For some it could seem counterintuitive to applaud a network-wide 1-2mph speed reduction. However, wide 20mph limits are highly cost effective. What seems, at first, like a small reduction in speeds benefits society at large. Shared gains add up. Wide 20mph limits are efficient, affordable, cost-effective, fair, understandable, rapid to install and healthy.

Interventions which assist everyone’s safety deliver a greater overall value to society than those changes which greatly aid only a special few.  This is known as the ‘prevention paradox’ – and is well understood in public health circles[1].  Consider the economics of vaccination. Everyone is offered a preventative intervention so that society and the NHS benefit from fewer cases requiring intensive treatment.  Road danger reduction which limits drivers to 20mph on most built up streets prevents around 20% of casualties and delivers a multitude of quality of life benefits e.g. better air quality, quietness and community cohesion.

Introducing 20mph limits typically brings reductions of 1-2mph in average speeds - levels which, at first, might not seem to offer a good enough return.  Yet research has found that every 1mph fall in average speeds reduces casualties by 5-6%.  As the benefit is at total population level the overall rate of return is huge. 

Cost effectiveness is key. Though we might ideally want to engineer every road afresh - e.g. narrow it, put in drive slower cues, plant trees etc. to make it ‘appear 20mph’ - this isn’t possible on a wide scale due to funding restraints. Plus it would take ages!  Meanwhile people would continue to face risks which are easily preventable.

Most of the biggest UK authorities have chosen wide 20mph limits.  Councillors weighed up their choices. Either do nothing and risk litigation/falling behind on best practice in protecting the vulnerable or do something for residents who want safer, healthier, people-friendly streets and to be able to walk and cycle. Alternatives are:

• Spend 50 times the cost for engineering zones per mile versus bulk buying signs and lines 20mph limits.
• Spend 7.5 times the cost for humps per mile reduced versus community wide 20mph.
• Spend millions on a patchwork of non-adjoining zones versus £3 per head for limits for most streets.
• A 20mph engineering policy taking up to 15 more years versus implementation of limits within 1-2 years.
• 10mph speed reductions for a few engineered roads versus a 1-2mph reduction across the network.
• Residents in a few streets benefit versus most residents in every ward get wide limit benefits.
• Postcode lottery of areas engineered safer versus near universal benefit of wide 20mph limits.
• A ‘this area’ is especially dangerous so slow down message versus let’s all slow down because it’s fair.
• Little/no marketing of 20mph zones versus driver education on why 20’s Plenty to improve compliance.
• Limited changes in walking and cycling or network-wide sustainable travel and traffic reduction.

Rod King MBE, Founder of 20’s Plenty for Us commented:-

“A little bit slower makes a big difference to community safety when area-wide. It’s because people feel, and are, safer in 20mph areas that they start to walk and cycle more.  Write to ask your councillors for 20mph limits today”.

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