Not in Iowa you haven't.

That way of using of "miles long" implies at least two miles. A 16% average gradient over a 2-mile stretch implies an elevation gain of 1,690 feet. The highest point in Iowa is 1,670 feet above sea level. So in order for a 2-mile stretch of road in Iowa to average 16%, you'd need to start twenty feet below sea level. Which you certainly can't do in Iowa without digging an extraordinarily deep hole: the lowest point in the state is 480 feet above sea level, and it's not like it's geographically right next to the highest point.