My friend told me he sometimes goes on this piece of land on the 55, where it meets the 1. It's right next to the gas station and you can supposedly take the dirt road to get on it.
You might try looking at the Wake County RE site to locate these parcels of undeveloped land. The owners are companies and trust (I think) out of state so it's unlikely that they will give you trouble for driving on their land
Have fun ...
The center of the map where you see the dirt road going south, that would be the way to get in.
Link
here (Google map)