Top 10 questions answered

Here are answers to the questions that we are asked most frequently about 20mph, including a short video. Answers are fully referenced with more detail. The most common questions are about the benefits of 20mph (fewer casualties, lower emissions, supports active travel), whether 20mph is popular (it is), can it be enforced (yes) and whether it creates more emissions (it doesn't). It's cost effective and has little or no impact on journey times (even buses).

You can also view the FAQs as a video in the link below 


Response detail

1. Why 20mph?

Multiple benefits for all road users include fewer casualties, lower emissions and more active travel.  Places that have introduced 20mph have seen casualties reduce by 20% - 30% [1]

2. Popularity

Surveys consistently show >70% of residents support 20mph [2]; popularity increases after their introduction. Social Media overstates opposition. Nearly 30 million people in the UK now live in Local Authorities committed to 20mph.

3. Enforcement

20mph is as enforceable as any speed limit. A Freedom of Information request by 20’s Plenty for Us revealed that 41 Police Forces (out of 44) [3] in Great Britain issued 3 million Notices of Intended Prosecution in 2022, including ¼ million on 20mph streets.

4. Cleaner

Accelerating to the limit takes 2.25x the energy to get to 30mph versus 20mph. As a result, emissions are around 30% higher in 30mph urban driving [4]. NICE recommends 20mph without speed humps for better air quality [5] and less noise.

5. Compliance

Most change behaviour, using the new speed limit as a reference point. The DfT says most drive slower in a 20mph speed limit [6]. Speeds can reduce up to 6 – 7mph [7].  In Bristol, compliance with 30mph limits also increased when 20mph was implemented [8].

6. Cost

At +/- £10 per person, 20mph is excellent value for money.  Signed 20mph schemes typically pay back in under a year: fewer casualties / more active travel [9]. Doing nothing costs more.

7. Journey times

No significant change, even for bus journeys.  In most town and cities, average speeds are less than 20mph due to congestion and other factors, such as lights, junctions etc

8. What about Wales?

Following the implementation of a 20mph national default in September 2023, overall compliance is good. Speeds are down by 2-3 mph[10] - more on faster roads.

9. A / B roads

20mph is allowed. Before excluding a major street, the Highway Authority must fully consider the needs of vulnerable road users [11].

10. Roads >24mph

Many Highways Authorities, such as Scottish Borders, Cornwall and Oxfordshire [12] implement 20mph on faster roads without reference to a 24mph threshold.


A PDF of the questions and answers is here



[2] page 10






[8] Bornioli, Pilkington et al





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