9 September 2025. Speed | Energy

Why lower speed limits are good public policy // Mapping low carbon electricity [J2T #649]

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1: Why lower speed limits are good public policy

It is two years since Wales introduced mandatory 20mph (30km/h) speed limits in built-up areas, with a small number of exceptions, to a wall of hostility on social media and some mainstream media. Now Will Hayward, who runs a Substack newsletter reporting on Welsh politics, has gone through the data to assess the impact of the change. Some of this is behind a paywall, since Hayward is a working journalist and needs to eat, but there’s enough in front of the paywall to get the picture.

TL:DR—it turns out to have been an effective, and cost-effective, piece of public policy. This is despite the level of rage that accompanied the introduction of the policy. When Hayward wrote a piece explaining the science behind the change in the law, his then office was plastered with ‘Wanted’ posters, one of which appears in his article.

(Source: The Will Hayward Newsletter)

Here’s what’s happened.¹

One, in the 20mph and 30mph (50km/h) areas, traffic has slowed down. And this has reduced casualties, on average by more than a quarter. (Hayward has had to combine 20mph and 30mph data because Wales government data doesn’t yet strip out the different speed limits. All the same, in comparison, casualties on faster roads have increased. This isn’t about drivers suddenly becoming better at driving.) This change, incidentally, is consistent with findings elsewhere in Britain.

Wales is a small country, and the data shows that five fewer people were killed, seventy five fewer people were seriously injured, and 550 fewer people were slightly injured. When Hayward multiplies that up against the cost to society of traffic deaths and injuries, it comes to more than £43 million, or a bit more than the cost of introducing the policy (somewhere between £32m and £39m, depending which figures you use.) Not many public policies pay for themselves in less than a year.

There’s also been a benefit in terms of lower insurance payments charged to Welsh drivers—a fall of £45 per year per driver—because of the lower collision levels. If that’s true across Wales, allowing for the fact that insurance companies are inconsistent, it would be worth around £90m a year.

Ah yes, but what about the effect on journey times? Well, they have gone up. The Wales Government has been monitoring 60 routes, and found that journey times increased on 57 of them. On 44 of these routes this was by less than two minutes. The largest increase was less than four minutes, on the 61kph route from Aberystwth to Cardigan in the morning peak. Curiously, though, the variation in journey times reduced: the 20mph limits may have smoothed out traffic speeds.

(Source: Rhondda Cynon Taf County Borough Council)

One of the criticisms when the policy was introduced was about how it was introduced. The legislation in 2023 mandated a reduction from 30mph to 20mph unless the local authority exempted a road. After a public consultation on the speed limits, local authorities had the opportunity to revert roads from 20mph to 30mph if they wished.

Those councils that made the most exemptions back in 2023 ended up needing to make the fewest changes in 2025 after the public consultation on the issue. Blaenau Gwent Council exempted the most roads in Wales in 2023 at 10.5%. Since then they have only ended up changing back one road.

Of course, these overall outcomes shouldn’t be a surprise. A research paper last year in Sustainability conducted a literature review on studies of the effect of introducing 30kph (20mph) in multiple countries across Europe. The researchers looked at safety, environmental impact, health effects, traffic speeds and congestion, and livability. Usually in these studies you get caveats about how the results vary according to local factors, but not here. As the abstract says:

Results from 40 different cities across Europe, including Brussels, Paris, and Zurich, indicated that reductions in speed limits improved road safety by decreasing the likelihood of crash risk and the severity of crashes that do occur. On average, the implementation of 30 km/h speed limits in European cities demonstrated a 23%, 37%, and 38% reduction in road crashes, fatalities, and injuries, respectively.

Lower speed limits also yielded environmental benefits, with emissions decreasing on average by 18%, noise pollution levels by 2.5 dB, and fuel consumption by 7%… Encouraging citizens to embrace walking, cycling and utilizing public transit services can further contribute to a safer and environmentally sustainable urban environment.

It turns out that if you can persuade powered and wheel-based two tonne weights to slow down a bit in the vicinity of people, they tend to do the people less damage. (And well: who knew?) But when you read the versions of this policy from its opponents, you get all sorts of claims being made:

The discussion and introduction of 30 km/h speed limits often encounter strong opposition and entrenched resistance, while the voices of supporters tend to be relatively muted and ineffective. It is evident that many people believe that 30 km/h speed limit is pretty low for a standard. In particular, opponents of the 30 km/h speed limit in cities argued that it would prolong travel times, diminishing overall commuting and transport efficiency.

Opponents claim that slower speeds will also worsen fuel consumption figures, and there’s nothing like an apparent policy paradox to halt a policy maker in their tracks. But these things are inventions:

[C]laims that 30 km/h speed limit leads to increased traffic congestion and higher congestion costs is a myth, unsupported by evidence. In urban centers, traffic flows most smoothly at speeds of 20–30 km/h. Despite shorter following distances, this speed range facilitates the entry of traffic from side streets, ensuring continuous flow. Examination of traffic patterns in Switzerland revealed that 30 km/h allowed the road system to accommodate more cars efficiently, resulting in faster overall travel times.

And smoother traffic flows also improve fuel efficiency. The paper is open access, so you can read it for yourself.

But all of this is also a reminder of something else. Car culture is essentially individualistic and asocial. Once in their vehicles, drivers get detached from the whole. They become more entitled. This sense of entitlement has been reinforced because car use has been privileged by public policy for 70 years in Europe, and more like one hundred in the United States. And that combination of privilege and entitlement is being articulated every time you hear the phrase “the war on the motorist.” And just by way of evidence of how that plays out, here’s a leaflet that was stuffed through my door complaining about my local council on the day I was writing this piece.

(Photo: Andrew Curry. CC BY-NC-SA 4.0)

Because, although people often think of cars as being broadly and evenly distributed across the population, there’s actually a strong social gradient. When car drivers complain about 20 mph (30 km/h) limits, despite the general safety, health and other benefits they convey, what’s being expressed is resentment about some modest and belated reduction of privilege.²

2: Mapping low carbon electricity

I saw a striking map of the share of world electricity that is generated in different countries by renewable energy. It is maintained by Electricity Maps, and is interactive, which allows you to look at how this has changed over time, and also at different time periods within this. It has a couple of striking features. (H/t to The Browser, who shared it one of their excellent daily emails.)

Here’s a non-interactive screenshot of the map, showing data for the last 12 months, going back from August 2025.

Source: Electricity Maps.

Broadly, the colour coding here is that green is good and brown is bad (unless you are ideologically opposed to renewables, in which case it is the other way around). Darker green is better than lighter green; darker brown (or black) is worse than lighter brown. The colour coding is also influenced by the specific energy mix (measured by the level of emissions per KilowattHour [KwH]: more on this later).

The main point that jumps off the screen here is how much politics and policy matters, maybe along with legacy investment and infrastructure. The difference between Canada and the United States is striking; as is the difference between New Zealand and Australia.

Even within Canada, the difference between the ‘oil provinces’ of Alberta and Saskatchewan, in the middle, and British Columbia and Manitoba on either side of them, is worth noting. Or, staying in that part of the world, the contrast between Canada’s Yukon (66% low carbon, all from renewables) and its American neighbour Alaska (29% low carbon, all from renewables.)

The reason I’m able to roll off this data is that in the interactive version every marked territory generates a little data map when you click on it. This allows me to tell you that the reason that Ethiopia has 100% renewable-produced electricity is almost all down to hydro, as is also the case for Democratic Republic of Congo. Kenya, at 91% (and with higher emissions per KwH) has a low-carbon mix that includes geothermal and hydro, as well as wind, solar and biomass.

These charts also allow you to look at the energy and emissions mix of different countries. I’ve pulled out below the comparison between Germany and Britain on the left, in the image below: their energy mix isn’t that different, except that Germany still burns coal and Britain has some nuclear. Britain burns more gas.

And by toggling to the ‘emissions’ button, we can see the effect that has on carbon intensity: Germany’s electricity generates significantly more emissions per KwH than Great Britain. Also worth noting that biomass isn’t that clean, even if it is renewable. And maybe nuclear would have higher emissions if the lifetime costs of construction were included: there’s a lot of concrete involved in a nuclear power plant.

Source: Electricity Maps

There’s a whole lot more data in these country pop-ups, including monthly emissions, monthly prices, and monthly load.

So it’s worth also saying something about data sources. Basically, Electricity Maps is gathering live electricity data through public APIs, and applying an algorithm to it to calculate energy mix. (There are notes on methodology here.) The business model is that there’s enough information here for people who need this kind of data to pay for a subscription so they can utilise it more fully. The mission of the business is

to decarbonise electricity by building the world’s most comprehensive, granular and useful electricity data platform.

It’s also worth noting that in the free to air version, you can see the electricity patterns in countries over the last 24 hours, including the amount of electricity generated, its carbon intensity, its source, and the ratio of renewables to fossil fuels. Staying with the UK, for example, at time of writing—on a fairly wet but windy day—there’s not a single hour when fossil fuels provided more than half of UK energy, and at times it’s a lot lower than this.

Of course, if you listen to the British hard-right as they cheerlead for fossil fuels, they would have you believe that this is why British electricity prices are higher than they are in Europe. The myth they choose to circulate is that because we have paid a surcharge to establish the renewables industry, electricity would be cheaper without renewables.

Chris Goodall has a recent piece on his Carbon Commentary blog that uses UK electricity market data to demonstrate how plain wrong, and wrong-headed, this argument is. His methodology is disarmingly straightforward.

It involves plotting the ‘next day’ electricity price—which represents about 20% of all wholesale market transactions—against the share of renewables (solar and wind only, excluding other low carbon sources) and seeing whether there is a correlation or not.³ (I have copied Goodall’s footnote on ‘next day’ prices into my own footnote here. The sample size is 220 days, from January to August 2025.

And here’s the top line chart. The more renewable electricity in the mix, the lower the next day electricity price.

(Source: Chris Goodall, Carbon Commentary blog.)

This is just the simple regression analysis, but there’s a lot more detail in a stack of other charts in the blog. The striking thing about the numbers is the size of the impact on prices as the share of renewables moves from 20% of energy mix to 50% of energy mix. It varies from month to month, but it is always large and positive:

In March and April, the fall in wholesale electricity price as a result of a 50% renewables share compared to a 20% share was £29 and £19 per MWh respectively. These were the low months; in May and July the figures were £58 and £64 respectively. January showed the largest effect of of a fall of £87 per MWh resulting from a renewables increase from 20% to 50% of all power production.

In proportional terms, by the way, these are all significant reductions. While I’m not sure if Reform UK employs policy analysts, although it seems unlikely, and if it does, it also seems unlikely that they will be reading this newsletter. But on the small chance that <Reform policy wonks>+<Readers of Just Two Things> is not a null set, let me just quote Goodall’s conclusion here:

More renewables on the GB grid unambiguously reduces the wholesale price, typically by a large percentage. Absent any other effect as the UK continues to add wind and solar, power prices will come down sharply. This is, of course, what we have seen already in many countries around the world.

But the reason that right wing parties in Britain who are in the pocket of interests promoting fossil fuels can spout nonsense about energy prices is because the way UK consumer energy prices are calculated is tied to the price of gas, which is now typically the most expensive element in the energy mix. (Gas is needed, at the moment, because wind and solar supplies are variable and energy systems need a reliable baseload that can be turned on and off to stabilise the overall electricity grid at any given moment.)

It’s hard to explain this, and I sometimes think it has been made deliberately complex to make it harder to understand. But imagine that you are at the greengrocer, and you buy two cheap vegetables (potatoes and onions, say) and an expensive one (asparagus, say). You’re a bit surprised when the greengrocer charges you the price of the asparagus for all of your vegetables. But that, approximately, is what happens in Britain’s consumer electricity market. (There’s a good less vegetable-based explanation here.) The so-called ‘price cap’ is a feeble attempt to mitigate the worst effects of this.

I don’t think that this is a deliberate scheme to extract money from households and give it to energy companies. That’s just its effect. It’s a legacy of a couple of things: one, the rapid fall in the costs of renewable energy, which outpaced industry forecasts, and two, the increase in gas prices since Russia’s invasion of Ukraine.

What’s less forgivable is the lack of any short-term or medium-term policy response to this. The short-term response would have been a windfall tax on energy company profits to bring energy costs for households back down a bit, especially for poorer households on paid meter tariffs.

One medium-term response is to ringfence the gas market so that we’re playing high prices only for the amount of gas needed at any given time by the grid, while also expanding renewable investment to increase the share of lower-cost production, and changing the wholesale auction system so it reflects the average weighted price, not the expensive cost of the last unit of power needed to supply demand.⁴ The British government has published some proposals that might do some of these things. But the pace is grindingly slow, given how many UK households are under economic pressure.

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j2t#649

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1. He’s looking at year on year data, comparing the 12 months before implementation with the 12 months following it. Because that’s the data we currently have.

2. Readers can probably think of other areas of life where a small reduction in deeply embedded privilege produces a similarly disproportionate reaction.

3. “The ‘day ahead’ price of electricity is the auction price agreed between suppliers and purchasers of electricity for power to be delivered the following day. I couldn’t find an accurate figure but the estimates I came across suggested this market represents about 20% of all wholesale electricity transactions.”

4. If I buy lots of cheap potatoes, and a tiny bit of expensive asparagus, I’ll pay less for my vegetables, in other words.