In September 2023 Wales lowered the default limit on most roads that had been 30 miles per hour to 20. A provisional government release later said that casualties on roads recorded as 20 or 30 miles per hour were 28 percent lower in the first year than in the year before. This release asks whether that fall can be credited to the policy.

In August, Evidence Press published an evidence package auditing that reading, but without a paper. This release is the companion manuscript. It adds no new data analysis; it sets out the argument in full.

The 28 percent figure does reproduce: 2,402 casualties before and 1,725 after. Later official versions of the same comparison give smaller falls, about 26 and 24 percent. But the figure answers a different question from the causal one. It counts every road recorded at 20 or 30, including roads that never changed. And nobody observes what would have happened on the changed roads without the policy.

The manuscript makes that gap exact. It describes three imaginary worlds that produce the same published totals, yet in which the policy cut casualties on changed roads by half, did nothing, or doubled them. Because all three fit the data, the published totals cannot tell us the size of the effect, or even its direction.

That does not mean the policy failed, or that it worked. It means a causal answer needs different evidence: a historical map of which roads changed, linked to where collisions happened, and a defensible comparison.

This unrefereed candidate was released on the twelfth of October, twenty twenty-six. The manuscript, review record and the original evidence package are linked on the page. This synthetic briefing is not additional evidence.