Granbury Road question

Kimberly Norton of FW asks:

Construction is taking place on Grandbury Road (Between Dirks Road and Hulen street but closer to the Dirks Road intersection).  There are signs that say "Construction 20 MPH," the original speed limit is 45 mph.  Is this construction posting of 20 mph only for when workers are present?  In other words, at night when no one is working is it legal to drive the original 45 mph?