Road humps and 20mph speed limits

In recent years there has been a growing trend for local highway authorities to reduce the speed limit to 20mph, particularly around schools. The Welsh Government recently introduced a default 20mph limit on ‘restricted’ roads. Restricted roads are those with streetlamps placed not more than 200 yards apart, which are usually found in residential and built-up areas. In London, over half of roads have a 20mph speed limit. There are also plans to introduce more 20mph speed limits in other parts of the UK, including Cornwall, Wirral and Scotland.

Measures reducing the maximum permitted speed of vehicles to 20mph are typically introduced in one of two ways:

• 20mph limits: this is where the speed limit has been reduced to 20mph as indicated with repeater signs, but there are no physical measures to reduce vehicle speed.
• 20mph zones: in addition to the reduced limit, traffic calming measures are also employed (for example road humps).

The Royal Society for the Prevention of Accidents (ROSPA) argues [PDF] that (signed only) 20mph limits do not reduce traffic speeds as much as 20mph zones with physical traffic calming measures. However, creating 20mph limits is usually cheaper and quicker than installing additional traffic calming measures.

In its Plan for Drivers policy paper, published on 2 October 2023, the UK Government committed to updating ‘’20mph zone guidance for England to help prevent inappropriate blanket use.”

New 20mph speed limit on restricted roads in Wales

In 2022, the Welsh Government used powers under Section 81 of the Road Traffic Regulation Act 1984 to change the default speed limit on all restricted roads in Wales from 30 to 20mph, via The Restricted Roads (20 mph Speed Limit) (Wales) Order 2022.

This change came into force on 17 September 2023. Welsh highway authorities can designate and retain a 30mph limit where there is a case for doing so. These 30mph roads will be marked by signs in the same way that variations from the current default speed limit are used.

In a July 2023 press release, the Welsh Government said:

• the new 20mph default speed limit will save lives
• people living in communities that already have a 20mph limit support it
• it will improve the environment and help create safer communities
• the new speed limits are reducing speeds
• it is not a blanket speed limit and there are local exemptions from it
• 20mph speed limits are already used in other countries

A BBC News article on 20mph speed limits in Wales quoted Vale of Glamorgan council that the average cost of installing a standard speed limit post and sign, together with necessary traffic management, is £1,044.52. The estimated total cost of the 20mph policy in Wales is £34.4 million (to date).

A Welsh Government impact assessment, published alongside the legislation which enabled Wales’ 20 mph speed limit, estimated the economic impact of the policy to be a “negative £4.54 billion” over 30 years. The Welsh First Minister Mark Drakeford said that the £4.5 billion figure was ‘notional’ and that the policy would save the NHS money every year.

In a poll carried out in October 2023, about a month after the speed limit was introduced, 59% of Welsh voters, including 51% of those who voted Labour in the 2021 Senedd Election, said they oppose the introduction of the 20mph speed limit, up from 34% who said they opposed the new speed limit in mid-September. Support for the new, reduced speed limit has correspondingly fallen from 46% to 29%.

A Welsh Government spokesperson told Wales Online on 17 October that it was too early to measure the benefits of the speed limit change.

In September 2023, seventeen charities and organisations signed an open letter supporting the Welsh Government’s policy, saying the benefits went beyond road safety:

It’s not just a road safety benefit. It also supports broader health, climate and societal goals such as helping the vulnerable to get about, improving social connection, reducing air and noise pollution, and more.

Research on the impact of 20mph limits

There have been a number of studies evaluating the impact of 20mph limits in parts of England and Wales, carried out prior to the introduction of the 20mph speed limit in Wales. Key findings include:

Public Support

Once the 20mph limit has been implemented, a 2018 DfT review of signed-only 20mph limits found high levels of support amongst cyclists (81%), residents living in the area with the new 20mph limit (drivers and non-drivers) (75%) and drivers not resident in the 20mph area (66%), but less support amongst residents of neighbouring areas (44%) and opposition from motorcyclists (29% supportive, 49% unsupportive).

Speed reduction

In advance of Wales’ national roll-out, eight trial 20mph areas were implemented during 2021/22, referred to as Phase 1 of the 20mph programme. In March 2023, the Phase 1 Interim Monitoring Report found that 64% vehicles were travelling at or below 24mph compared to 45% pre-implementation, and that mean speed was reduced by 3mph. A Transport for London (TfL) press release (February 2023) noted that monitoring of 20mph schemes since they were introduced found reductions of 1.7-5mph “across most sites surveyed” in London. Across England, a 2018 Department for Transport (DfT) review of signed-only 20mph limits found a small reduction in median speed (less than 1mph).

Casualties

Lower speeds resulted in both fewer and less severe collisions, leading to fewer casualties. In London, a TfL press release (February 2023) noted that both the total number of collisions and collisions resulting in death or serious injury decreased by 25% following the introduction of 20mph limits. Collisions involving vulnerable road users decreased by 36% (from 453 to 290), while collisions involving people walking decreased by 63% (from 124 to 46).

Journey times

The impact on journey times is likely to be slight. A 2018 DfT review of signed-only 20mph limits found that journey times are estimated to have increased by 3% in residential areas and 5% in city centre areas. This adds less than half a minute to a two mile trip and less than a minute to a five mile trip.

Active travel

There is some evidence that 20mph limits lead to more walking and cycling. Transport for Wales’ 20mph Task Force Group report (July 2020) noted that:

Counts during the piloting of 20mph in Bristol found small increases in walking and cycling. Self-reported increases in walking and cycling were also noted after implementation of the pilot 20mph speed limit in Edinburgh.

Air quality

Changes in air quality are expected to be negligible or show a slight improvement. Transport for Wales’ 20mph Task Force Group report (July 2020) noted that, while there is relatively little evidence for the impacts of 20mph limits on air quality, a study modelling the impacts of a 20mph default speed limit for restricted roads across Wales concluded there would be an overall improvement in air quality.

Noise

Slower speeds are likely to result in lower noise levels. Transport for Wales’ 20mph Task Force Group report (July 2020) quoted research that found: “higher motor vehicle speeds always lead to greater annoyance”.