Percent grades are the amount of rise per 100 linear units of measurement run. Usually expressed as feet in the USA.
BTW, the now-closed Saluda Grade in North Carolina (owned by Norfolk Southern) has a maximum grade of 4.9 percent, and an average grade of 4.24 percent.
Around Victorville in CA, there are ‘exits’ that take you right up the face of steep hills. The purpose is to give trucks with failed brakes a way to stop their rigs from barreling down an incline out of control. I took my Cherokee up one to see how far I could get. About at a forty degree angle, I chickened out.