In Fairfax County, are passengers in a vehicle allowed to drink alcohol? ("Drinking and Riding")

Under Virginia State Law, yes, but we don't recommend it.   

Such passengers may be accused of violating local ordinances involving public consumption.  Further, the cops will usually roust the driver even if he or she is not drinking, once they smell any alcohol.    Since cops in Virginia, including Fairfax County, can't always tell an intoxicated person from a sober one, this creates a troubling situation.