Transport for the West Midlands: Understanding the Real Impact of 20mph
Customer success story
Transport for the West Midlands (TfWM) is responsible for delivering a safer and more effective transport network across a large and complex urban region. As part of its Regional Road Safety Action Plan, TfWM has committed to an evidence-led Safe System approach, one that prioritises interventions proven to reduce harm.
Within this context, 20mph plays a key role. It is widely recognised as an important tool for improving safety outcomes, particularly in urban environments where people and vehicles interact closely.
However, as rollout has expanded, the strategic question has shifted. The focus is no longer simply whether to introduce 20mph, but how to understand its real-world impact, where it delivers the greatest benefit, and how future programmes should be targeted.
TfWM commissioned Agilysis to provide that clarity by combining a comprehensive review of the evidence with a detailed analysis of local outcomes.The Challenge
Like many combined authorities, TfWM faced a common but complex challenge. 20mph limits had been implemented across multiple local authorities over an extended period, often with differing approaches, timescales and levels of supporting activity. At the same time, broader trends in road safety (such as long-term reductions in collisions) made it difficult to isolate the intervention's specific impact.
TfWM needed to understand not just what had changed, but why, and whether those changes aligned with expectations from the wider evidence base. Without that understanding, it would be difficult to:
- target future interventions effectively.
- prioritise investment in a constrained funding environment.
- and demonstrate the value of 20mph within a broader Safe System strategy.
The challenge was therefore not a lack of data, but a lack of integrated insight. Bringing together evidence, local context and real-world outcomes in a way that could inform decision-making.
The Approach
Agilysis addressed this challenge through a structured, two-part approach designed to connect national evidence with local performance.
The first phase examined contemporary UK and international research on 20mph schemes. This included large-scale city and national programmes, as well as the latest meta-analysis studies.
The review established a clear baseline for expected outcomes, particularly around the relationship between speed reduction and safety benefits. It also identified a critical insight: the effectiveness of 20mph is strongly influenced by baseline operating speed, with greater benefits achieved where pre-implementation speeds are higher.
The second phase focused on the West Midlands itself. Agilysis combined multiple datasets to build a detailed, evidence-based assessment of outcomes:
- STATS19 collision data, covering an 18-year period and over 6,700 collisions linked to 20mph roads
- TomTom connected vehicle data, providing consistent, network-wide insight into real-world speeds before and after implementation
Using a robust before-and-after methodology, the analysis compared three years pre- and post-implementation for safety outcomes, alongside 12-month comparisons for speed data.
The use of connected vehicle data from TomTom was particularly important. It enabled a more complete view of driver behaviour across the network, rather than relying on isolated point measurements, and allowed patterns to be analysed at scale.
The Outcome
The results provide a clear and credible picture of how 20mph is performing across the West Midlands and, importantly, why.
At a headline level, the findings align closely with both UK and international evidence. Across the region, baseline speeds were already low prior to implementation. In Birmingham, for example, average speeds were around 16.8mph. In this context, the observed outcomes were consistent with expectations:
- small but statistically significant reductions in speed
- larger reductions on higher-speed (classified) roads
- no statistically significant short-term change in collision numbers on 20mph roads
At the same time, wider network trends showed a long-term reduction in collisions of around 43%, indicating an overall improvement in road safety outcomes across the study area.
The critical insight from the project is that these results do not indicate underperformance. Instead, they reflect the underlying conditions of the network. Where speeds are already low, the scope for additional safety gains from speed reduction alone is limited.
This reinforces a key principle for decision-makers: the impact of 20mph depends on where it is applied.
Where higher speeds persist, 20mph has greater potential to reduce risk and deliver measurable safety improvements. Where speeds are already aligned with safer outcomes, the intervention plays a different role - supporting consistency, compliance and long-term behavioural change.
For TfWM, this work has provided a clear strategic foundation. It enables the organisation to move from a broad application of 20mph towards a more targeted, evidence-led approach, focusing on locations where the greatest impact can be achieved.
More broadly, the project demonstrates the value of combining high-quality data with robust evaluation. By understanding baseline conditions, measuring outcomes accurately, and interpreting results in context, authorities can ensure that interventions are both effective and defensible.
