Title: Do 20mph Speed Limits Improve Air Quality? Leading Scientist Raises Concerns

Introduction

The recent implementation of 20mph speed limits on most residential roads in Wales has raised questions about its effectiveness in improving air quality. Despite the Welsh government’s intentions to make roads safer and reduce vehicle emissions, a leading scientist warns that the move may not have the desired impact. This article explores the concerns raised by Professor Frank Kelly and examines the research behind the effects of lower speed limits on fuel efficiency and emissions.

The Impact of 20mph Speed Limits on Air Quality

Professor Frank Kelly, a renowned scientist from Imperial College London, argues that the new 20mph speed limits are unlikely to significantly improve air quality in towns and cities. He points out that many urban roads already experience congestion during rush hours, resulting in speeds of 20mph or less. Therefore, he suggests that little further benefit to air quality can be expected from this new rule.

The Welsh government implemented the 20mph limit on most roads that previously had a 30mph limit. The primary objectives were to enhance road safety, encourage active transportation, and reduce individual vehicle emissions. The government claims that lower speeds result in reduced fuel consumption and emissions due to the energy required for acceleration. However, Professor Kelly highlights that vehicles on urban roads are often already restricted by congestion to speeds of 20mph, rendering this claim less impactful.

Research on Fuel Efficiency and Emissions

To determine the impact of speed limits on fuel efficiency and emissions, the Transport Research Laboratory (TRL) conducted a study in 2022. The research focused on urban traffic conditions, speed limits, and driving styles. It found that the energy required for acceleration was the largest contributor to fuel consumption and emissions in such conditions.

The TRL study revealed that vehicles are generally more efficient at higher speeds. However, this efficiency advantage does not account for the energy needed to reach those speeds. For example, the study found that a cruising speed of 30mph was only more efficient than 20mph if the vehicle could drive for 0.3 miles without stopping, which is unlikely under urban driving conditions.

Optimum Cruising Speed and Future Considerations

According to Nick Molden, founder and CEO of Emissions Analytics, the TRL study considered 25mph to be the optimum cruising speed, assuming it could be maintained. This suggests that a slightly higher speed limit may strike a better balance between fuel efficiency and emissions reduction.

In England, Prime Minister Rishi Sunak is also considering curbing 20mph zones as part of his pro-motorist agenda. This highlights the ongoing debate surrounding the effectiveness of lower speed limits in improving air quality.

Exploring Alternative Approaches: 60mph Limits

In an effort to reduce emissions and improve air quality, National Highways in the UK is conducting trials of 60mph limits on short sections of its network, including the M1, M4, and M6. The focus of these trials is to assess the impact of reducing the speed limit from 70mph to 60mph on NO2 levels, particularly in areas exceeding the legal annual limit.

While the results of these trials have yet to be published due to data complexity, early indications suggest that the lower limit reduces emissions by 17%. However, it is worth noting that different speed limits may have varying effects on NOx emissions, as highlighted by a review of methodology conducted in 2019.

Conclusion

The implementation of 20mph speed limits on most residential roads in Wales aimed to improve road safety and reduce vehicle emissions. However, concerns raised by Professor Frank Kelly and the ongoing research into fuel efficiency and emissions highlight the need for a comprehensive assessment of the impact of lower speed limits on air quality.

As discussions continue on the effectiveness of 20mph speed limits, alternative approaches such as 60mph limits are being explored to address air quality concerns. It is crucial to consider a range of factors, including congestion levels, driving conditions, and vehicle efficiency, when formulating policies aimed at improving air quality and reducing emissions on our roads.