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 mean, going the speed of traffic is safer than not. So yeah, it's okay when everyone else is doing it.

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.

It bothers me when people don't go the speed limit. Faster or slower, the speed limit is to set a consistent speed. Consistent means predictable which results in safer conditions. This would result in less traffic and less fuel consumption.

However, following the flow of traffic is safer than trying to go the speed limit. If you're the one person going 20 units per hour slower than everyone, you are the hazard.

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

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.

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.

I don't. I go the speed limit to the best of my ability. I only speed by accident and when safety suggests I need to follow traffic.