Once upon a time, highway signs, lighting, etc. were designed so that no
car could damage them by running into them. Cars hit these things head-on
and people died.

Modern highway design makes these things so they will "break away" should a
car hit them. The thinking is that a life is more important than material
things.

With that said, I would *not* place anything in the path of the cars which
would make them stop instantly such as boulders. concrete barrier, etc. On
some highways, they have plastic containers filled with sand which are
placed in front of bridge piers and other things which would stop a car
instantly. When a car hits the containers, the sand goes flying and the car
gradually slows down. I'm sure they use a specific type of sand and may or
may not fill the containers to different heights???

Also some "run-away" ramps (for when breaks fail on downgrades) use deep
sand to slow vehicles down. The tires sink into the sand. I don't know how
deep the sand goes or what type of sand they use, but I assure you the
government has probably conducted numerous tests to find the best depth and
best sand to use.

I have seen some private barbed wire fences on curves where the land owner
tied white plastic strips onto the fence every 4 inches or so. This makes
the curve more visible at night.

In any case, I would consult with a state or county highway department
engineer before doing anything. They would be able to give you safe
solutions to this problem and might fix the problem for you. If you could
take pictures of the approach (from both directions), curve, your yard,
etc. and meet with an engineer in person, you might get better results and
answers than just a phone call...