Whilst it must be annoying if you're a local, I'd suggest that slowing down and rubber necking is exactly the correct response to have when passing something as

Whilst it must be annoying if you're a local, I'd suggest that slowing down and rubber necking is exactly the correct response to have when passing something as classically awesome as the henge. What would solve the problem easily (rather than trying to hide it with a hedge), would be to do as they do up in Scotland in the national parks: Provide plenty of parking places anywhere the henge is in view. I drove through Galloway National Park a few weeks ago and every 200 yards the scenery changes to another amazing view and every 200 yards or so, there's somehwre you can safely stop and enjoy it. People can now safely stop to have a look and take photos, and traffic on the road can drive. If people are amazed enough at a piece of national heritage that they want to admire it, hiding it from view seems like entirely the wrong response.