Grade, commonly referred to as slope, is simply rise/run. In order to find %grade we can use the equation 100(rise/run) and that would give us how much we fall or rise over a certain distance. For example, if you had a 10% grade that means that you would rise or fall 10ft for every 100ft of distance ran. The equation can also be expressed as tan a= triangle h/d

We can now see why handling a 9% grade may be cumbersome, but a 100% grade is just plain "suicidal" because taking into my example above, if we had an 100% grade that would mean rising 10ft for every 10ft! If I am out biking or jogging I want a 0% grade from now on because that's just plain flat ground. 

Quick story on how this concept may have helped my family in the past. One camping trip we decide it would be nice to go up into the peaceful mountains of Colorado and wanted a new campsite for a change instead of the one we always went to called "Redondo". So instead of taking a turn at the Redondo campsite we tried a new one called "Iron Gate". Now, the road to the campsite may have been perfectly fine for any old car carrying some light tents, but we were pulling a full size pop up camper! There were no %grade signs anywhere in sight and had we have seen them I think we would have thought differently before journeying up that hill. For the first 15 mins or so our Bronco handled itself just fine, but then the hill started to get steeper and steeper. We had no choice now but to continue on, we had already made it this far. Bad choice. The next 45 mins were filled with cursing, stopped cars and screeching tires, as we slowly made our way up the hill. We made it to the top with smoking brakes and exhausted campers. I don't think we will ever camp up there again and from now on I'll try to see if there is a %grade sign on the road BEFORE we spend an hour trying to get up it.