Daily Mail says 21% accident reduction in 20mph zones "not significant"

A report in today’s Daily Mail claims to show how a Department for Transport report demonstrates “Why death rates INCREASED in 20mph zones”. It’s one of those articles that makes you sigh wearily, the way you might when faced with a wilfully contrary child. But to be fair, it’s not just us. Even the Mail’s own readers seem to be having a bit of trouble swallowing their line, at least if some of the comments below the story on their website are anything to go by.

The DfT-commissioned report entitled “Interim evaluation of the implementation of 20 mph speed limits in Portsmouth”, draws the following conclusion: “Early figures suggest that the implementation of the 20 mph Speed Limit scheme has been associated with reductions in road casualty numbers. The scheme has reduced average speeds and been well-supported during its first two years of operation.”

Fairly unequivocal then. But no, that conclusion comes in stark contrast to the opening line of the Mail’s article which states: “Reducing the speed limit to 20mph in all residential streets does not significantly improve road safety.”

We can only assume that the Mail’s Chris Brooke has contacted the report’s authors - Atkins, the UK's largest engineering and design consultancy and the world's 11th largest design firm - to tell them they have made a terrible mistake.

Brooke’s conclusion seems to have been based on the statistic for people killed or seriously injured (KSI) which the DfT report states rose from an average 18.3 to 19.9 when the figures for the three years before the scheme are compared to the two years of its operation covered by the report.

But as the report clearly explains: “Because the total numbers of deaths and serious injuries of casualties by road user type and cause are relatively low, few inferences about the scheme’s impacts should be drawn from these figures.” In other words, the figures are statistically less significant and are the kind  that could vary from year to year as a result of individual incidents (a triple fatality accident occurred in Portsmouth in the consultation period, for example) or factors such as protracted periods of  wintry weather.

More statistically significant perhaps, is the reduction in slight injuries – a far more common type of accident – which when combined with the KSI figures shows an overall 21% reduction in the total number of accidents. The Mail apparently deems this figure “not significant.”

The Portsmouth speed reduction initiative is important because it is the first time a local authority in England has implemented such an extensive scheme covering most of its residential roads using signage alone. There are no other traffic calming measures in place. The signage-only issue means the scheme is, to a large extent. self-policing, relying on motorists to recognise the benefits of reducing their speed to the wider community and specifically in reducing accidents and injuries.

The Mail, however, latches on to the figure of an average reduction in speed across the 20mph residential street zones of just 1.3mph. The article fails to point out that while many of the streets already had average speeds below 20mph due to their narrowness and the presence of parked cars, on those streets where traffic averaged above 24mph before the scheme, the reduction was an impressive 6.3. So where the need for a reduction in speed was greatest, the introduction of the 20mph limit was at its most effective.

The report should make a strong case for the wider implementation of urban 20mph zones when studied by those responsible for town planning and traffic management in other British cities. Or at least the ones who don’t believe what they read in the Daily Mail.