Re: Researchers defend 20 mph speed limits despite rise in casualties
Your recent news item on 20 mph speed limits  gave an impression that new evidence had emerged against 20 mph speed limits. Nothing could be further from the truth.
Firstly here are some key facts:
• Detailed analysis of 20 years worth of data by Chris Grundy, published in the BMJ in 2009, showed that the introduction of 20 mph zones was associated with a 41.9% reduction in road casualties (95% confidence interval 36.0% to 47.8%) .
• The key aim of 20 speed limits is to encourage more considerate driving, unlocking the potential for walking and cycling, leading to better health, less traffic congestion, more social interaction, less noise pollution and stronger communities.
• Raw numbers of reported casualties are wholly inadequate for drawing conclusions about the impact of policy. If we succeed in allowing our streets to be populated by people of all ages, the very young to the very old, on foot and on bikes, then reported casualties will rise in number because more people are using the road space, even though the individual risk and danger has reduced.
The basis for the recent claims against 20 mph speed limits seems to be an article from The Sun newspaper , which in turn seems to have been prompted by a “Safe Speed” campaigner. Safe Speed is a campaign that maintains that to improve road safety we must abolish the enforcement of speed limits and remove all speed cameras.
Here are the actual numbers (table 1) :
Between 2010 and 2011 the number of reported casualties on 20 mph streets rose by 384 to 1,935, and on 30 mph streets fell by 1,057 to 99,117. This rise of reported casualties in 20 mph streets is almost undoubtedly due to the fact that the number of 20 mph speed limit streets has increased.
We would also expect a concomitant fall in the reported casualties on 30 mph roads. However the opposite is true. The number of reported fatalities on 20 mph roads rose from 6 to 7, whilst on 30 mph roads they rose by 67 to 612. This unexpected rise of fatalities on 30mph roads is by far more worrying than the increase in casualties on 20 mph roads.
Our experience in Bristol has shown us that a signs only 20 mph pilot scheme resulted in a 12% increase in walking and cycling, a reduction in day time road speeds and no impact on journey times or bus reliability . There was also increased local support for the scheme after implementation, by both road users and residents. Casualty numbers are far too small to draw any conclusions from.
With regard to vehicle speed in populated areas, we are at the beginning of a culture change. Europe is ahead of us, with 30 kph (18.6 mph) speed limits the norm in urban areas and excess speed in these locations is regarded as unacceptable. It took 12 years before “clunk click, every trip” resulted in the legal requirement of front seat drivers to wear seatbelts and it took years of slow and careful work to build public understanding of Smokefree in indoor public places prior to the legal change.
Considerate and calm driving will take time to become established as the social norm, but it is the right policy for an active population, for sociability and for safety.
Dr Sarah Ingamells, Foundation Doctor NHS Bristol
Dr Angela Raffle, Consultant in Public Health – Wider Determinants of Health, NHS Bristol.
 Limb, M,. 16 Aug 2012. Researchers defend 20 mph speed limits despite rise in casualties. BMJ 2012;345:e5580 [online]. Available at http://www.bmj.com/content/345/bmj.e5580?sso [Accessed on 3 Sept 2012]
 Grundy C, Steinback R, Edwards P Green J, Armstrong B, Wilkinson P. Effect of 20 mph traffic speed zones on road injuries in London, 1986-2006: controlled interrepted time series analysis. BMJ 2009;339:b446
 Jones, M,. 11 Aug 2012. Official: Casualties soaring in 20 mph zones. The Sun [online]. Available at: http://www.thesun.co.uk/sol/homepage/news/4481938/Official-Casualties-so... [Accessed on 3 Sept 2012]
 Department for Transport, 2011. Reported road casualties Great Britain: main results 2011. London; Department for Transport. Available online at: http://assets.dft.gov.uk/statistics/tables/ras10001.xls [Accessed on 3 Sept 2012]
 Bristol City Council, 2012. 20 mph speed limit pilot areas: Monitoring report. Bristol, Bristol City Council [pdf]. Available at: http://www.bristol.gov.uk/sites/default/files/documents/transport_and_st... [Accessed on 3 Sept 2012]
Competing interests: No competing interests