Summary

On 17 September 2023, the Welsh Government introduced a default 20mph speed limit on restricted roads (usually residential or busy pedestrian streets with streetlights). A policy evaluation has been commissioned by the Welsh Government to assess the impact of the 20mph policy. In the interim, Transport for Wales (TfW) is responsible for monitoring the initial effects of the 20mph policy, and publishes regular national monitoring reports.

TfW’s monitoring framework for the default 20mph speed limit provides detail about the key performance indicators (KPIs), which are the metrics used to assess the effects of implementing the policy. These include metrics relating to:

• Causalities
• Traffic speeds
• Journey time reliability
• Vehicle and pedestrian interactions
• Air quality

The Welsh Government publishes two sets of accredited official statistics that include figures for casualties in relation to speed limits. The police recorded road collisions statistics are published in annual and (provisional) quarterly publications, and the underlying data are available on StatsWales and through an interactive dashboard. The reported road casualties statistics are published in an annual publication, which represents more detailed analyses of casualties using the police recorded road collisions data. The police recorded data are also provided to the Department for Transport for statistical releases concerning casualties in Great Britain.

TfW publishes vehicle speed monitoring reports, which include statistics based on data from traffic speed monitoring devices. Data from an Ordnance Survey dataset are also used to compare mean speeds on A and B roads from a relatively small sample of in-vehicle telematics. Both datasets include statistics from before and after the 20mph policy implementation.

Things to consider

There is a lack of official data on the volume of traffic on roads with different speed limits. Traffic volume is an important factor to consider, as it likely impacts the findings relating to several KPIs (e.g. collisions, air quality).

Other considerations concerning the metrics used in TfW’s monitoring framework are as follows.

Casualties

• To make meaningful comparisons of casualties, it is recommended to use three years’ worth of collision data before and after implementation of the 20mph policy. Comparisons between shorter periods can be problematic as other factors (e.g. traffic volume, weather) can have a large impact on the results. As a minimum, quarterly data should be compared with data from the same quarter in other years.
• The police forces in Wales differ in terms of the tool used to report collisions. Most police forces use a non-injury-based reporting system, whereas Dyfed Powys moved to using an injury-based reporting system in May 2023. The injury-based reporting system automatically classifies the severity level of injuries, whereas the non-injury-based reporting system relies on officers’ judgement. Analysis indicates that collisions are more likely to be classified as severe than slight when using an injury-based rather than a non-injury-based reporting system. This difference affects both the comparability within Wales and over time. However, the difference only affects comparisons of the severity of casualties and does not affect comparisons of the total number of casualties.
• After the implementation of the 20mph policy, the number of roads that had a 20mph speed limit increased and the number with a 30mph speed limit decreased – the number of collisions on these roads are therefore expected to increase and decrease, respectively. The police recorded road collisions and reported road casualties statistical publications present findings about collisions for 20mph and 30mph roads combined, which enables more reasonable comparisons to be made over time.
• Police recorded statistics are not necessarily coherent with other estimates of collisions and casualties, such as those based on survey data and data concerning hospital attendance and admissions, and insurance claims.

Traffic speeds

• For the metric concerning the percentage of vehicles complying with the 20mph speed limit, compliance is defined as anyone driving at or below 24mph. The 24mph threshold aligns with UK enforcement, although the current Welsh enforcement threshold is under 26mph.
• The vehicle speed monitoring data collected and published by TfW is based on 38 monitoring sites across 10 areas of Wales. Care should be taken when these data are used to make statements about the speeds regionally or nationally.
• The Ordnance Survey data provide mean speeds for roads. As such, the data are affected by factors other than speed limits, such as vehicles slowing down when approaching a junction.

Air quality

• Factors other than speed limits affect traffic volume and vehicle acceleration, which influence air quality. Such factors include construction works, traffic management measures, temporary road closures and closeness to vegetation.
• Some air quality data collection was commissioned by TfW, focused on the phase 1 trial areas where 20mph speed limits were implemented during 2021/22. This air quality data was collected using lower-cost air quality sensors. These sensors are appropriate for making relative comparisons of air quality between sensors inside and outside 20mph speed limits. However, the data collected from these sensors should not be compared with national air quality objectives, which rely on assessment using different equipment. TfW has published the full technical phase 1 air quality monitoring report.
• TfW has commissioned further specialist air quality analysis using data obtained from existing Wales Air Quality Network sensors. These sensors are higher-cost and have been in place for several years for general air quality monitoring rather than being specific to 20mph monitoring. This analysis is not yet complete.

What to look out for

• Small samples are used to generate the statistical evidence to assess several metrics, including traffic speeds, journey time reliability, vehicle and pedestrian interactions, and air quality. These metrics should not be used to make broader conclusions, such as at a regional or national level.
• Users should also be sceptical about very confident statements that draw conclusions about causation. Factors other than speed limits (e.g. traffic volume or weather conditions) are likely to affect findings. As such, particular attention and scrutiny should be given to statements suggesting that the introduction of 20mph speed limits caused an effect.
• As the 20mph policy affected total length for roads with both speed limits – total road length increasing for 20mph and decreasing for 30mph – the number of casualties or collisions are expected to change in line with the change in total lengths. Comparisons before and after implementation of the 20mph speed limit should therefore be made using the 20mph and 30mph combined figure.
• Given the difference in reporting systems across police forces in Wales, comparisons across police forces in Wales, or before and after implementation of the 20mph policy, should be made based on the total numbers of casualties and collisions, and not on the numbers of slight or severe injuries.
• Data relating to carbon dioxide emissions are not currently available, and even if data were to be obtained it would be difficult to make a direct causal link between 20mph and carbon dioxide.

Wider support

The Welsh Government’s FAQs about the 20mph policy are a good starting point for a general understanding of the speed limit changes. The FAQs cover a broader range of topics than the monitoring reports, including costs and economic impacts – we have published regulatory guidance about statements on public funding to aid interpretation of such statements.

TfW’s monitoring framework document explains the approach taken to monitoring the initial effects of implementing the 20mph policy.

Two Welsh Government resources provide more detail about road collision and casualty statistics. The Welsh Government’s Chief Statistician published a blogpost that assists in understanding these statistics. The quality report for the police recorded road collision statistics also provides important context for understanding the data.

The UK Parliamentary Office of Science and Technology produced a helpful briefing on the public health impacts of 20mph speed limits, including information about challenges with interpreting the evidence from evaluations of introducing 20mph speed limits.