Its illegal all speeds cant go up or down more than 10mph at one time it has to be stepped down and has to be so far apart (can't remember the footage off top of head) with the exception of a school zone and i think it is a 20mph drop not sure on that would have to go back to my manual for that one since i dont use it very often most of my school zones drop 10 mph.
I don't know what the law says, but I have to go through two bridge construction zones to the Farm. They start the reduction in speed two miles away which is stupid.
70 to 65, then to 55, then to 45, and finally 35. Don't know if that's how the law works, but its stupid, and only a trap.