I was wrong in my previous comment, because I accidentally used a calculator that did percentages wrong (it converted percentages of a circle, but in measuring grade, percent is rise over run (or height over width, if you imagine it as a right-angled triangle—you can take the tangent of the angle to get the grade). So 100% would be 45°.

So 15° is actually just under 27%.

The steepest hill in my city is 31%, so that's not outside the realm of possibility. But it is very, very extreme. Heck, even 15% is an extremely steep climb. An extremely steep popular climb near me averages out to less than 9%. It gets decent stretches which probably average to 12%, and those are exhausting to get up.