Why do drivers not slow down when they see signs indicating road work?
Presumably the decision of drivers to slow down in response to work zone signage is influenced by many factors.


Signage and road factors: Presumably there are a wide range of factors related to the nature of the signs and the structure of the road setting that influence whether people slow down. For example, I've seen road work signage on freeways that were often in place even when no actual road work is occurring. In these cases I've seen many drivers slow down little if at all. In contrast, I typically see much greater reduction in speeds when actual road work is occurring. Or for example, I imagine that a worker standing with a sign would be more effective than a static sign.
Cultural and norm factors: Presumably countries and regions  differ in cultural attitudes to driving, the acceptability of speeding, and the general tolerance for risk taking. Road safety advertising employ a wide range of strategies for reducing speeding including targeting personal interest (e.g., you'll get fined if you speed) or highlighting consequences (e.g., speeding increases the likelihood of killing yourself and others). 


A few empirical studies

Jarvis (1983) performed empirical research on Australian highways and found that 


  There is a clear indication that work area speed limits reduce vehicle speed on entry into the work zone; however, the large majority of drivers still exceed the speed limit posted. 


Garber and Patel (1995) found that signs triggered by radars indicating excessive speed were more effective in slowing cars than static signs.

References


Garber, N. J., &amp; Patel, S. T. (1995). Control of vehicle speeds in temporary traffic control zones (work zones) using changeable message signs with radar. Transportation Research Record, (1509), 73-81.
Jarvis, J. R. (1983). The Effectiveness of Road Work Speed Limit Signs. Australian Road Research, 13(3).