In another thread, there was a comment made about Police cars in Fairfax County and their windows being tined dark - possibly outside the gudelines of privately owned vehicles in accordance to the VA Code.
The fact of the matter is this, Law Enforcement vehicles, are expempt as outlined in Sec.K of: § 46.2-1052.
They can be as dark as they like, and be within the bounds of the law.
"K. The provisions of this section shall not apply to law-enforcement vehicles."
(
http://leg1.state.va.us/cgi-bin/legp504.exe?000+cod+46.2-1052 )
I hope this helps shed light on the matter.