A simple line-drawing or diagram of slope percentages can be deceptive when compared to the actual experience of walking on such a slope.
Here is such a diagram showing the different slopes expressed as percentages - and for comparison a picture of a San Francisco cable-car. According to USGS, the steepest cable-car route in San Francisco is 22%. We don't know exactly where that piece of the route is, but have chosen a representative picture of one of the steeper parts to give you some perspective of what such a slope entails in "real life".
So the slope shown here is considerably less than the "30% slope" under discussion.
The designations "Flat to gentle", "Gentle to moderate", "Moderate to steep", etc. are those generally used in Land-type Studies, Parcel Data Mapping etc. as well as by U.S. Geological Survey.