"Amazed" I'm in agreement with you.
I have a measured 100 meters that is on a 15% grade. I'm pretty sure that is accurate too.
When I run this I can hear the gearboxes straining in the cars that pass by.
I can't imagine running at any kind of speed at twice this grade.
I hike quite a bit and if you get much over 30% grade you basically start looking for hand holds and checking your foot placement.
15% is not so steep. It is steep enough to get a good pre-stretch of the ankle, but not unmanageable.
All I can say is, the hills I use (there are 3 that can be considered "ramps") are steep enough so that, as you said, it is difficult to drive a car up it.
When I first started running this hill using short sprints, it was literally difficult to get up it. I had to absolutely power my arms strongly, get way up on the balls of the feet and push HARD.
I cannot imagine these types of hills can be less than 25 foot rise for each 100 feet horizontal, i.e., 25% grade.
If you hold up your hand vertically, consider this a 90% grade or sheer drop off a cliff. Then simply push your hand down slowly and look at each "hill" you make. When you get to 25-30% you will see it is not impossible, but very difficult.