . .

Lastly, the report also compares the speed reductions achieved in Portsmouth with those in two other speed reduction schemes, in London and Hull. It is worth mentioning that these schemes included were much better funded than the Portsmouth scheme and included traffic calming measures:

So there you go, a bunch or relevant data all in one concise package. Shame the reporting in the mainstream media isn’t like this. . . .

BFTF will leave you to draw your own conclusions about the effectiveness of 20mph limits, as with most things in life, it’s complicated.


Update Nov 2018 : Noticed this RoSPA newsletter on the subject. Also new research summary from DoT here (shame none of the news articles (e.g. The Guardian, The Evening Express, The Manchester Evening News) linked back to the original study.