Some of those little farming towns are now rapidly frowning suburbs, teeming with housing tracts, and those fields are now shopping center upon shopping center.
This is utter absurd, it's definitely based in the reality of 1/2 m*v^2. Decreasing the speed decreases the kinetic energy of impacts, decreasing injuries and deaths. That's the reality.
The fact that American roads were designed as the beastly stroads and 6-lanes inner city avenues that are present all around doesn't mean their speed limits should follow that design, the speed limit should follow what utilisation occurs around the road.
You might be right that in some places speed limits can be a political stunt of sorts. Basic physics and the examples of multiple Western countries which achieved a less deadly traffic by implementing stricter speed limits (among other measures) should show you part of the reality you feel is lacking.
Look at Norway, Sweden, the Netherlands and so on. USA's road fatalities per billion km-vehicle are on average double, you can do better and kill less people, in total amount of road fatalities the USA is only behind Brazil.
No road in a city centre should allow for faster than 40 km/h traffic around people. A road (non-motorway) with major separation between automobiles and people in a city shouldn't exceed 60 km/h. Traffic even flows better with larger windows for smart traffic control, lower the speed, lower the average travel time.
USA infatuation with left turns across roads posted at 45 mph or higher lead to deadly T-bone collisions. European roads often have roundabouts which naturally slow traffic and a collision would be a glancing side kiss. USA foot gun left turns kill people by design.
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.