If I recall correctly, the speed for everyone else on that road is 70 mph. How stupid and dangerous is that?
Not sure if that was a rhetorical question, but I'll bite since I saw a bunch of those signs while I was out there.
For me, the dangerous - more accurately disruptive - aspect of those signs is their potential for being misread from a distance. Before I'd got used to them, I'd see the "55" from way back and think "oh, better start slowing down". As you get closer, you then notice it isn't referring to you anyway.
Danger depends entirely on how busy the road is. On a huge number of the highways I drove on that sign could read 25 and not be dangerous, since the left lane is perpetually empty and you could see any slow-moving traffic from miles away.
The speed difference is only really dangerous if you don't have the room to pass. Though really, that's up for the driver to decide. If something is going slowly it's your responsibility to ensure you either pass it safely, or slow down in sufficient time and pass when there's a gap. It's only dangerous if you barrel along at the same speed and then realize you've not left enough time to make a maneuver.
On a busier road? Potentially more dangerous. Again, I only have limited experience to go on, but I didn't encounter any speed differential-related issues while out there. Even when big 18-wheelers were doing 30-odd up steep hills while everything else passed at 80. Because again, the trucks keep right, drivers are aware of this, and move left earlier.
It is of course also the responsibility of the slower-moving driver to ensure they don't disrupt traffic flow. I don't advocate the annoying arses that sit on motorways at 50 mph therefore forcing large, marginally faster vehicles like trucks (limited by EU law to 56) to pass, and in those scenarios I'd advocate a minimum speed limit (which several states I passed through had - bravo). But the responsible driver should be able to go slowly without disrupting traffic. Again, you guys have pull-over lanes on single-lane roads so slower traffic can move if they start to form a queue. We don't have that in the UK, but we really should.
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On the subject of the thread in general: I've probably mentioned this before, but I've no real problem with speed limits, at least in the UK, and not particularly bothered about them raising it to 80.
Four reasons:
1) Everyone does 80 anyway. An 80 limit wouldn't have any effective differences.
2) You're lucky if you get near that speed most of the time anyway. You may as well set the limit to 180 for all the difference it'd make.
3) While cars are entirely capable of doing 80 safely these days, I'm still unsure of drivers' ability to do likewise.
4) Fuel. If people want to waste an extra 10-20% of fuel getting to their destination a handful of percent quicker, that's their prerogative. But even without my eco-leanings I don't make much money, and I can think of better things to use fuel on than zipping down a motorway at a largely imperceptible 10 mph quicker.
As a result I tend to just cruise along at 70 regardless of the car I'm driving - whether it's a Smart Fortwo or a big luxury saloon or SUV.
