Good question as it's somewhat counter intuitive. I think that all roads are not equally looked at by the police. So you would have lets say 10 major roads tha

Good question as it's somewhat counter intuitive. I think that all roads are not equally looked at by the police. So you would have lets say 10 major roads that get a patrol twice a day, and a deployable manned speed camera between them. If we add 20 minor roads to the jobs, then the police would need extra manpower to drive down these roads and another couple of speed cameras and crew to cover this increased demand. The assumption I am making is that many of the 20 mph roads were not previously under active patrols / enforcements when they were 30mph. Perhaps it's a kind of active enforcement versus a passive kind.