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