20mph limits 'more effective with traffic calming'

29 May 2023

New research has confirmed that 20mph limits backed by physical measures have substantially greater speed and casualty reduction benefits – with compliance found to be 'poor' where no physical measures are in place.

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

It looked at evidence from six countries in mainland Europe as well as in the UK and found that the extent to which 20mph (or 30km/h) schemes deliver actual speed and casualty reductions depends on whether they are supported by measures such as road humps and/or changes in relative road width.

When a speed limit of 20mph is introduced with such measures, speed is normally reduced to less than 20mph, provided it was less than about 30mph before the measures were implemented.

However, schemes without other measures result in speed reductions of around 1-2mph where ‘before’ speeds are approximately 25mph, and 3-5mph where ‘before’ speeds are approximately 30mph.

In the UK, signed-only schemes result in approximately 11% fewer casualties than before they were introduced but this rises to 40% in schemes where physical measures are introduced.

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 but do not automatically prevent speeding unless drivers select a mandatory system.