Questions about 20’s Plenty - Answered
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1. Does 20mph work? Yes
Lower speeds reduce casualties and lower limits reduce speeds
NICE (National Institute for Health and Care Excellence), WHO (World Health Organisation) and 
the Stockholm Declaration (Feb 2020) recommend 20mph where people and motor vehicles mix.
The Global Network of Road Safety Legislators notes: [where motor]
“traffic mixes with vulnerable road users…the speed limit should be…under [20mph]”
2. Is 20mph popular? Yes
Surveys consistently show more than 70% of residents support 20mph*; popularity increases after their introduction
3. Is 20mph just used to generate income? No
There are many reasons for introducing 20mph, but financial incentives for Local Authorities or the Police are not the reason, since fines go to the Treasury. Although Fixed Penalty Notices are sometimes given, it is more common to have warning letters or for drivers to attend a speed awareness course.
Introducing speed limiters on new cars from 2022 will reduce the compliance issue.
4. Do you need to wait for several road casualties to justify 20mph? No
Rationing 20mph to places with high casualties contravenes the Department for Transport guidance and WHO/NICE recommendations that 20mph is a safe speed where people and vehicles mix. Highway Authorities cannot create arbitrary reasons to justify higher speed limits which ignore other factors, such as the needs of vulnerable road users.
5. Do the police need to support the new limit?
Police are consulted as a stakeholders, just like local communities. It is the Highway Authority’s obligation to set the appropriate speed limit. Once the scheme design is agreed, the police should consider appropriate enforcement measures (if any). Vertical physical calming should be used as a last resort as they impede emergency services.
6. What is the pollution impact of 20mph speed limits?
Overall pollution reduces, particularly from diesel*.
Less accelerating/braking reduces brake and tyre particulates*. NICE recommends 20mph without speed humps for better air quality, lower noise levels, vibration and road wear.
7. Is 20mph expensive?
Not really – about £3–£5 per head and it’s good value for money. Doing nothing costs more.
Costs depend on the scheme size, the amount of driver education and the extent of physical calming. Signed 20mph schemes typically pay back in under a year in fewer casualties and more active travel.
8. What’s the effect on Business of 20mph? Positive
Businesses thrive where it is safe to walk and cycle and more money is available to the local economy. 
It also has benefits for businesses’ own staff.
9. What about overall journey times / bus timetables?
There is often no significant difference on cross town trip times. Many bus companies have found no difference in their timetables.
10. Do Pedestrians take less care in a 20mph limit zone? No
There is no evidence for this, whilst there is substantial evidence from Bristol, Edinburgh, Calderdale, Brighton and other places that 20mph reduces casualties.
A 1mph reduction in speed on an urban road reduces casualties by 6%.
*To see the full document and list of sources for all data quoted, visit www.20splenty.org/20_questions_about_20_s_plenty