Originally Posted by
Tourist in MSN
The distance markers should be one mile apart on horizontal ground, but if you were going down a hill, the elevation would add some distance too. If you recall Pythagorean Theorem from high school trig, ... ... ... But if the hill is quite shallow, only a percent grade, then no big deal, no correction should be needed.
These were being placed before GPS.
It seems the only practical way to place so many of them is rolling-out a mile on the road surface.