I've driven all over the Western Hemisphere, under all sorts of conditions. This is a sign you see in a lot of places, indicating there are curves ahead. Usually they mean just a handful of curves on a moderately winding road.
In San Francisco the sign may be taken literally if you're facing Lombard Street.
600-ish feet in length, paved with bricks and featuring 8 curves along a
one-way (downhill) road with a better than 25% grade.
"Curves ahead." In this case the sign is an understatement.
Drive carefully, and test your brakes before turning in.