So here's some light background reading on Interstate 66...
http://www.virginiadot.org/projects/idea66/downloads/Coleman-Decision.pdf
To make a long story very short; Arlington bitched and moaned and did everything it could to stop 66 from going through at all. A compromise was reached that allowed 66 to continue so long as certain restrictions were put in place
1 - Only 4 lanes total
2 - No trucks ever
3 - HOV during rush hour
That's how VA gets the power to levy HOV restrictions on I66. FYI, it doesn't matter whether you agree or think it's right, you still have to abide by it.