Field Guide to Math in Humboldt
No. 29 Percentage Grade on Roadways 

It is hard to travel in Humboldt County without seeing a down hill warning to truck drivers about percentage grade. The percentage basically describes the change in altitude over a 100 foot stretch. i.e. a 10% grade drops, on average, 10 feet for every 100 feet traveled or traveled horizontally (the calculations are virtually the same) Thanks to Tami Matsumoto for suggesting the topic and info. Photos are from a Hwy 36 ridge top at mile marker 32.00.
More info. http://theclimbingcyclist.com/gradients-and-cycling-an-introduction/​