These reductions in speed limits are primarily political, while corruptly funneling money to overpriced contractors and police running deceptive speed traps.

I'm talking generally about speed limit reductions here. Not just 30 to 20, but 60 to 50, 40 to 50 or 40 to 30. Sometimes it's done with valid safety intentions, backed up by data. More often than not it's done as part of some bullshit political project.

From another of my comments:

The limits are assigned so politicians can pat themselves on the back and maybe score some votes. Sometimes also so some new speed trap locations can be created, catching people out in areas where the road feels like it has a higher speed limit (although this is perhaps less true for 20 zones).

I'm not aware of police extensively enforcing 20 zones, but I am aware of police enforcing speed limits in areas where it has been reduced for arbitrary reasons. Quite often these involve civil works that are ludicrously overpriced and under-delivered, which reeks of corruption.