1. The grade of road is established by the steepness of the road. The steeper the road, the higher the grade. A 100% grade would mean that tan x= 1 where x would be the angle in degrees.
2. To have an X % grade means that the grade is not known.
3. The slope of a line tells you the direction that the line is going. Whether it is increasing, decreasing, horizontal, or vertical.
4. The slope of a line is related to the grade of a road because like the grade of  a road the slope determines how the line will look.


I have seen a couple grade of road signs, Usually by the mountains warning trucks. The highest percentage of a grade of road I have seen was a 10% grade. Grade of roads are important because without them drivers would not have a proper warning of the steepness of roads and how to adjust their cars to prevent accidents.