> Or should we realize how silly this all is?
Why? It's an important question to have data on. It's essential to designing neighborhoods
> The solution has always been to physically isolate pedestrian traffic from vehicles at anything above trivial speeds
How in residential roads? By installing overpasses everywhere (they suck for pedestrian traffic and accessibility), installing traffic lights everywhere (they're already way to common in America and really suck for traffic flow) or do the same thing other countries have done, which is to slow drivers down (by design & by law) in residential roads.
The first two options are also pretty expensive, can take a lot of land and many counties already suffer financially from their road infrastructure.
In general, roads with speed limits above 30mph are not common inside urban environments in germany (with quite a few exceptions, especially for large arteries, which are often being put underground or getting their speeds reduced).
Installing sidewalks more commonly in the US would already be great, but this data is from germany, where a street without a sidewalk is an uncommon (although not totally rare) occurrence.
Having >30mph traffic inside urban areas is mostly stupid. It increases land use, costs, discourages anything other than car use and is really dangerous. There should be at most only a handful of roads with higher speeds in cities with >500.000 people.
> US pedestrian traffic is nonexistent
There are somewhat obvious reasons for that, among them hostile road design & high speeds. It's close to suicide to try to walk anywhere in many places in the US.
> So low even the cops don’t really enforce them
I don't know if you've looked at other countries, but enforcing speed limits, especially in such a way to act as an effective deterrent, isn't commonly done (with exceptions like switzerland).
Regarding the highways: That's the reason I've searched for data in the US regarding the change of the speed limit from 55mph to ~65mph, which has a very significant (but not as large as in residential roads) effect. I can't imagine further increasing it not having a similar effect. That also shows that the speed limit does have a somewhat large effect on travel speed (at least in the top end), because otherwise there wouldn't be that many issues.
I'd also be interested in changes for the average travel speed in the US, because it doesn't increase as much as you'd think in the >100km/h in many european places, even without significant amounts of speeding, due to cars scaling really badly at density with higher speeds.