Road.cc – a cycling website – have nicely pulled-apart the latest distortions printed in the Daily Mail about road safety:
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…
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.”…
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.”