Even with the caveats about limited data and untangling causation and correlation, the statistics are striking: the first year of a scheme in Wales where the speed limit on urban roads was lowered to 20mph resulted in about 100 fewer people killed or seriously injured.

Introduced in September 2023 as a major road safety project of the Labour-run Welsh government, it made 20mph the default limit for any built-up area, defined as roads where lamp-posts were no more than about 180 metres apart. This took in about a third of all roads, with the Conservative opposition saying the definition was far too broad.

At the time, other controversies were raging over roads policies, including low-traffic neighbourhoods and the expansion of London’s ultra-low emission zone – a culture war embraced by some Tory politicians.

Countering this, increasingly, is hard evidence. While Welsh government statisticians warn that at least three years’ data will be needed for a meaningful conclusion, the road casualty figures – showing an estimated 10 fewer deaths and a nearly one-third drop in overall casualties – follow research from insurers indicating that 20mph zones appear to be bringing down the number of claims.

Peter Fox, who covers transport issues for the Conservatives in the Senedd Cymru, insists his worries about 20mph zones are not about the general idea, just how it was implemented, with the lower limit taken as the standard for built-up streets.

“None of us are against 20mph around schools, busy residential areas, or anywhere where there’s quite a lot of pedestrian footfall,” he said. “But we didn’t need to change the default position.”

Lee Waters, who was transport minister in the Labour-run Welsh government when the policy was introduced, has called it “the most successful road safety intervention in modern times”. But even he concedes that with retrospect it could have been done differently.

“You have to apply the sniff test,” he said. “If someone is being asked to drive at 20mph and it doesn’t make sense as to why, you have a problem. There wasn’t sufficient capacity or willingness locally to use the flexibility within the guidance, to apply it on a place-specific basis.”

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This was subsequently addressed in Welsh government guidance to councils to help them identify areas where 20mph schemes were inappropriate and the speed limit widely flouted. Officials say this assessment process is coming to an end, and changes have been made.

Now 20mph zones are being proposed and implemented beyond Wales, and not just from Labour politicians. Cornwall council, in south-west England, which is run by the Conservatives, began a programme of reducing speed limits to 20mph in urban areas in 2022, and it is still being expanded.

Connor Donnithorne, the councillor who leads on transport matters in Cornwall, said their policy had been different to Wales, including a more targeted approach and a focus on encouraging people to reduce their speed using signs rather than enforcing it with cameras.

“The idea is to do all this in a pragmatic way,” he said. “The issue is that if you force people to go 20mph on a road where it doesn’t make sense, it’s very difficult to change driver behaviour and patterns. That’s why we’ve worked very hard with the highways team to make sure that it is done in the right places, because that brings the community along with us.”

For all the progress, a key question remains: as more and more evidence arrives, why is all this still even a debate? As Waters puts it: “There is still this idea that 70 or so road deaths a year in Wales is acceptable. But we would never accept 70 deaths a year on the ferry to Ireland, or on buses.”

Some argue that it is because of what is called motornormativity, an idea devised by Ian Walker, a professor of environmental psychology at Swansea University. This sets out that car ownership and use is so ubiquitous that people are all too often unable to think beyond it.

One result, he argues, is that people implicitly accept deaths and injuries on the roads as this is seen as essentially random, and so in a curious way almost fair, when in fact the odds are notably worse for pedestrians and cyclists.

“This isn’t generally done in bad faith,” Walker said. “It’s just a very car-centric mindset of thinking the risk is distributed equally, when for some it’s much higher and systemic.”

Another aspect of Walker’s research has an even more pertinent lesson for politicians: policies such as 20mph zones are often more popular than people think.

A study he led, based in the UK, US and the Netherlands, found that two-thirds of people believed they personally supported non-car travel more than the public at large. This is an example of a social-psychological phenomenon called pluralistic ignorance, where people automatically assume the consensus must differ from their own views.

In this case, it seemingly happens “because people rarely see changes to the car-first status quo being seriously discussed or supported, let alone implemented”, Walker argues. As such, he says, consultations on 20mph zones tend to be dominated by dissent, with supporters silent as they assume they are in the minority.

There is, perhaps, a lesson here for politicians, although not an easy one: sometimes it can be best to ignore the noisiest voices, and take the evidence-based course.