You could do a "volleyball pit" by the edge of the road. Won't really trap cars that get in it too effectively, but it'll slow 'em down very quickly and it's easy to repair. Even if you let it grow over with grass it will probably still work. A rock garden with lots of gravel and a few big rocks would probably work too and you could make it look nice.
Friend of my dad's lived at the bottom of a hill on a sharp corner and people were always driving into his yard, aimed at his bedroom. He looked into concrete barriers but discovered there are all sorts of rules about artificial barriers. So he put the word out to everybody that he'd take their big rocks (took 4 of us to get my dad's biggest one into a pickup). He built a nice rock garden with BIG rocks (some the size of small cars) on that end of his lot and the cars stopped coming in. Made a hell of a noise once in a while, but nobody ever came over, and, because it was "gardening material" not "an artificial barrier" he was safe from lawyers. You better check this out in your area though.
Actually, the old Buick idea is a good one, as long as your local cops don't try to ticket it for being abandoned.
Jim
Hows the erosion in your area?
Actually, I think posts 4' deep with only 1' (12") sticking out will do even better.
Instead of the hassel of insuring it and tagging it and risking a ticket get a convertible or cut the top off and make a planter out of it. Put up a sign that sez "Planter boxes for sale"
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