Maximum walkable slope

by Kay
(San Francisco CA)

I need to provide a walkable path, minimum width 4 feet, down a 17 degree slope. The plot is 10 feet wide by 90 feet long, and the slope is almost straight down from top to bottom. My plan is to use a series of four-foot-wide cuts across the path on contour, connected by four-foot-wide paths along alternate sides of the the plot. It appears that these paths would have to have slopes in the range of 20 to 25 degrees. Is this slope walkable? What alternatives would you suggest?