Is there a law that states how far apart speed limit signs are?

I live in Missouri and I have been told by more than one person that there is a written law that states speed limit signs must be posted every x number of feet/miles. I was pushed onto a detour one day and thought the speed limit was the same as the other road and ...well, it wasn't. Problem is there are no posted speed limit signs for a full two miles of road and I think I have the grounds to have the ticket dismissed.