20mph zones – how effective are they?

In recent years, 20mph speed restrictions have become a familiar sight across the UK, particularly in urban areas, in an effort to improve road safety and reduce accidents. While evidence suggests that the rollout has had positive results, it has equally faced significant backlash, with some citing its impact on travel time, small businesses and congestion.

So, how effective are these measures in reality? Let’s take a closer look and see if 20 really is plenty.

Where have 20mph speed limit zones been introduced in the UK?

If you’ve driven through a residential area recently, chances are you’ve come across a 20mph zone. These limits are now a common feature in many urban areas, aiming to make streets safer for everyone. The map below details major cities in the UK that have implemented 20mph speed limits in residential and high-pedestrian areas:

There are two types of 20mph speed restrictions that are used across the UK:

20mph speed limits: Areas where the speed limit has been lowered to 20mph (enforced by repeater signs) without the use of physical measures like speed bumps.

20mph speed zones: Use traffic calming measures to reduce the negative impacts of motor vehicles in urban areas. The idea is that these measures, such as speed humps, chicanes and road narrowing, naturally slow vehicles to speeds below the limit, effectively making the zone ‘self-enforcing.’

London has taken significant steps to implement 20mph speed limits – over half of the city’s roads, including those within the Congestion Charging Zone, now have these a 20mph speed limit in place.

Transport for London (TfL) has also recently announced that they are set to expand these zones, with the aim to include over 137 miles of roads by the end of 2024. Other major UK cities shown above have adopted similar changes – primarily in city centres and residential areas – while Bristol and Edinburgh have adopted a city-wide 20mph limit.

Welsh uproar after national speed restrictions imposed

In 2023, Wales became the first country in the UK to roll out a national 20mph default speed limit on all restricted roads. Driver’s caught speeding in 20mph zones could face a minimum fine of £100 and three penalty points.

Despite overwhelming evidence that these measures will save lives, it was met with huge backlash from Welsh drivers – a petition protesting the 20mph mandate became the most signed petition in Senedd (Welsh parliament) history, reaching almost 470,000 signatures.

Protesters cited the cost of the policy (£32 million, even though the amount of money saved in the long term will be far greater), increased emissions and congestion and the potential for a ‘spike in road rage incidents’ among reasons for their fury. So, do these claims hold any truth?

Massive reduction in accidents and casualties

Perhaps the strongest argument for the use of 20mph speed restrictions is the reduction of accidents and casualties. The evidence is hard to ignore: data suggests that a pedestrian is seven times more likely to be killed if hit by a vehicle travelling at 30mph compared to one travelling at 20mph. That’s a staggering difference – and it’s backed up by plenty of research:

The first widespread study on the effectiveness of 20mph zones was carried out by TRL (Transport Research Library) in 1996. It found a 60% reduction in injury collisions and a 67% reduction in child injury collisions in 20mph zones, nationwide. More recently, a 2007 review of 78 speed restriction zones in London found a 42% reduction in injury collisions and a 53% reduction in fatal/serious collisions in 20mph zones.

Yet not all the evidence points this way – a study of Belfast published in 2022 stated that ‘restricting speed limits to 20mph in town and city centres doesn’t seem to reduce road traffic collisions, casualties, or driver speed’. This particular study is frequently cited by opponents of the Welsh scheme. However, it focussed on results from a small-scale rollout covering around 70 streets in the city centre of Belfast, where the average vehicle speed was already 17mph before the new limits were implemented.

It’s clear that these unusual results from a smaller study don’t hold up when you look at the bigger picture. Larger, more comprehensive studies show that 20mph zones make a real difference—improving road safety and ensuring fewer trips to the hospital.

Public health and encouraging sustainable transport

Another key reason for the use of 20mph speed restrictions is to promote a more sustainable, healthy lifestyle by encouraging more people to walk and cycle. This has been proven to work in several areas of the UK; in Edinburgh, data collected before and after the introduction of 20mph limits showed a 7% rise in walking, a 5% increase in cycling, and a 3% drop in car use.

Similarly, in Bristol, 20mph limits reduced vehicle speeds and increased walking and cycling. Residents of both cities are largely in favour of the changes. Lastly, lower speeds in residential areas reduce traffic noise – who can complain about that?

Have the rollout of 20mph zones impacted congestion and travel times?

One of the biggest concerns about 20mph zones is their impact on traffic and travel times. There is also debate as to whether this affects HGV drivers and van drivers. And for small businesses, particularly those relying on delivery drivers, this can be a real headache.

Impact on small businesses

In an article by the BBC, a business owner in Wales claims that the new restrictions are  “destroying companies and putting so much stress on people”. The man, who owns a bed and mattress company, says his deliveries were “taking an hour-and-a-half longer than usual.”

Driving at 20mph in these larger vehicles is harder than your average car. And it’s even more difficult with a full load, going downhill or dealing with chopping and changing speed limits in quick succession.

Insurance consultant Luke Morley said, “I’ve dealt with many commercial clients who are facing fine after fine because they keep getting caught going over the speed limit by one or two miles per hour, and it’s massively impacting their careers.”

“Of course, driving safely is an absolute must, no one is denying that. And while we must have a line in the sand for appropriate driving speeds, I can’t help but feel that HGV and van drivers face a tougher challenge than other motorists, whose vehicles are more suited to these slower zones.”

Does it affect emergency services?

Another concern for many is how these new speed limits will affect our emergency services. Of course, like with all speed limits, these zones have exemptions for ambulances, fire engines and police on emergency calls. Yet for zones with traffic calming measures like speed bumps and chicanes that are unavoidable, it has caused minor delays to response times.

While this might seem concerning, if we take a step back, the cost of accidents that have been prevented because of 20mph speed restrictions would have put a much larger strain on emergency services than minor delays to response times. So, in the long term, these measures will save money and lives.

Is there really an impact on air pollution?

While research on the effects of 20mph speed zones and limits on pollution levels is limited, the Royal Society for the Prevention of Accidents (ROSPA) stated that it is ‘possible that 20mph zones and limits could increase pollution, as some vehicles will operate inefficiently at low speeds. However, it is also possible that reduced speeds encourage smoother driving and reduced acceleration, which reduces emissions.’

A study by Imperial College London found that some traffic calming measures can cause more vehicle pollution than others. For example, it was found that vehicles can emit up to 98% more nitrogen oxides and up to 64% more carbon dioxide when driving over speed bumps compared to speed cushions.

However, the study concluded that overall, 20mph speed restrictions are unlikely to worsen local air quality and have no net negative impact on exhaust emissions. 

So, are 20mph speed zones and limits really a bad thing?

While they can be an inconvenience, it seems the lives saved vastly outweighs the cons of 20mph speed restrictions. The impact it might have on small businesses, van and lorry drivers shouldn’t be understated and perhaps special exemptions should be implemented in these cases.

However, the proven impact of these measures on road safety, reduced accidents and collisions, and greener transport has made them widely effective. So maybe it’s not such a bad thing after all…

Sources: BBC, ROSPA, Imperial College London, The Guardian, Sustrans

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