This type of sign is often seen on mountain roads, especially on high mountain passes. The grade percentage varies widely, of course; the vast majority are single-digit in the 5-8 range but I've seen them as high as 21%.
What is that sign above really telling us? 8% of what? I'd never really thought about it. If asked, I would have said it meant an 8 degree angle. I would have been wrong. The sign is telling us that for every 100' of horizontal distance, the road loses 8' in vertical distance.
So, a 100% grade would mean that for every 100' of horizontal distance, there's a loss of 100' of vertical distance. If you sketched this on graph paper, you'd find that a 100% grade yields a 45 degree angle.
Any questions? No? Okay, it's time for recess. You may go outside and play now.
