"Preferred" Parking = reserved, cannot be used by others?

I have a project where we only have 3 parking spaces provided, resulting in a substantial reduction from the baseline. However, the client is reluctant to reserve these exclusively for carpools/vanpools, since there are so few spaces. The language in the LEED credit / Reference Guide does not specifically say that these have to be reserved for only carpools / vanpools, just that they should be "preferred" or prioritized, and that they need to be the nearest one, etc. However, examples of signage I've found, including the one in the description at the top of this page, seem to say that they are reserved only for carpool/vanpool. Can someone confirm either way, and if it does have to be reserved for only these vehicles, can you show me the citation where it says that? Thanks