It would depend on what strategy you're using. Is it a permanent roadblock, or a temporary one you can take down at need? If the former, then a good, solid tree across the road will do if you cannot build anything, plus it looks natural (if suspicious). Anything man-made on the road will only signify that something of possible worth is on the other side of the obstacle.
I also suppose that, in a permanent roadblock (if it's manned), you could make a series of very deep ditches that make it impossible for almost any civilian vehicle to cross, and have hidden, fortified LP/OPs in close enough range to cover the roadblock should someone attempt a strategy to overcome the obstacle (such as filling in a ditch, or placing a makeshift bridge over it strong enough for a vehicle to cross. You prevent the latter, you could also place barbed wire, and perhaps booby traps around it to dissuade infantry. You could place caltrops along the road starting at 50 yards or so from your barrier to dissuade infantry and blow tires from oncoming vehicles (however, I'd say that a sign warning intruders away, placed at 100 yards, would prevent angering any innocent neighbors).
Another option, usually readily available to us all, is to use vehicles to block the roads. Sure, it screams "I'M HERE! COME AND TAKE IT!" However, I don't think anybody is going to move three or four vehicles placed in a reinforced wedge, with no tires/slashed tires and possibly with the rims in pre-dug holes in the ground that cause the vehicle to sink down to it's belly. The down side to this is that you have just given any enemy potentially good cover from your infantry defenses.
I'm not too sure what your options would be if you require a removable barrier. Perhaps you could use the multiple ditch system w/infantry booby traps again, except you have a reliable, reusable bridge you can extend from your side to allow your own personnel and vehicles to cross. You could also (in an urban area) attempt to wall in your perimeter, installing only one gate large enough for vehicles pass through. You could put the gate between two sturdy buildings, and strategically place debris/manmade obstacles/whatever will prevent vehicle movement in a winding pattern that would make it impossible for a vehicle to just charge straight in. They'd have to take it very slow on entrance, making the turns, and making them vulnerable to infantry assault by you. This defense, though, would be very manpower intensive and would mean the existence of a well-organized community that has enough manpower to delegate workers from other tasks such as active perimeter defense, food gathering, repairs, etc.
Any other ideas, guys? I thought about dragon's teeth, but it depends on the ability to weld and transport huge metal beams, and I don't think that most people will have the ability to do that.
