You need to know both the grade and the length of the grade. Some people brag about 30% grades, but if they are only 10 feet long, they are over in just 1 or 2 pedal strokes. On a long climb, the average grade over 1 mile or more is much more interesting. Few paved roads (anywhere in the world) are steeper than 10% for more than 2 miles. In the USA, I believe the interstate highways and railroad tracks are never steeper than 6% for any distance.

On the other hand, sometimes you see stuff like this on state highways:

State Highway 108 in eastern California.