20 Questions About 20’s Plenty Answered

We have put together the most common questions that we get asked about 20mph limits. Here we also separate the myths from the reality. References are provided to all our answers.

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20mph Questions




Does 20mph work?

20mph benefit

Yes. Lower speed limits reduce casualties by lowering speeds.  NICE[1] and WHO recommend 20mph where people and motor vehicles mix[2].  The Global Network of Road Safety Legislators notes “[where motor] traffic mixes with vulnerable road users…the speed limit should be…under [20mph]”[3]


Is 20mph popular?

20mph benefit

Yes.  Surveys consistently show >70% of residents support 20mph[4]; popularity increases after their introduction[5].


Do you need several road casualties to justify 20mph?

Common myth

No. Rationing 20mph to places with high casualties contravenes DfT 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.


Do the police need to support a new limit?

Common myth

No. It is the Highway Authority’s duty to set the appropriate speed limit, although police are consulted as a stakeholders as are local communities. Once a scheme is agreed, the police should consider appropriate enforcement measures. Vertical physical calming should be used as a last resort.[6]


Have casualties risen anywhere a 20mph limit has been implemented?

Common myth

No. Some people mistakenly quote a Bath & NE Somerset Scrutiny committee report from May-17[7].  Evidence in Bath shows that casualties reduced after implementing 20mph by 23%[8]


What does the Atkins report say about 20mph and casualty reductions?

Common myth

It's unconvincing[9]. Case study samples were small (<20 casualties pa)[10]. Atkins did not consider other evidence where casualties HAVE reduced on 20mph roads and underplayed the significant benefits in Brighton where faster roads were included.


Do drivers obey limits without enforcement?

Common myth

Most do. For persistent offenders, enforcement is welcome. However, the evidence is that lower speed limits bring speeds down[11]. In Bristol, compliance with 30mph limits also increased when 20mph was implemented[12]. DfT compliance figures show that the majority drive slower when the speed limit is 20mph[13].


Won’t police decline to enforce?

Common myth

Police can enforce 20mph as much as any other speed limit[14] and some forces are pro-active – West Midlands[15], Avon & Somerset[16]. Speed limiters on new cars will soon reduce the need for police enforcement.


Do drivers ignore a 20mph limit without traffic calming?

Common myth

Most comply without additional measures.  The ‘population paradox’ shows how small speed reductions over wide areas bring more benefits than big interventions in specific places. Inexpensive traffic calming – signs, lines, staggered parking, planters – are often sufficient to solve residual compliance issues.


Can you have 20mph on an A or B road?

Technical question

Yes.  Many Local Authorities have 20mph on A roads. A Highway Authority cannot exclude a street unless it has considered the needs of vulnerable road users and has shown that journeys on foot will be insignificant[17].


Can you have 20mph on roads with a mean speed >24mph?

Technical question

Yes, particularly where the mean speed across a number of roads is under 24mph or where including a faster road would make a logical area[18].  The cost of physical calming should not be used to prevent 20mph, unless 30mph can be shown to be as safe.


What’s the pollution impact of 20mph limits?

20mph benefit

Pollution reduces, particularly from diesel[19]. Less accelerating / braking reduces brake and tyre particulates. NICE recommends 20mph without speed humps for better air quality[20], lower noise levels, vibration and road wear.


Is 20mph expensive?

20mph benefit

No. It’s good value for money - c. £5 per head. Doing nothing costs more.  Costs depend on the scheme size, marketing spend and the extent of physical calming.  Signed 20mph schemes typically pay back in under a year - fewer casualties and more active travel.


What’s the effect on business of 20mph?

20mph benefit

Positive. Businesses thrive where it is safe to walk and cycle and more money is available to the local economy. It’s also for businesses’ own staff.


How much does speed reduce without physical calming?

Technical question

It depends on the existing speeds. Speeds will reduce more on faster roads and less on slower roads. Data from the Scottish Borders[21] showed an reduction of 3mph, with up to 6pmph on faster roads. Similar reductions were seen in Bristol[22] and Portsmouth[23].


Do trip times for bus increase?

Common myth

Not significantly for cross-town journeys. Many bus companies found no difference in their timetables.


Do pedestrians take less care in a 20mph limit?

Common myth

No.  Evidence from Bristol, Edinburgh, Calderdale, Brighton and others shows how 20mph reduces casualties. A 1mph reduction in speed on an urban road reduces casualties by 6%


Can the 85th centile speed be used to judge a 20mph scheme?

Technical question

No. DfT guidance says that mean speeds should be used[24], but note that focussing on average speeds is often misleading.


Do you need road humps if speeds are above 24mph?

Technical question

No.  Highway Authorities, such as Brighton[25] Bristol[26] and Portsmouth[27], set 20mph on faster roads without physical traffic calming because that's where most casualties occur and where speeds reduce the most. Using the “need to self-enforce” to prevent 20mph doesn’t comply with DfT guidance on lower speed limits where there could be significant pedestrian or cycle movements.


Is 20mph just used to generate income?

Common myth

No.  Of many reasons for introducing 20mph, financial incentive is not one. While any fines from Fixed Penalty Notices go to the Treasury, it is more common for drivers to receive a warning letter or attend a speed awareness course. Introducing speed limiters on new cars from 2022 will reduce the compliance issue anyway



[1] https://www.nice.org.uk/guidance/ng70

[2] https://bit.ly/2GwS4l3

[3] www.4roadsafety.org/wp-content/uploads/2017/05/4roadsafety-Manifesto.pdf

[4] https://bit.ly/2UykFeG

[5] YouGov surveys https://www.roadsafetyobservatory.com/Evidence/Details/11651

[6] DfT guidance on setting local speed limits January 2013 

[7] https://bit.ly/2VihTik

[8] https://bit.ly/2O2DSGo

[9] https://bit.ly/2UykFeG

[10] http://www.20splenty.org/dft_20mph_evaluation

[11] DfT Freeflow speed statistics https://bit.ly/2XLhfYh

[12] Bornioli, Pilkington et al http://eprints.uwe.ac.uk/37939/

[13] DfT Freeflow speed statistics https://bit.ly/2XLhfYh

[14] NPCC guidance https://bit.ly/2IUSfcI

[15] https://bit.ly/2L1Wlm5

[16] https://bit.ly/2GGCd4L

[17] DfT guidance on setting local speed limits January 2013 

[18] DfT guidance on setting local speed limits January 2013 

[19] Report by Imperial College for City of London April 2013 https://www.cityoflondon.gov.uk/business/environmental-health/environmental-protection/air-quality/Documents/speed-restriction-air-quality-report-2013-for-web.pdf

[20] https://www.nice.org.uk/guidance/ng70

[21] 20mph Road to permanence

[22] BRITE study, Pilkington, Bornioli et al http://eprints.uwe.ac.uk/34851/ and http://eprints.uwe.ac.uk/37939/

[23] WS Atkins interim evaluation http://www.wirralpedestrians.org.uk/files/20mphzoneresearch.pdf

[24] DfT guidance on setting local speed limits January 2013 

[25] WS Atkins: effectiveness of 20mph speed limits https://www.gov.uk/government/publications/20-mph-speed-limits-on-roads

[26] BRITE study, Pilkington, Bornioli et al http://eprints.uwe.ac.uk/34851/ and http://eprints.uwe.ac.uk/37939/

[27] WS Atkins interim evaluation http://www.wirralpedestrians.org.uk/files/20mphzoneresearch.pdf





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