Twenty’s plenty

This is one of our asks for safer streets across the borough of Enfield:

A 20mph speed limit on all roads where people live, work or shop

This page was written by Jeremy Leach of Twenty’s Plenty For Us, who campaign for the adoption of 20mph limits as the default speed limit in built-up areas.

Why do lower vehicle speeds matter?

There are a number of reasons why, in built-up areas, lower maximum speeds for vehicles are a good thing. These are linked to reducing road casualties, encouraging more people to be active and to walk, cycle and use public transport, reducing noise levels and helping to improve air quality.

TfL is now introducing 20mph limits on much of its Red Route network.

Road casualties

Speed plays a big part in road casualties and that is why there is so much effort to try to reduce vehicle speeds in places where large numbers of people are out and about.

  1. People hit by a vehicle travelling at 30mph are four times more likely to die than those hit at 20mph[1]

  1. Braking distances are far greater at higher speeds. When driving at 30 mph you have far less time to react to any incident. In addition, the kinetic energy in your motor vehicle is proportional to the square of the speed. The stopping distance is also proportional to the speed squared so that means that a car braking from 30 mph will still be travelling at 22mph when one braking from 20mph will have stopped[2].


  1. Even small reductions in speed make a real difference to the numbers of road casualties. Each 1mph reduction in mean traffic speed is associated with a 5% reduction in accidents; the exact reduction depends on the type of road: thus: a 1mph reduction in average speed would reduce accident frequency by 6% on urban main roads and residential roads with low average speeds.[3]

Research has shown that where the maximum vehicle speed is reduced to 20mph, casualty levels are cut by almost a half. The study 20mph Zones and Road Safety in London, which TfL commissioned from the London School of Hygiene and Tropical Medicine in 2009,[4] found that in the 399 20mph zones implemented in London between 1990 -91 and 2007-08, there was “a 42% reduction in all casualties within 20 mph zones compared with outside areas, adjusting for an annual background decline in casualties of 1.7% on all roads in London.”

Where 20mph limits are introduced on their own without any traffic calming, studies have found that there is an estimated 20% decline in casualty numbers.

Although the numbers of serious and fatal road casualties have fallen in recent years, TfL data[5] shows that the total numbers of people injured on the roads in Enfield remains stubbornly high, at over 1,000 people per year.

Year Total number of road casualties in LB Enfield
2005  1,205
2006  1,054
2007  1,030
2008  854
2009  1,022
2010  1,075
2011  1,109
2012  1,038
2013  1,012
2014  1,003
2015  1,051

Although it is important to calm residential roads, the reality is that casualties occur most frequently where vehicles and people interact. In London, this means that it is the more major streets and roads and town centres and high streets in particular where casualty numbers are greatest. Lower speeds are vital on these more major roads where large numbers of people live, work and spend their leisure time.

Below are the casualty maps[6] of the main population centres in Enfield for the three years 2013 to 2015 (showing casualties of all severities). These show how the vast majority of casualties occur on the more major roads. Reducing speeds on these roads is just as vital as calming residential areas.




Edmonton Green




Enfield Town


Improving public health – encouraging people to walk and cycle 

Lower speeds also encourage more people to walk and cycle and be active. Fear of motor traffic is one of the biggest factors in why people do not choose to cycle, with 44% of people saying they would cycle more if the roads were safer[7]. In Edinburgh, research into trials of 20mph limits found that the proportion of children cycling to school rose from 4% to 12%, those considering cycling to be unsafe fell from 26% to 18%, and those feeling speeds were unsafe for walking fell from 17% to 12%[8].

Creating an environment where more people are active and feel able to walk and cycle can really help improve public health in Enfield.  Statistics shows that excess weight in both adults and children is a real issue in the borough, and both are often significantly higher than the London and England averages[9].

England London Region Enfield
2.06i – Child excess weight in 4-5 year olds 22.1% 22.0% 24.3%
2.06ii – Child excess weight in 10-11 year olds 34.2% 38.1% 41.5%
2.12 – Excess weight in Adults 64.8% 58.8% 63.5%

Reducing noise

Research in Sweden[10] has shown noise decreased between 2 and 4 dB for cars when speeds were reduced from 50kph (32mph) to 30kph (18 mph). Excessive traffic noise has been linked to disrupted sleep and even heart disease.

Improving air quality

The overall effect of lower speeds is to reduce air pollution. It can also reduce stopping and starting and thus reduce fuel consumption.

Although there is a small increase in NOx and CO2 emissions when petrol vehicles travel at 20mph rather than 30mph, the position is opposite for diesel vehicles, where emissions are lower at 20mph than 30mph. Given the much larger quantities of dangerous emissions from diesels and the current distribution of diesel/petrol cars in London, 20mph results in lower levels of NOx and PM10 overall[11].

The introduction of 30kph zones in Germany led to drivers changing gear less, braking less, and requiring 12% less fuel.[12]

The current picture in London

All of these benefits have led a large number of the London boroughs to bring in 20mph limits across much or all of the borough. The current position in relation to 20mph limits in summer 2017 is shown in the map below, where 9 out of 13 Inner London boroughs have introduced 20mph speed limits and many Outer boroughs are also taking up 20mph limits.

The latest estimates show that more than two-fifths (43%) of all Londoners are living on roads and streets with a 20mph speed limit (3.7 million out of total 2015 population of 8.7 million). In Inner London (inside the green line below) almost three-quarters (72%) of the population is living on 20mph streets and roads (2.2m out of 3.1m population).

How can we enforce 20mph?

So these are some of the benefits of introducing 20mph speed limits – but how can we get more people to stick to them once they have been introduced? Boroughs that have introduced 20mph limits and Transport for London are grappling with this as they strive to implement Vision Zero policies in relation to road casualties. The recently published draft Mayor’s Transport Strategy[13] is aiming for no one to be killed or seriously injured on London’s roads by 2041.

There are lots of steps that are now being considered to reduce vehicle speeds across the capital in search of the Vision Zero targets. Some of these include:

  1. Introduce 20mph limits: average speed reduction 1.0 to 1.5mph

Camden Street


  1. Remove the centre white line on a road: average speed reduction about 3mph (TfL Research)

Kennington Park Road, Lambeth


  1. Remove excess carriageway: for example by introducing segregated cycle lanes

North-South Cycle Superhighway (CS6) – St George’s Rd SE1

On Enfield’s A105, the new semi-segregated cycle lanes have reduced the width of the carriageway to 6.5 metres (the minimum for a bus route). Anecdotally, this seems to be having a calming effect on traffic on the finished sections, without causing traffic congestion.

4. Fit buses with mandatory speed limiters

A trial by TfL in 2015 trial found that buses which were fitted with Intelligent Speed Adaptation (ISA) stayed within the 20mph speed limit “97-99 per cent of the time”. TfL now intends to require all new buses from 2018 to have this technology fitted. If this approach was extended to the other fleets that TfL oversees, such as taxis. private hire vehicles, goods vehicles and delivery vans, it could have an enormous impact of compliance with speed limits in London in the longer term.


5. Wider use of camera enforcement

TfL research has found an average 58% fall in the number of people killed or seriously injured (KSI) where safety cameras operate in London. Better Streets for Enfield supports installing average speed cameras on the A10, where outdated speed cameras have been removed.


5. Community Roadwatch

Herne Hill, Southwark

This is a joint initiative between TfL and the Metropolitan Police where members of the community suggest locations where vehicles speed are high and enforcement is needed. The scheme is being rolled out to all 33 London boroughs. See

Why not go 20?

So…we know that reducing vehicle speeds to a maximum of 20mph on streets and roads in built-up areas can make an enormous difference to the numbers of people who are killed and injured on our roads and they can also reduce the intimidation from traffic that people who are walking and cycling often feel.

Transport policy in London with the Healthy Streets initiative in the Mayor’s Transport Strategy and a Vision Zero approach to road casualties is moving towards a lower speed environment on London’s roads where 20mph is highly likely, in time, to become the default on all streets and roads where people are out and about in large numbers. Plus the boroughs and TfL are now gaining lots more ways to increase compliance with 20mph limits.




[3] TRL Report 421: M Taylor, D Lynam and A Baruya – Effects of drivers’ speed on frequency of road accidents