The 35mph posting is usually implemented when driveways, entrances, and intersections occur at a frequency that makes it difficult to consider each subsequent circumstance while driving at some higher speed.
 
I can usually guess the posted speed limit simply by observing the environment I am driving through.
 
Highway 319 through Omega Georgia, which is a very small town with driveways, entrances, and intersections occurring every 100 or so feet is a good example of that sort of implementation.
 
best regards,
mike