
actually,  teh D means the registered driver has diplomatic immunity.
That means they can do as they damn well please on the roads, and you
have only God as your protection.   

The state Department Issues Saa-XXX  plates for personnel  who work
at the embassies but haven't been granted  immunity.  Most embassies
have restricted parking for embassy personell  street side.  

The S plates allow them to use  those parking areas as well as the
restricted lots at National and State dept,  without a lot of crap.