That would, in fact, be an extremely steep slope. An 80% grade is steep enough that if you fall you do not stop until you reach the bottom.
http://www.foxnews.com/travel/2017/01/30/5-most-dangerous-sk...
http://www.foxnews.com/travel/2017/01/30/5-most-dangerous-sk...
> In general, beginner slopes (green circle) are between 6% and 25%. Intermediate slopes (blue square) are between 25% and 40%. Difficult slopes (black diamond) are 40% and up. ...
Also generally with ski slopes the quoted gradient is some tiny portion of the run. Top to bottom the actual gradient is much less.