Hi @ylouka - this information is found in the site planning and design handbook, second edition, on table 7.4. We have that information listed as the reference for the question - I’d always suggest checking out the reference if a topic is confusing!
I agree that 20% seems a bit steep for parking, but the question is asking about the maximum allowable slope (not the best-practice slope) and the answer (20%) is clearly listed in one of the ARE 5.0 reference materials.
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