Should we adopt 20mph speed limits?

In Britain, the usual speed limit in built-up areas is 30 miles per hour. In some places it is lowered to 20, particularly around schools. A growing number of councils are recognising the benefits of the lower limit in urban areas, and the number of 20mph zones is rising. Around half of Britain’s biggest cities now have 20mph limits as the default speed in residential areas. Luton isn’t among them, though there are several 20mph zones around the area where I live. Here are five things to consider:

Less traffic – drivers might consider lower speeds to be inconvenient, but there are benefits for drivers that offset the loss in speed. For example, cars can travel closer together at lower speeds. That gets more cars in the same space, and keeps them moving. Traffic is less likely to come to a standstill. It’s much easier to turn or merge into traffic at 20mph, which again keeps traffic moving. A Department for Transport study last year looked at journey times for drivers in areas where 20mph limits had been introduced. They had increased by just 3%.

Safety – Nine times out of ten, a pedestrian hit by a car traveling at 40 miles per hour will be killed. At 30mph, their chances improve to 50/50. At 20mph, they have a nine out of ten chance of surviving. Since stopping distances are shorter at lower speeds, drivers are also less likely to hit anyone in the first place. The majority of fatal accidents happen in 30mph areas, so a reduction in speed limits would save lives.

Active travel – 20mph roads are both safer and perceived as safer, making people more likely to walk or cycle. That’s better for health, and of course every family that chooses walking or cycling helps to reduce car traffic for everyone else too.

Unfortunately, where this has been monitored the rise in active travel is actually very modest – usually in the single figures. There are many factors involved in how people choose to travel, and it’s more accurate to say that 20mph zones remove barriers to active travel rather than claiming that they improve active travel rates.  However, it’s worth noting that the biggest change is in children cycling to school. Our cities do not serve children well, and we should give serious attention to anything that improves childrens’ lives and increases their freedom and independence.

Pollution and CO2 – cars produce more air pollution when idling, or when stopping and starting. Where 20mph zones help to keep traffic moving, there will be less pollution. If more people switch to active modes of travel, that will also lower pollution and CO2. On the other hand, most manufacturers tune their cars to be most efficient at ‘normal’ speeds. Many cars produce more CO2 and pollution at low and high speeds, with an efficiency sweet spot in the middle. So greenhouse gas emissions and pollution are actually marginally worse at 20 than at 30mph.

Does the lower efficiency cancel out the benefits of smoother traffic movement? There are lots of variables and few studies, but at the moment there isn’t a compelling environmental argument for a 20mph speed limit. (Lower speed limits on motorways definitely would improve environmental performance, but that’s not today’s topic.) This will change as the number of internal combustion engines falls. There will come a point when there are enough hybrid and low emissions cars on the roads for 20mph to be better for the environment too.

Improved quality of life – finally, people feel safer on their streets and neighbourhood with lower speed limits. The area feels more relaxed as people literally move at a slower pace. There is less noise pollution. Residents are more likely to spend time in the streets, and there is a correlation between car speed and the number of neighbours that people know by name. More children play outside in 20mph zones. Lower speeds improve community.

In summary, there are a number of things to consider with 20mph zones, and they have to be held in balance. The main arguments are safety and wellbeing, with little affect on drivers. It also lays the foundations for more active travel, though how many people actually switch would depend on all sorts of other things. I suspect that whether it’s a necessary measure will depend very much on local conditions. For more detail, see the 20’s plenty campaign.


  1. The only real problem I see with the 20 mph limits in the area of Luton where I live is the lack of enforcement. Most motorists I see grossly exceed those limits unless there are humps, which have their own disadvantages.

      1. While that is true, it will be many years before such vehicles are the norm. New cars sold today, without that technology, will in some cases still be in use in 20 years.

  2. I’m unfamiliar with street layout in Britain. Are neighborhoods usually set off from main thoroughfares, or do the main streets usually pass through residential neighboorhoods with limited shoulder space?
    Also, do crosswalks usually have stop signals or traffic lights?

    1. All of the above! There’s a huge variety in Britain. Very few cities are laid out on a grid, and some towns are planned better than others. Generally speaking neighbourhoods are not on main roads, and more modern estates are designed with arterial roads and quieter residential streets. Some crossings have signals, some give priority to pedestrians.

      And there are no jaywalking laws in Britain, you can cross wherever its safe. On my first visit to the US, I was amazed to find that you can’t just cross the road when you need to.

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