Fact is that it’s counter intuitive to say reducing speed from 30 to 20 makes no difference. Yes there are reports that say that but they point out it’s because of non compliance with the lower speed limit.
For it to work, save lives and reduce serious accidents it needs to have robust law enforcement. Personally I would welcome that as it should result in less idiots on the road. The more I see of the arguments against the more I think they are just political!