Well, coming over any hill onto the down-slope, you're going to have decreased weight on the tires due to the bike's trajectory. An extreme case of this is a jump, when your trajectory carries you high enough to leave the ground altogether.
Frictional force (provided by your tires for accelerating, braking, turning, etc) is equal to the coefficient of friction between the surfaces (your tire on the road) times the normal force pushing DOWN (the weight of the bike and rider). Decreasing either of these will result in less available frictional force (traction) to control the bike. Cresting a hill (and momentarily decreasing the weight on your tires) is the same as going over an oily patch (which would decrease the coefficient of friction), as far as traction is concerned. Both situations warrant planning to avoid sudden turns, braking, or whacking the throttle, if you like your bike and bones the way they are.
In Australia our 'advisory' posted speeds for corners that might be of issue can be really misleading (as I'm sure they are in the US). In the areas that recieve alot of rain or ice or snow in the winter the signs are posted accordingly so most of the time you can safely go 50%-100% over them but I've been caught out sooooo many times on corners with decreasing radius or an off camber that I usually play it safe unless I know the road...
Not to mention the thought of encountering a dead wombat or kangaroo in my chosen line plus a 4x4 with a caravan coming the other way just doesnt really make for a good ride !
I've got a few of those decreasing-radius-followed-by-elevation-changes things where I ride, too. Things I do to deal with them are fairly basic...
One technique that helps me is to keep some weight on my feet (more than when flying straight-and-level) so I can keep the bike going where I want it to.
I try to always look 'way into the turn...far beyond where you think you're looking. It's easy in a DR turn to let the eyes drift towards the outer edge of the road -- and that's where you'll go. On blind turns (even ones I'm familiar with) I mentally 'imagine' the road well beyond where I can see it and turn into this vision, it really helps.
One last caution about DR turns, especially right-handers going downhill. Where I ride, this usually occurs where the road is cut into a hillside and is graded (pitched) towards the left to help drain off water. On these turns, the ground clearance on the right is reduced due to the pitch so I take it easier than I might on a level road with the same dynamics.
The speed warning signs I've seen have such a large variation that I think it would be a mistake to use them to determine the correct cornering speed. Most of them seem to have been set to give the correct speed for an improperly loaded U-Haul truck - on packed snow. I did notice that the signs in Oregon are more conservative than those in California, but that whole state is like that, and in more than just speed signs.
When I see a yellow sign indicating 20 mph or below, I pay more attention to it. The 10 mph signs usually indicate a very tight curve. Sometimes they're there to indicate a different sort of hazard, like maybe a driveway around a blind corner.
I judge all unknown corners based on what I can see and usually do the same on roads I'm familiar with as well. I always try to ride with the frame of mind that the worst possible thing is around that next corner that I can't see around. Occasionally I'm right, and there is a moving van three feet over the center line, or a cow standing in the road, or a stalled truck that can't pull off to the side, or a bunch of gravel in an unexpected place. Riding so that I can stop in time as soon as I can identify the hazard allows me to continue riding rather than picking up my bike out of the ditch or worse.
Use the little yellow speed signs as warnings for how difficult the turn might be, but learn to judge the road for yourself and don't count on them to accurately indicate the true safe speed for any given turn.
Good answer Scott. I agree most of the signs > 20mph are of dubious value, and anything below that means look out. But I suspect they only put those signs up when there have been accidents, so seeing no sign is not a guarantee of smooth sailing. Always be able to come to a reasonable stop.
Oh, and not strictly a speed sign, but one to watch out for is a left intersection sign (looks like a T rotated 90 degrees) when the intersection isn't visible. I got bitten by one of those where there were two cars waiting to take a left just past a sharp rise in the road, so weren't visible until I crested and had 30 feet to stop. (Going downhill on a washboard road surface!) So definitely keep an eye on those signs and decided for yourself.
On roads that I do not know while riding alone about 20 MPH over is usually a safe bet. I also like to enter a curve late so I can see further around it and have more space coming out if my speed is misjudged. If I am riding with someone who knows the road, I usually like to go through a couple of MPH faster than they are going...... just to let them know I am there..... heh.....heh.....heh.
In N. Georgia, TN, SC, NC and Alabama when I'm on a road I've never or infrequently ridden I find it a safe general rule to double the caution sign recommended safe speed. It's easy and I don't have to do big math. I've never regretted this technique. I can always circle back and hit a section again which I've often done. I'm approaching 60 years old and there's a lot more road to ride if I play by the rules. 😊
I don’t have a rule. Depending on what I see, and how far I can see, I’m aiming at the other side, not looking at my speedo. Scaring myself being stupid got old a long time ago. When I’m familiar with a curve my speed goes up. I have no depth perception at all though. It might be different if I did.