Does Slow Driving Cause More Pollution?

Driving at lower speeds is generally considered safer, but does adhering to a 20mph speed limit cause more pollution? This question has sparked debate, with some arguing that slower speeds increase emissions and air pollution, while others claim that the smoother driving patterns at 20mph reduce particulate emissions from tyre and brake wear. There are also concerns about the potential for increased traffic tailbacks and the impact on journey times. While research suggests that lower speeds can reduce fatal accidents, the environmental impact of 20mph speed limits remains a subject of discussion.

The impact of 20mph speed limits on exhaust emissions

There are an estimated 450 20mph speed limit zones across the UK, many of which are located near schools. Since their introduction in 1991, they have become an increasingly popular traffic-calming measure. However, there is a concern that these lower speed limits could increase pollution (both carbon dioxide and particulates) and cause traffic tailbacks.

Research by Imperial College London suggests that 20mph speed limits do not negatively impact exhaust emissions. The research found that vehicles moved more smoothly, with fewer accelerations and decelerations, than in 30mph zones, reducing particulate emissions from tyre and brake wear. The study also indicated that by eliminating sharp acceleration manoeuvres, significant energy and emission savings could be achieved.

A case study on traffic circles (not roundabouts) showed that they produced the least increase in vehicle fuel consumption and emissions. The results demonstrated that traffic circles allow smoother driving patterns with milder acceleration behaviour when compared to speed humps and stop signs.

The implementation of 20mph speed limits is primarily focused on reducing fatal accidents, particularly those involving children, rather than reducing pollution. However, any reduction in tailpipe emissions is a beneficial outcome of these measures.

To enforce these speed limits and improve road safety, measures such as raised pedestrian crossings are being introduced, allowing motor vehicles to travel over them safely without slowing down if driving at 20mph. Additionally, police enforcement powers are being designated to community support officers to stop speeding vehicles and take action against drivers.

The effectiveness of traffic-calming measures

Traffic calming measures are an effective way to improve safety for motorists, car drivers, pedestrians, and cyclists. They are also used to combat speeding and other unsafe behaviours of drivers. These measures aim to encourage safer and more responsible driving and potentially reduce traffic flow.

Urban planners and traffic engineers use various strategies for traffic calming, including speed humps, chicanes, curb extensions, modal filters, and living street and shared space schemes. Speed humps, for instance, are raised sections of pavement with a curved or flat top that extends across the road, alerting drivers to slow down, especially before pedestrian crossings. Similarly, raised pedestrian crossings slow down vehicles and make pedestrians more visible.

Traffic calming measures are particularly common in Australia and Europe, especially Northern Europe, but less so in North America. The concept originated in the Dutch "Woonerf" schemes of the 1970s and has since been refined, especially in Germany and the Netherlands. The "Woonerf" schemes introduced the idea of shared space between vehicles and pedestrians, giving priority to the residential function of the street and reducing the dominance of motor vehicles.

The effectiveness of traffic calming measures in reducing vehicle speeds and improving safety is well-documented. Studies have shown that they can reduce crashes, injuries, and fatalities by up to 40%. Additionally, they can significantly reduce the severity of accidents and improve community cohesion. For example, the town of Hilden in Germany achieved a rate of 24% of trips being made on two wheels through the use of 20 mph zones and other traffic calming measures.

Beyond safety, traffic calming measures can also provide environmental benefits by reducing noise and air pollution from traffic. Slower speeds can decrease noise levels by up to five decibels. While there is limited research on the impact of traffic calming on air quality, studies suggest that lower speeds, such as 20 mph or 30 km/h, can reduce tailpipe emissions and improve air quality.

However, it is important to consider the style of driving, as "calm" driving in third gear can result in lower emissions than "aggressive" driving in second gear. Additionally, certain traffic calming measures, such as rumble strips, can cause noise pollution, especially on roads with a high proportion of HGV traffic.

The safety of 20mph zones

While 20mph speed limits might help reduce road-related deaths, they have also been criticized for potentially increasing pollution from carbon dioxide and particulate matter. However, the primary objective of implementing such limits is to enhance road safety, particularly in built-up and residential areas, and this goal has been largely successful.

Cheshire West and Chester Council's four-year programme to increase 20mph speed limits on residential roads resulted in a notable improvement in road safety. An analysis of the first three years revealed a 43% decrease in vehicle collisions, a 14% drop in fatal or severe injury collisions, and a 47% reduction in minor injury collisions. This aligns with the National Statistics, which showed that a significant number of road collisions, including fatalities and serious injuries, occurred on residential roads in built-up areas.

The effectiveness of 20mph zones is particularly evident in reducing accidents involving children. A review of 72 schemes by the Royal Society for the Prevention of Accidents (RoSPA) found a 70% decrease in child pedestrian accidents and a 48% reduction in child cyclist accidents. This reinforces the argument that the primary benefit of 20mph speed limits is improved safety, especially for vulnerable road users like children.

The impact of 20mph zones extends beyond accident reduction. These zones also encourage active travel by making walking and cycling more attractive options for shorter journeys. The slower speeds create a perception of safer roads, making people more inclined to leave their cars at home and opt for healthier and more environmentally friendly modes of transportation. This shift can contribute to reducing traffic congestion and improving overall community well-being.

To minimize potential increases in emissions, the implementation of 20mph zones should be carefully planned, utilizing appropriate traffic-calming measures. Traffic circles, for instance, have been shown to produce the least increase in vehicle fuel consumption and emissions compared to speed humps and stop signs. They allow for smoother driving patterns with milder acceleration behaviour. Therefore, the choice of traffic-calming measures plays a crucial role in ensuring that the safety benefits of 20mph zones are not offset by unintended environmental consequences.

The impact on driver concentration

Driving at lower speeds generally requires more concentration, and 20mph speed limits are no exception. Some drivers find these limits frustrating, particularly in hilly areas, and they can cause congestion. This frustration can lead to drivers paying less attention and being more easily distracted, which could potentially be more dangerous.

However, there are also arguments that 20mph speed limits can improve driver concentration. For example, one source suggests that driving at 1800 to 2000rpm in third gear or second gear in an automatic car will make it easier to maintain 20mph and will put the engine in a more economical and environmentally acceptable range. The fact that drivers are concentrating better at this speed will make it less likely for them to have an incident.

Furthermore, 20mph speed limits can reduce the number of times a driver has to accelerate and decelerate, which can lead to a smoother driving style with milder acceleration behaviour. This can reduce particulate emissions from tyre and brake wear, which is a significant cause of air pollution from zero-emission vehicles.

Overall, while 20mph speed limits may require more concentration from drivers, the reduced speed can also lead to smoother driving and potentially improved concentration due to a more economical engine range.

The role of enforcement in ensuring compliance with speed limits

While a well-chosen speed restriction should ideally be self-enforcing, enforcement is often required to ensure compliance with speed limits. The engineering community and transportation professionals play a crucial role in speed enforcement by providing data and insights to enforcement officials, as well as maintaining and deploying automated speed enforcement equipment and technologies.

Automated speed enforcement, or ASE, has a high rate of violation detection and increases the safety of first responders. While it may be limited in range and ability to educate offenders, it is particularly useful when engineering measures cannot be installed to calm traffic. Examples of ASE include speed cameras, which have been shown to be effective at reducing accidents and injuries. A review of evidence by Richard Allsop found that speed cameras led to an 11% to 44% reduction in fatal and serious injury crashes. The UK Department for Transport estimated that cameras led to a 22% reduction in personal injury collisions and a 42% reduction in people being killed or seriously injured at camera sites.

In addition to ASE, traditional enforcement methods involve patrol officers monitoring locations where crashes or speeding violations have occurred. To be effective, these measures must be consistent and sustained, and the data collected must be provided back to transportation professionals to assist in the evaluation of road designs and speed limits. Transportation professionals should remain involved in traditional enforcement to monitor the proper posting of signs and other elements of maintaining a speed management program.

To further improve compliance with speed limits, it is important to educate the public about why and how speed limits are set and to promote responsible driver behaviour through advertising. Special enforcement activities, such as saturation patrols, sobriety checkpoints, and impaired driving crackdowns, can also be utilised to improve compliance. Additionally, collaboration with highway safety partners, traffic safety stakeholders, and health and medical communities can help to prioritise speed management as a safety, economic, and public health issue.

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