20mph more effective when accompanied by traffic calming

12.05 | 22 May 2023 | | 2 comments

New research confirms that 20mph limits backed by physical measures have substantially greater speed and casualty reduction benefits than those without.

The research project was carried out by the Parliamentary Advisory Council for Transport Safety (PACTS), with together an international team of road safety experts, and was made possible by funding from The Road Safety Trust.

The project set out to explore the evidence from six countries in mainland Europe as well as in the UK where more and more towns and cities are introducing 20mph limits.

The PACTS report notes that lower urban speeds can play an important role in delivering casualty reductions and increasing active travel.

Indeed, ‘safe speeds’ is one of the five foundations of the Safe System approach to road safety. Safe System is aims to manage the magnitude of crash forces so that, in the event of a collision on the road network, no one should be killed or seriously injured. It recognises that people sometimes make mistakes, and that all elements of the road system, for example road design, vehicle construction and user behaviour, contribute to safety outcomes.

The report finds that the extent to which 20mph schemes deliver actual speed and casualty reductions depends on whether they are supported by other measures, such as road humps and/or changes in relative road width.

Schemes without other measures result in modest speed reductions – typically 1-2mph where before speeds are approximately 25mph, and 3-5mph where before speeds are approximately 30mph. When a speed limit of 20mph is introduced with physical measures, speed is normally reduced to less than 20mph, provided it was less than about 30mph before the measures were implemented.

In the UK, signed only schemes result in approximately 11% fewer casualties than before they were introduced.

This figure rises to 40% in schemes where physical measures are introduced, as the speed reduction is substantially greater.

The report also finds that compliance with 20mph limits where no physical measures are in place is poor.

The report notes that Intelligent Speed Assistance (ISA) – which is aimed at increasing compliance with speed limits – is seen as the most effective in-vehicle system for reducing speeding and improving road safety. ISA systems alert the motorist if they exceed the speed limit and encourage compliance. They do not automatically prevent speeding unless the driver selects a mandatory system.

Margaret Winchcomb, deputy executive director at PACTS, said: “20mph is now generally accepted as the safe speed for streets used by people walking, cycling or wheeling. At 20mph a pedestrian is likely to survive an impact with a motor vehicle whereas at 30mph the pedestrian is significantly more likely to be killed.

“Traffic speeds of around 20mph also make walking and cycling more appealing – a crucial step towards the Government’s desire that 50% of journeys will be walked or cycled by 2030.

“This report shows that public money spent on self-enforcing 20mph zones has substantially greater effects than when it is spent on just the signs and road markings of 20mph limits.

“Longer-term, technology in vehicles, rather than the road layout will help drivers stay within speed limits.”

Sonya Hurt, chief executive of The Road Safety Trust, said: “20mph speed limits are becoming increasingly commonplace across the UK, in a bid to reduce speeds and save lives.

“The Road Safety Trust was delighted to fund this research which builds on the evidence base and confirms the importance of accompanying traffic calming measures.

“The findings will help ensure the continued roll-out of 20mph schemes can be backed by the latest evidence, and be as effective as possible in terms of reducing casualties.”


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    The unspoken logic seems to be “let’s not do this good and affordable thing because it’s not as good as this other but sadly unaffordable thing”.

    Philip Jones, Stourbridge
    Agree (1) | Disagree (2)

    We have also been saying the same for years but recognise the huge difference in cost whereby community-wide 20mph limits with engagement are 7 times more cost-effective than 20mph with physical calming. See https://www.20splenty.org/20mph_limits_vs_isolated_20mph_zones

    Rod King, Lymm
    Agree (13) | Disagree (5)

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