2010 ADA Standards for Accessible Design Pocket Guide
503.4 Floor and Ground Surfaces.
Vehicle pull-up spaces and access aisles serving them shall comply with 302. Access aisles shall be at the same level as the vehicle pull-up space they serve. Changes in level are not permitted. [S.H. 4.6.6]
EXCEPTION: Slopes not steeper than 1:48 shall be permitted.