I was having the same problem a couple of years ago. My mailbox was on a 4"x4" post right on the edge of the gravel road I live on and was snapped off three times in one year. I ended up installing a 6"x6" post about 5 feet back from the edge of the road and mounted a 2"x6" on top of the post out to the road with a brace on the underside. This basically cantilevered the mailbox out to the road. The snow plow wing now goes under my mailbox and nobody has run it over either. If they do run it over, they are probably going to roll their car as my ditch drops off pretty fast.