VA only has HOV allowance!
The federal highway system requires
that for federally-funded highways with
HOV lanes, that they have the
prerequisite number of people in the
vehicle to ride in the HOV lanes, or
be a SULEV AND ILEV vehicle.
The Prius is a SULEV (as is a number
of other non-hybrid cars), but since
the Prius uses gasoline it does not
meet the ILEV specification (Inherently
Low Emission Vehicle).
VA somehow managed to get their
hybrids grandfathered in to use the
HOV lanes, but the feds aren't happy
about it (the allowance is supposed
to be phased out).
Other states have bills stating that
hybrids can use the HOV lanes, should
the federal government decide to
allow it.
Currently, only VA vehicles with the
VA Clean Fuels license plate are
allowed to drive solo in the VA HOV
lanes.
Otherwise, you need to have the
correct number of people in your
Prius to ride in HOV lanes. (until
the federal government changes the
ILEV regulation, anyways...)