Originally Posted by DrIsotope

Mapping software will routinely spit out grades of +20%, even though such conditions likely never occur. Seeing even 15% for a few hundred yards on publicly traveled roads is unusual. Here in SoCal, there's virtually nothing aside from those narrow mountain roads that are above 12%. Now the OP is in Japan, so there might indeed be some ridiculous grades over there.

Regardless, 4,800 feet in 21 miles is plenty steep. I hope those guys are in good shape.