17 May 2021


Many streets in the UK have roads which restrict drivers to a 20mph limit. The main road and feeder roads around these 20mph zones usually have a 30mph speed limit so this usually results in areas with variable speed limits.

The 20mph speed signs are an important indicator to drivers that they are in a zone where there are schools and other institutions and they have to drive with more attention and care. These 20 mph zones also tend to have an associated speed hump in order to reinforce the lower speeds.

Even with these restrictions in place, do 20mph zones help prevent crashes and fatalities? Do 20mph zones really work?

Why were 20mph restrictions implemented?

Towns and cities all over the UK have seen an increased number of restricted speed zones implemented in high foot traffic areas such as near schools, nurseries and other residential zones. These restricted speed areas were brought in as a traffic calming measure to reduce traffic accidents and fatalities on roads that are used regularly by pedestrians, cyclists and other road users.

Today, it’s estimated that one in three towns in the UK either have, or are planning to implement, 20mph zones in areas with high pedestrian traffic. In Edinburgh, for example, almost every street in the city centre is now under a 20mph limit, with only a handful of key roads in a 30mph zone.

The number of pedestrian, cyclist and other road user accidents are well documented, and evidence shows that it is roads in built up areas, such as residential areas, B –roads and other low speed minor roads, where accidents are most prevalent. In 2016, the majority of pedestrian casualties occurred in built up areas, with 29 of the 34 child pedestrian fatalities and 302 of the 413 adult pedestrian fatalities due to traffic accidents taking place in these areas. The story is similar for cyclists with over half of cyclist deaths (58 of 102) and the vast majority of cyclist casualties (16,934 of 18,477) occurring on roads in built-up areas.

Whilst pedestrians and cyclists are considerably more vulnerable, drivers are also more likely to be injured in collisions in built up areas due to the high number of vehicles on the road, even if they aren’t going particularly fast. In 2016, there were 789 fatalities, 15,993 serious injuries and 113,055 minor injuries from collisions, with a large portion of these taking place on residential roads, B-roads and other lower-speed minor roads.

It makes sense that driving at higher speeds increases the chances of both drivers and pedestrians being injured in an accident, which is a strong argument for creating 20mph zones. As the speed restriction rises, so does the risk of severe injury to both pedestrian and driver. However, there is alternative research available to suggest that 20mph restrictions have a lesser impact than first thought. For example, one study found that collisions at 20mph had a 2.5% chance of fatal injury for pedestrians, compared to 20% at 30mph, while another study found the same risk to be 1.5% at 20mph and 8% at 30mph. In both cases, it’s clear that lowering speeds does improve survival rate in the case of collisions with pedestrians, so for many local authorities, introducing 20mph zones isn’t a difficult decision.

So, do 20mph zones work? The research

Several studies have shown that minor accidents actually increased in 20 mph zones by nearly 17%. Studies by the Institute of Advanced Motorists, based on data collected by the government, has shown that minor casualties in 20 mph zones increased by 19% while major casualties in these zones saw a 29% increase. However, other studies from the Netherlands show that after six years of low speed zones, the number of accidents dropped more than 80%.

The Theory

There have been several theories put across as to why 20mph zones are not working as effectively as they should.

One theory is that the 20mph sign does not alter the behaviour of drivers because the driver still feels that the road is a 30mph zone. This could be because both the roads feel and look the same, especially as a lot of feeder roads are 30mph. This variable change can go unnoticed by drivers.

Another theory of 20mph zones not working is distractions. When the driver keeps checking his/her speedometer to ensure compliance with the speed limit of 20 mph less attention is paid to the road, potentially causing accidents.

In addition to these theories, it has been reported that traffic congestion and pollution increase when the speeds are lower. This conflict will be experienced more by cyclists in a 20mph zone when they find that cars are going at the same speed as them, instead of going by them. This could potentially lead to tailgating accidents between cyclists and vehicles in a 20mph zone.

20mph Limits vs. 20mph Zones

There are lots of different ways of creating and enforcing 20mph areas in towns and cities, but the most common methods in the UK are using blanket 20mph limits, and creating 20mph zones.

In areas with a 20mph speed limit, the rules are exactly the same as any other speed limit on the roads, with fines, points and disqualification of your license should you be caught speeding. Many of the areas with 20mph limits actually don’t have any additional elements in place to encourage road users to slow down. Particularly for drivers who have been driving on these roads for a long time, this means it can be very easy to forget the new limits, which is where features of 20mph zones can be incredibly helpful.

In lots of areas, 20mph zones don’t actually have a legally enforceable speed limit, and instead rely on physical cues to encourage drivers to slow down and obey the 20mph limit themselves. If you see a speed limit sign with a green circle, rather than the standard red speed limit sign, then it’s an advisory measure, and not a hard limit.

One thing to be aware of though, is that while these limits are advisory, so driving faster than displayed isn’t illegal on its own, if you were to get into an accident driving over 20mph in one of these areas, it could still be held against you in court. Essentially, in 20mph advisory zones, the roads should be designed to make drivers want to move more slowly, which can be done in lots of different ways.

How to change

Overall, these theories suggest that roads themselves should be self-regulating, i.e., have obvious limitations that suggest to the driver that they need to drive slower. This could be more effective than putting up a sign that indicates the speed at which to drive.

When roads are designed to appeal to the intuitive speed limits of drivers, there could be a much lesser chance of fatalities and casualties. So one way to make sure drivers comply with the speed limit in 20 mph zones would be to change the structure or character of the roads per se. This is one of the key differences between 20mph limits and 20mph zones, with physically changed roads without hard limits being arguably more effective than simply enforcing a 20mph speed limit.

The zones which are 20mph could potentially have further self-regulating traffic controlling methods that are not just restricted to speed humps to address the issues discussed. There could be narrower roads, chicanes, planting of trees and shrubs, etc. Of course, this is a much more significant investment for local authorities, and requires significantly more budget than simply installing new signage and updating the area guidance. However, it’s an investment that is very wise, and can contribute massively to the effectiveness of 20mph zones in built-up areas.

Several studies have shown that when 20mph zones are introduced with associated traffic-controlling methods mentioned above, there is a significant reduction in traffic accidents, particularly compared to areas where there is only a 20mph speed limit.

So the way to address these issues could be to introduce these traffic-controlling measures even on roads which are wider, leading to a potentially significant reduction in road traffic accidents. Effective traffic calming measures are usually found when city planners and transport departments create roads with pedestrians and cyclists in mind too, rather than just drivers.

Traffic calming schemes that leave space for other road users and focus on improving the appearance and safety of roads for all, such as wider footpaths, designated cycle lanes, central islands for pedestrians and other such measures, can be incredibly effective at helping reduce accidents, potentially even more so than 20mph speed limits alone.

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