There are roads where speeding is so common you will in fact be dangerously tailgated if you drive the speed limit

In my experience, American roads are built to support much higher speeds than the speed limit is set. Going 10-20mph over the limit doesn't feel particularly unsafe on the wide, straight roads.

On the other side, it felt crazy to drive the speed limit on (for example) the narrow, windy roads in Ireland. The limit there felt like a literal maximum, whereas in the US it feels like the minimum.

Often it occurs because the speed is poorly done. Engineers will come out and review road shape, surrounds, etc. and agree to new limits if it's clear drivers are, on average, using the road at the speed anyway.

But that's a big difference from a culture of people just speeding everywhere.

As noted car enthusiast NotJustBikes has said, driving is not a conscious activity. We drive to what feels right, to a degree. A road built to support 100kph will always have speeder problems until layers of traffic calming are put in place.

UK B road limits are in fact limits (as opposed to North American suggested minimums), often being little more than millennia old cart paths with asphalt applied

Just like not stopping at stap signs..

It’s because it is never enforced anymore.

Because roads are engineered to accommodate a certain speed limit then politicians artificially lower it so they can show how safety conscious they are. Then instead of admitting it was a stupid rule they blame everyone for breaking it just like they do with abstinence only education.

My entire commute

I would agree with the several comments on the engineering and design of the higher speed roads being able to handle increased limits, but I don't think the road or even most of the cars' ability to go that fast are the problem. It's the drivers. Raising the limits means people will go that much faster (the ones who are even bothering to stay close to it), and the risks will be higher and more deadly.

There is also the issue of fuel efficiency vs. speed and how little time is gained for a lot more gas used. Even with modern cars there's a limit of aerodynamics and how much more it takes to push air out of the way faster as you get higher in speed. Of course this pales compared to how many people I see idling in parking lots on their phone, and then complain about gas prices. There's a disconnect somewhere.

i have gotten tailgated by a semi truck in a highway construction zone, one of the scariest things i have experienced while driving. i couldnt even see his lights in my back window he was so close

If you look at the design documents for the roads in your area, usually available in plain old PDF files at your State DOT website... it usually states both the designed speed, and what the posted speed will be.

If it's a modern road, it is almost certainly designed for 5-10mph over what the posted limit will be. Because they know most traffic will speed, and that is accounted for in the road design, regardless of the posted limit sign.

many of the speeders could well be cops who don’t feel like enforcing traffic laws &, well, prefer to speed themselves

It's worse than that. If everyone speeds all the time, cops have a valid reason to pull over and harass anyone they choose, on the pretext of speeding. You'll never guess what demographic gets a disproportionate amount of police attention despite not speeding (or committing other traffic violations) significantly more than average.

You can add rural Japanese drivers to that. "Customary" is about 10kph over with some going 19 just to stay safely under one of the higher penalty tiers. I don't just mean on the expressways, I mean on rural roads marked 30-50.

Honestly, even if you raise limits, people will raise them further

So they set up speed camera(that is not Flock or mass surveillance) and people yell about constitutional right.

And of you're not speeding, it appears in the US they stop you for endangering others on the road

People will always drive at the speed that feels safe, it's not just Americans. Gradually here this is being recognized, and roads narrowed, speed bumps added, etc. in places where we need to go slowly. At my work complex the stated limit is 10 mph and even on my bike I am speeding, and cars are passing me, but it doesn't feel unsafe at all.

This isn't specific to America...

Here in Europe we occasionally get road construction sites on motorways where the speed limit is temporarily reduced to 80, 60 or even 40 because the lanes are narrower than usual and drivers need to navigate around the construction site. I'm never quite sure how fast to drive there: if I actually drive the posted limit, I'll have 0 cars in front of me, but it looks (from my perspective) that I'm holding up 67 cars behind me who would want to drive faster. If I keep up with what the driver in front of me is doing, I'm technically violating the law...