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20mph Bill will cut casualties, Parliament officials tell MSPs


Andrew Forster
15 March 2019

Area-wide signed-only 20mph limits “generally reduce road collision casualties”, Scottish Parliament officials have told  MSPs scrutinising the Restricted Roads (20mph sped limit) Scotland Bill (LTT 01 Mar).

The Scottish Parliament Information Centre (SPICe) reaches its  conclusion on the basis of evidence submitted to the rural economy and connectivity committee’s inquiry into Green MSP Mark Ruskell’s Bill. Most of the relevant evidence was submitted by supporters of the Bill, chiefly campaign group 20’s Plenty for Us and academic Adrian Davis. 

SPICe considered the casualty evidence about schemes in Bristol, Calderdale, Portsmouth, and Warrington, as well as two journal papers, and the DfT’s evaluation report on signed-only 20mph limits. 

The DfT report by consultants Atkins, AECOM and Professor Mike Maher, concluded there was “insufficient evidence to conclude that there has been a significant change in collisions and casualties following the introduction of 20mph limits in residential areas” (LTT 23 Nov 18).  

But SPICe says: “City-wide 20mph speed limits generally reduce road collision casualties, although some smaller schemes have not reduced casualty numbers.” 

Of the un-named journal articles SPICe examined, one is from the Journal of Public Health. “This umbrella review confirms these findings and that 20mph zones and limits can reduce accidents and casualties,” says SPICe. 

The other is from the Journal of Epidemiology and Public Health. SPICe’s summary of this paper says: “In Wales, between 1 January 2011 and 31 December 2013,?14,639 people were killed or injured on 30 mph limit roads. Reducing all current 30mph limit roads to 20mph limits could prevent 6–10 deaths and 1,203–1,978 injuries per year at a value of prevention of £58m–£94m.”

SPICe also cites the TRL report, The effects of drivers’ speed on the frequency of road accidents, as evidence to suggest that casualties should fall as a result of lower speed limits.

Report ‘a poor basis for decision-making

“The SPICe report is a disappointment, in that it merely presents some headline results from a variety of different studies / implementations of 20mph limits, in a variety of formats. There is no real attempt to bring some order to, and understanding of, these results on casualty reductions.

In particular, it does not sufficiently bring out that many of the studies are pure before / after comparisons without any comparator areas. It is all very well for a council such as Calderdale to state that there has been a 30 or 40 per cent reduction in casualties but, without an attempt to estimate what would have happened over the same period if nothing had been done, this is at best misleading. It is implicitly claiming that all of this reduction is attributable to the scheme. Seen against a background of generally reducing road casualties, either nationally or in neighbouring areas, this is a dangerous claim.

If we look in more detail at Calderdale Council’s report presenting its results, for example, we can see that there were reductions in casualties in the four neighbouring authorities of Bradford, Kirklees, Leeds and Wakefield over the same period. 

From the information provided by the council’s report, it is possible to model the data for the five areas over the six-year before period (2012-16) and the after period for Calderdale of 2017. 

What emerges from this analysis is that, using the other four areas as comparator areas, the relative reduction in killed and serious injuries in Calderdale is -24% but with a 95% confidence interval of -42% and 0%: so on the boundary of statistical significance.  

The result for all casualties is a 95% confidence interval of -12% to +9%: that is, no significant change. 

The point is that this result of no significant change in casualties, relative to the neighbouring areas, is very much at odds with the claimed result in the Calderdale report of a 30 or 40% reduction in casualties. 

As I say, it is disappointing that the SPICe report fails to challenge, or set in a proper context, such results.”

Mike Maher was a co-author of the 20mph research study commissioned by the DfT, and is an honorary professor in the department of civil, environmental and geomatic engineering at University College London.  

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