In relation to freeway signs: What does 18% downgrade mean??

Question

I was driving on the freeway and on the offramp it said 18% downgrade at 300 ft.....WTF???

Answer 1

18% means 18 ft downward when you've gone 100 ft horizontally. 18/100 = 18%. Capisce? :)

Answer 2

Steep downhill grade. The sign lets truckers know it's time to slow way down before exiting.