Will Welsh 20 mph speed limits actually lower emissions?

Image by 12138562O from Pixabay

One of the key arguments for introducing 20 mph speed restrictions on many Welsh roads has been that lower speeds will improve air quality and lower emissions.

Yet with claims of increased travel time and choked roads following the restrictions, some are finding that difficult to swallow.

In our third and final piece about the restrictions, we’ll take a look at the environmental argument for a 20 mph limit.

Hang on, don’t lower speeds = more emissions?

On the face of it, making traffic travel slower would seem likely to increase emissions, for a couple of reasons:

  • internal combustion engines are most efficient at speeds well above 20 mph. According to how stuff works: ‘…for most cars, the “sweet spot” on the speedometer is in the range of 40-60 mph.’ And though it notes that, ‘Cars with a higher road load will reach the sweet spot at a lower speed’, it’s safe to say that the sweet spot probably isn’t 20 mph. Given that lower efficiency means higher emissions, lowering the speed would seem counterproductive for the environment.
  • if speed restrictions lead to tailbacks, idling engines will quickly increase the amount of pollution.

This seems to be borne out by a 2013 study commissioned by the Edinburgh Centre for Carbon ­Innovation (ECCI), which suggested that lower speeds in towns would lead to increased emissions of particulates, NOx gases and carbon dioxide. However, the ECCI study mentioned above also points out that:

Traffic schemes that can smooth the flow will normally reduce emissions, save fuel and may also have other benefits.

And it turns out that smoothness also has an major impact on air quality.

Smoothly does it

One key factor that increases air pollution levels is the amount of stopping, starting, acceleration and deceleration that a vehicle does. Driving smoothly reduces all of these.

When Imperial College London looked at how speed restrictions impacted emissions in the city, they concluded that vehicles moved more smoothly in 20 mph zones. The details were complex:

  • Particulate emissions from petrol and diesel vehicles dropped by just over 8%.
  • CO2 emissions went up for petrol vehicles, but down for diesels.
  • NOx emissions went up for diesels, but down for petrol cars.

Where does that leave us? Overall, the report concluded that there was no negative environmental impact of lowering the speed limit. Looking specifically at the NOx, there was a net improvement on that particular pollutant, because diesel cars produce far more.

So, a lot of the environmental impact of 20 mph zones turns on whether they help smooth traffic flow or make it worse. If they create queues of idling traffic and herky-jerky driving, the zones will probably worsen pollution. If they do the opposite, then they’ll bring environmental benefits. In London, the evidence points to a net improvement, but that doesn’t mean the same results apply everywhere.

More bikes, fewer cars?

That leaves one more environmental argument for 20 mph zones: the claim that in the long term, they will get us out of cars and onto bike saddles. It’s argued that the 20 mph zones will make it safer and more pleasant to cycle, and less attractive to go by car, persuading more of us to leave the latter at home. There’s some evidence that this can happen. Research in Bristol showed increases of 12% in walking and cycling following the introduction of 20 mph zones. Outside of that, we’re not finding much that looks definitive in UK cities.

Science is difficult

We understand the frustration of Matthew Evans, a Newport Council committee member. In October 2023, as the local authority’s chief scientific officer presented findings on their data modelling, he commented:

“You’re saying the methodology [is suggesting 20mph] is going to make pollution worse, but you’re saying the reality is it won’t – how does that work?”

The scientist went on to explain that although the model suggested emissions could actually increase with a speed reduction, the limits of the data meant he wasn’t so sure that would happen in practice:

He gave the example of George Street, where modelling was done for road speeds at 30mph, but “we probably know, anecdotally, just walking around George Street, it rarely gets above 15mph between the two sets of traffic lights”.

Tearing your hair out yet? Maybe what this shows us is that real science is slow, complicated, cautious, expensive… and it can’t always give us the clear-cut answers we want.

A bit of honesty about bias

The 20 mph restrictions are, to put it mildly, controversial. A few weeks back, we wrote about the massive protests against the restrictions – but also that there’s a lot of public support.

These strong feelings mean that when it comes to the possible environmental benefits of these restrictions, most of us know the answer we want to hear:

  • If you think the 20 mph restrictions are a good idea, and you like them, then you’re motivated to think that they’re brilliant for the environment.
  • If you think 20 mph speed limits are stupid, and you hate them, you’re motivated to think that they don’t even help the environment.

Writing this, we’ve done our best to put our own feelings to one side. And overall, the best we can say about the environmental benefits for 20 mph restrictions is that they remain unproven.  We’re left with the impression that:

  1. 20 mph restrictions are so new that there’s not that much definitive work on their impact, and
  2. scientists are more ambivalent about their environmental benefits than you might think.

Of course, that doesn’t mean those benefits won’t appear in the long-term, but no one is really sure at the moment.

One thing is for certain, as 20 mph zones spread like wildfire across Europe (and they’re so popular that they probably won’t revert back), we’ll soon have a lot more to go on.

Whether you prefer to get here at 20 mph or 30 mph, either way, we’d love to see you. We are an independent garage specialising in all the VW group marques, including Audi, Volkswagen, Skoda and SEAT. WVS provides servicesrepairs and MOTs, delivering a main dealer level of care at affordable prices. To book your vehicle in, or for any enquiries, get in touch.