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...