Ignore the side road I added (opps!) Basically if I am driving down the screen what is the speed limit? It is a workzone but the sign is only posed going up. The road goes back to 55 after the work zone.
The speed limit is the last legally posted limit on the stretch of road, as I understand it. If 55 were the last speed posted, that would be the limit.
However, many (most? all?) states don't require that you be driving faster than the posted limit to be guilty of speeding or a similar violation. As I understand it, typically laws require that drivers drive at safe and prudent speeds given the road and conditions. In these cases, posted limits may be identified in the law as prima facie evidence of unsafe or imprudent driving. However, if the posted limit is 55 in a busy construction zone, you very well still might be violating the law - and be ticketed, and lose if you challenged that ticket - if 55 is too fast.
So - the speed limit can be 55 and you can be ticketed for doing 55. The absence of a correct posted limit does not absolve you of your legal responsibility to meet the "reasonable person" standard.