Yeah, "speed limits" are primarily a political game, not anything based in reality. Roads designed for 55 get labeled 45 (in 1990) then 40 (2000) then 35 (2010) and then "road dieted" to 25 (2020). Everyone's "speeding" on paper, but not in reality -- they're going the speed they're were always supposed to go. But politically, people just decided slower=nicer for their particular stretch of street (they already live there, so 'walkability' is just fine for them, strip out all the lanes, throw garbage into the street, whatever) without really caring about how badly it would impact public transportation for everyone in general. It's a textbook "tragedy of the commons" problem.
If you enforce this by vehicle, everyone's going to flip out (because the fake-posted-speed-limit is way below what any sensible person is actually safely driving) and you'll see a significant number of people instantly jailbreak their cars to fix it.