We usually don’t define pitches as walkable, but rather how accessible they are.
Generally, the range is as follows:
Less than 1/4:12 can be considered flat for walking purposes.
1/4:12–1:20 is considered an incline where even a disabled person can ascend without aid.
1:20–1:12 is considered to require handrails to assist a disabled person.
Normally, that’s where the list ends, but there is more information.
At 3:12 you need surfaces that aren’t smooth because the presence of water, even dew, creates slippery conditions.
At 5:12 vehicles need to be rated as off-road or there are unlikely to be able to ascend.
At 7:12 most people require stairs or additional equipment to ascend.
When you get past 7:12 most people will find a vertical climb (0:12) easier than hiking beyond 7:12.
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