Reducing speed by 20-30% at scale results in a very large loss of man-years of lives in the form of sitting in a car. Reduced earning capacity, lost time with their families, waking up earlier and risks to health associated with reduced sleep, less theoretical throughput of roadways, reduced money for education/food/childcare when they accidently go too fast for a moment and are fined, lack of discretion in issuing tickets for bona fide emergencies, people suddenly slowing down before camera causing accidents, etc.
The obvious win in places like the US is that being pulled over is one of the most dangerous thing that ever happens to the common person, as they are exposed to a psychopath with a gun who is trained that the most important thing is to optimize every interaction to maximize his chance of 'making it home to his family' and if a policeman shoots everything that moves (up to and including, falling acorns) because he 'fears for his life' he will largely get away with it. So it is a nice alternative to that.
The obvious win in places like the US is that being pulled over is one of the most dangerous thing that ever happens to the common person, as they are exposed to a psychopath with a gun who is trained that the most important thing is to optimize every interaction to maximize his chance of 'making it home to his family' and if a policeman shoots everything that moves (up to and including, falling acorns) because he 'fears for his life' he will largely get away with it. So it is a nice alternative to that.