You don't need to drive at any speed in any area. You drive at a speed that is safe for the road conditions. 30mph as a maximum speed is a perfectly safe speed to drive at in a residential area.Why do you need to drive 30mph in a residential area?
If you widen the debate to include air quality, which is what Drakeford has done, reducing the speed limit to 20mph makes air quality poorer as the vehicle is in 2nd gear and using more fuel, the likelihood of congestion increases further reducing air quality and leads to driver frustration which, whilst wrong, is a human trait and it does cause accidents.
All in all, a very poorly thought out policy.