Some more details on the data in this bbc article https://www.bbc.co.uk/news/articles/cjee04vlqglo
It isn’t as clear cut; the numbers injured in 20/30 limits has decreased a bit but not by that much (only very slightly lower than 2017). Over all roads, in the same period injuries actually increased by a similar factor in the same period (presumably just statistical variation).
Essentially, the decrease in 20/30 limits isn’t so big that the change obviously caused it. More fundamentally, 30 limits were already quite safe, so making them safer doesn’t have a big impact overall on road safety.
Finally to the point that there is no cost, there is. Time is money. If going more slowly cost someone six minutes a day and their time was valued at the minimum wage, then that would have cost them around £400 per year. If that was the average across the welsh population, it would have cost the population £1.2 billion each year. Made up numbers, but illustrate that this is actually quite an expensive policy for those routinely affected.
I’m certainly not set against the policy, but it is worth being aware that it isn’t such a clear win with no downsides as some want to portray.
