Around Richmond (central Va), I don't know of anywhere that is 100% legal unless it is on private property. I have gone out with some friends in the construction zones of Pocahontas Parkway (the new highway around Richmond) and that has been a lot of fun. Other than that, we have had some light fun on railroad trails and Va. Power clearings, but that is about it.
Best bet, head west to the NFs mentioned above (2-3 hours) or head to Outer Banks to get sandy (3 hours).
Let me know if you find anything great,
Robb