Hello James,
In regards to the recent post toward driveway security strips, or "spike strips" I think of another possibility to the construction in using a 2×6. I would consider fabricating the base out of treated 3/4" plywood. Not only is this already somewhat camouflage, it will take the years of abuse that weather will give it.
I envision a design in which the upper three or four layers that are glued together with construction adhesive, (preferably a polyurethane adhesive like PL Pro), and then drilled for insertion of spikes. Then, apply an additional layer with a width of approximately 18" more than the previous layers. This will serve two purposes.
1). It keeps the spikes from pushing through the bottom of the board when driven over thus helping penetration
2). Ideally the tire of the vehicle will be over the lowest layer thus stabilizing the setup without causing it to roll over with pressure. Assuming of course that you have dug a small trench to submerge the unit.
Plywood is much stronger than conventional Pine with much more resistance to breakage. More layers, and glued and screwed together will add to the strength. Keep in mind that different treatment processes require different fasteners that resist the corrosion factor of the chemical saturant of the wood/plywood.
Regards, – The Wanderer