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



1.        Does 20mph work?

Benefit of 20mph

Yes. Lower speeds reduce casualties and lower limits reduce 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]

2.        Is 20mph popular?

Benefit of 20mph

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

3.        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.

4.        Do the police need to support a new limit?

Common myth

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]

5.        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].  The report lacked rigour as it compared unlike places and used small data sets. Evidence in Bath shows that casualties reduced after implementing 20mph by 23%[8]

6.        Does the Atkins report on 20mph show a lack of casualty reductions?

Common myth

The Atkins report is unconvincing[9]. Case study samples were small (<20 casualties pa) compared with larger comparator areas[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.

7.        Do drivers obey limits without enforcement?

Common myth

For persistent offenders, enforcement is welcome. However, the evidence is that reducing a speed limit does 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].

8.        Won’t police decline to enforce?

Common myth

The National Police Chiefs Council says police will enforce 20mph as much as any other speed limit[14] and some forces are pro-active –West Midlands[15], Avon & Somerset[16]. The introduction of speed limiters on new cars by 2022 will reduce the need for police enforcement compliance issue.

9.        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. For residual compliance issues, inexpensive traffic calming – signs, lines, staggered parking bays, planters etc – are often sufficient.

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

Technical question

Yes.  Many Local Authorities already have 20mph on A roads. In order to EXCLUDE a major street, the Highway Authority has considered the needs of vulnerable road users and shown that journeys on foot will be insignificant[17].

11.     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 into a “Limit” scheme would make a logical area[18].  If a “Zone” with physical calming cannot be afforded, leaving the speed limit at 30mph is not acceptable unless it can be shown to be as safe.

12.     What’s the pollution impact of 20mph limits?

Benefit of 20mph

Overall 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.

13.     Is 20mph expensive?

Benefit of 20mph

Not really – about £3 - £5 per head and it’s good value for money.  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. Doing nothing costs more.

14.     What’s the effect on business of 20mph?

Benefit of 20mph

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.

15.     Are speed reductions in areas without physical calming significant?

Technical question

1 – 2 mph is a typical reduction, which is significant[21], but averages mislead. Roads with speeds around 20mph are unlikely to change.  Faster roads show speed reductions between 4mph (Bristol) [22] and 7mph (Portsmouth)[23].

16.     What about overall trip times / bus timetables?

Common myth

There is often no significant difference on cross town trip times. Many bus companies have found no difference in their timetables.

17.     Do pedestrians take less care in a 20mph limit?

Common myth

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%,

18.     Can the 85th centile speed be used to assess 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.

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

Technical question

No.  Many Highway Authorities, such as Brighton[25] Bristol[26] and Portsmouth[27] , set 20mph on faster roads, often without physical traffic calming as it’s where most casualties occur and where reducing limits bring the greatest speed reductions. Using the “need for self-enforcement” to prevent a 20mph scheme doesn’t comply with DfT guidance to have lower speed limits on roads where there could be significant pedestrian or cycle movements.

20.     Is 20mph just used to generate income?

Common myth

No.  There are many reasons for introducing 20mph, but financial incentives for LAs or the Police are not the reason, since fines go to the Treasury.  In any case, 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 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] DfT guidance on setting local speed limits January 2013 

[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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