Nikki has part of it right - enforcement presence DOES reduce speed… right around the cop. And often this cop presence creates a ripple effect of the Slam On Brakes And Then Floor It Once They’re Out Of Sight reaction.
If the goal is to reduce speeding in general, road design is the most effective. A massive, stick straight, multi lane, super wide highway with clear-cut everything around it encourages speeding (people feel safer going faster, or simply FEEL like they’re going slower than they are), whereas fewer lanes, narrower and more “landscaped” roads slow down drivers the entire way.
Ripping out our superhighways and making road unnavigable by the US’s giant OTR trucks (and horse trailers!) isn’t a solution, obviously. But law enforcement isn’t a perfect solution either (as anyone who has played the ‘why are we all stopping OH there’s a cop on the other side of the interstate great’ game can attest). Honestly, my opinion is that mass transit has a huge potential to fix a lot of these issues, but the US is notoriously crap at implementation of light rail and even just buses.