If it’s simply the case that the curve is too sharp for the speed, then the normal sign is a curve sign such as this one with possibly the addition of a speed plaque beneath it to designate what the safe approach speed is. If you feel like checking the MUTCD, try here.

If it’s dangerous because there’s a blind driveway or intersection, there are signs for that, too.

One of the nice things about the MUTCD is that you’re specifically allowed to create custom warning signs with messages tailored to the situation. So if the department who had jurisdiction over your curve wants to place a diamond shaped, black-on-yellow warning sign that says “DANGEROUS CURVE AHEAD”, they’d be within their rights to do so.

I wouldn’t recommend that practice, though. It’s unlikely that a sign that said that would do any good.

With respect to the “new traffic patter” or similar sign, those are usually used when placing a new type of restriction at an intersection, such as going from a two-way stop to an all-way stop, or moving from an all-way stop to a signal. The most critical time to inform the public of a new traffic pattern is when you have an in-place flasher with red in one direction but yellow in the other. If you move to an all-way stop, or a signal, it can be difficult to retrain the drivers on the yellow approach. Normal procedure for a lot of traffic engineers is to put up variable message boards for 30-60 days prior to the change, plus leaving them there for another 30-60 days afterwards. That’s a dangerous situation.

My personal favorite “new” sign was a traffic signal in west Texas. There was a Signal Ahead sign on the approach with a plaque beneath it saying “new signal.” Unfortunately, the dirt and grime on this sign indicated it was anything but new.

Is there something to be said in defense of signs which may be intended to Command Attention from visitors/non-locals than the people who are more familiar with the road way. I don’t need the sign on the hump to tell me to keep right, if I’m familiar with the traffic pattern (e.g. divided road). Of course, I still laugh at the usefulness of the “new traffic pattern” or “new control device” signs. If we were really paying attention to the signage, we wouldn’t need a sign to tell us they have changed!