Great Point! So according to ARE and Architectural practice if references have directly contradicting requirements architect should follow the most stringent one. Since ARE Handbook lists as a reference Site Planning and Design as well as International Building Code 2018 they are equally valid references. While Site Planning and Design states in Table 3.4 Typical Grading Standards that Parking lots maximum grading is 20%
International Building Code 2018 states in 406.4.3 that the max slope should be 6.67%.
While IBC prescribes the slope for parking structures, not sites. The logic suggests that the slope on the site, exposed to the elements, won’t be steeper than the one inside of the structure.