If the advisory speed (say 55 mph) equals the posted speed limit (55 mph), do you have to place a horizontal curve warning sign?
You do not have to place the sign based upon the speed limit, but if the 85th-percentile speed is greater than 55 mph it may be recommended or required. If the traffic is greater than 1,000 AADT and the highway is an arterial or collector then Table 2C-5 shall be followed.
The key is the standard in the MUTCD that states you should use the higher of the two speeds, statutory speed limit or 85th-percentile speed. Without knowing the 85th-percentile speed you may use the speed limit, but if you know the 85th-percentile speed is greater than 55 then you need to follow table 2C-5.
You can put a plaque under the curve sign if you use it, but it is not required. This is a may condition in the State Supplement.
Finally, if you have evidence of problems with traffic negotiating the curve, you can always place a curve sign and possibly chevrons or arrows to help them navigate the corner.
In advance of horizontal curves on freeways, on expressways, and on roadways with more than 1,000 AADT that are functionally classified as arterials or collectors, horizontal alignment warning signs shall be used in accordance with Table 2C-5 based on the speed differential between the roadway's posted or statutory speed limit or 85th-percentile speed, whichever is higher, or the prevailing speed on the approach to the curve, and the horizontal curve's advisory speed.
Where the advisory speed is less than the legal speed limit, the horizontal alignment sign shall be supplemented with an Advisory Speed plaque (see Section 2C.08).
Where the advisory speed is greater than the legal speed limit, an Advisory Speed plaque shall not be used.
Where the advisory speed is equal to the legal speed limit, an Advisory Speed plaque may be used.