Lower speed limits save 5x more road casualties than targeted interventions

A guest blog from Adrian Berendt of 20's Plenty for Kent and Campaigner of the year - 2018


Road safety professionals often prefer targeting road safety improvements on particularly dangerous roads and junctions? Others consider it better to make more general interventions with a wider impact, known as the ‘prevention paradox’, e.g. health impact of immunisation.

We assess which approach is better by comparing two alternative approaches[1].

1) Targeted interventions on rural A roads. We use the example of the £100m fund for specific interventions as proposed by the UK government; and

2) Prevention Paradox – lowering overall speeds by a few miles per hour with wide area 20mph on urban non-A roads.

Our analysis shows that spending the same money on Alternative 2, the “Prevention Paradox” saves 5 times the number of KSIs as targeted rural A road interventions.


[1] We eliminated three other road types as potential candidates:

  • Motorways: few casualties and atypical of the UK road network;
  • Rural non-A roads: extensive, carry little traffic and have few casualties; and
  • Urban A roads: mixed characteristics of each of the road types chosen; might need a combination measures.

Targeted interventions on rural A roads

According to DfT figures, rural A roads carry 30% of motor traffic in the UK and account for 23% of Killed and Seriously Injured (KSI) casualties – some 5,800.  The UK government is investing £100m on the most dangerous sections of rural A roads with the aim of saving 72 KSIs per year[1] - a rate of 0.72 KSIs per £1m spent.

Widescale 20mph on non-A urban roads

We estimate that implementing wide-area 20 mph limit schemes, without physical traffic calming would save 3.36 KSIs per £1m spent.

A typical spend for such schemes is £5 per head of population – £330m for the UK population of 66m. On a different basis, a recent report for the DfT has a cost of £4,000 per mile[2] or £340m for the 85,000 miles of non-A urban roads . On either basis, £1m would cover 3% of non-A urban roads[3]

Potential KSIs: The DfT guideline is that a mean speed reduction of 1mph reduces casualties by 6%[4]. A recent study for Bristol, which included faster roads in a large wide area scheme, showed a reduction in mean speed of 2.7mph[5], implying 16% fewer casualties, based on a 6% saving per 1mph.  Bristol, Brighton, Edinburgh, Calderdale and others report KSI savings of around 20%.

In 2017, there were 8,500 KSIs on non-A urban roads[6]. A conservative reduction of 15% on 3% of those roads would mean 3.36 fewer KSIs, nearly 5 times the reduction achieved under Alternative 1 – targeted interventions.

Other benefits of wide area 20mph

In addition to fewer road casualties, there are many other benefits of 20mph:

  • A whole of range of improved outcomes under wide area 20mph schemes: increased walking and cycling, leading to better health, greater social interaction – see Donald Appleyard study – lower air and noise pollution.
  • Implementing a national default speed limit of 20mph, with exceptions would enable £100m to cover a greater number of roads.
  • AS 20mph becomes more widespread, more people will obey the limit, lowering speeds in urban areas and saving even more casualties.


Not only does 20mph bring multiple benefits that targeted interventions can never bring – more Active Travel, less pollution, better health – it is clearly more cost effective in reducing casualties.

The government praises a reduction of 0.72 KSI per £1m spent on targeted rural A-roads.

We believe that there is a far greater case for gaining a reduction of 3.36 KSI per £1m spent on wide-area urban 20mph limits.

Wide area 20mph are FIVE TIMES more effective in reducing KSIs than targeted interventions and provide far wider societal benefits.


[1] https://roadsafetyfoundation.org/safer-roads-fund-results/

[2] https://bit.ly/2Vk9kQh

[3] Changing the national default speed on urban roads from 30mph to 20mph would reduce costs significantly, since less than 10% of the urban road network are A roads, the most likely roads that would remain as 30mph.

[4] https://bit.ly/2OXh2fK

[5] https://www.sciencedirect.com/science/article/pii/S2214140518303141

[6] excluding roads with a current speed limit above 30mph

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