traffic question

This isn't really so much a question as to operations of a CMV more than a question for any driver on the road. I started noticing this after driving a truck and remember someone telling me the facts but can't remember where I seen it at. Ok those of you older grizzled trucker veterans out there who over the years have met and made friends (or enemies) with various representatives of DOT whether it be inspectors or the highway patrol in various states might know the answer to this.

scenario: you are driving along a highway (doesn't matter 4 lane divided or two lane) current speed limit is 65mph. you see the yellow diamond sign that has 55mph with an arrow pointing up indicating that the speed limit is about to drop. You pass the 55mph sign. How far must the next sign (whether it is an increase or decrease in speed) be placed? If I remember right, the drop or increase can't be more than 10mph and has to be a minimum of 1/10 of a mile. In other words, you pass the 55mph sign, the next sign shouldn't be 100ft from the 55mph sign correct? if it is 1/10 of a mile that would be approximately 528ft. I am asking this cause I am driving here in Oklahoma and I have noticed a few areas where the speeds drop like this and sure enough folks that are unaware of the change (out of towners from what I have seen) are inevitably sitting on the shoulder with a black and white local sitting behind them with lights flashing.