I get the idea of grade ratings, such as 6%, 9%, etc. and the higher %, the steeper incline. However, how are they derived, and why aren't angles used? Does the percent grade indicate the ratio of elevation change (in feet, for example) over a horizontal distance (also in feet)? Such as 90 ft rise over 900ft is a 10% grade?

What are the steepest grades allowed on Interstate Highways, and other US Routes?