North of lake Ontario is all hills. We had a hard time finding a road to make an impromptu drag strip when we were teenagers.
Ride planning is top priority around here. 12% and even 17% can be ridden daily. I ride repeats on a hill that shows 21% in places. The ride down more than makes up for the ride up.
This is the perfect hill. It is over 1Km long, steep and a few fairly gentle curves. (Just enough that you can't see the cars on the way up.)
Our flat TT route has a section about 2Km long with 3% and 4% sections. We consider anything with in 3 Km of Lake Ontario flat. Anything North of that is hilly. Not Mountainous just hilly