Here’s how my dad (who’s a retired County Engineer) explained the yellow speed signs to me:
When you see the yellow speed signs under curve signs, that’s the maximum rated safe speed for that particular stretch. IOW, when you see a curve sign with a “45mph” sign directly under it, you’ll be able to safely make the curve under normal driving conditions as long as you are going 45 mph or less. Sure you can take it faster, but at your own risk. They’re not enforcable legally, but posted there as a warning to drivers.
Try this experiment sometime: find one of these curves, go around it at the recommended speed. Then go around it again slightly faster (5 mph faster should be plenty). You’ll notice that ‘seat-of-the-pants’ feeling, where your body seems to lean towards the outer edge of the curve. That’s the sign that you’re taking the curve too fast to be safe.
Similar signs are posted near freeway offramps for the same reasons; if they weren’t there, how many drivers wouldn’t slow down and would go off the road trying to take a 25 mph curve at 60?