Note: This is assuming itÃƒÂ¢Ã‚â‚¬Ã‚â„¢s a properly posted sign. IÃƒÂ¢Ã‚â‚¬Ã‚â„¢m of the belief that orange and black speed limit signs you sometimes see at construction sites are not legally enforceable (not thatÃƒÂ¢Ã‚â‚¬Ã‚â„¢s IÃƒÂ¢Ã‚â‚¬Ã‚â„¢d recommend ignoring them). The signage should be the typical black and white signs, and there should be signage advising youÃƒÂ¢Ã‚â‚¬Ã‚â„¢re entering a construction zone.
The temporary speed limit isn't solely dependent on workers being present. Construction brings different conditions to that portion of the road that may require a lower limit (raised manholes, redirection of traffic, etc). These factors don't change once a workers shift is over for the day.