At least two or three times a year, I have consulting clients ask me about anti-personnel and anti-vehicular obstacles.
In heavily-wooded country, dropping some trees to form an abatis (as shown in this illustration from US Army Field Manual FM 90-7) is a viable expedient. But keep in mind that obstacles often work both ways. they will keep the bad guys out, but also keep you in. That is why my favorite roadblock is a Caterpillar (“Cat”) or similar tracked tractor, parked perpendicular at a narrow spot on a road, with its blade dropped and ignition system disabled. That will stop just about any vehicle short of another Cat. The biggest advantage of this method is that a Cat can be moved quickly, to allow the passage of “friendlies.”
If you don’t own a Cat, then parking cars or trucks perpendicular at a narrow spot works fairly well. Remember: In most foreseeable circumstances, emplacing multiple obstacles of marginal utility is as good as emplacing just one massive obstacle. One fairly inexpensive technique is to emplace multiple 5/8″ diameter steel cables at 20 to 50 foot intervals strung 18inches above the ground, secured with heavy duty padlocks. To gain entry, even someone equipped with large bolt cutters would have to repeatedly reduce each obstacle. And during that time, they could be warned off or directly engaged with rifle fire.
As I’ve mentioned several times before in SurvivalBlog, an obstacle is only useful in defense if it is under observation from defenders, and can be fired upon by them. Otherwise, the obstacle can be quickly reduced or bypassed by attackers and rendered useless.
I’ve already discussed anti-personnel obstacles at some length in SurvivalBlog, including tanglefoot wire, razor wire, and concertina wire. I recommend that you store defensive wire, but that you delay emplacing it until the situation warrants it. In essence, you should wait for the time when your neighbors will no longer say, “Gee, what a nut case!”, and instead say: “Gee, I wish that I had thought of that!”