My first house was on a busy road, and I had problems with trucks and trailers hitting my mailbox.
I thought I came up with a clever way to solve the problem. I planted a six inch pipe in concrete. I welded a flange near the top. I got another slightly larger piece of pipe, and cut a short section off. I dropped this steel cylinder over the post so it rode on the flange. I then welded another smaller pipe on the side of the flange. I mounted my mailbox on this pipe. I drilled a hole through the flange and into the pipe. I tapped the hole in the vertical pipe, and used a nylon bolt to keep it in positon. When something hit the mailbox, the bolt would shear and the arm would swing away.
The post office didn't like it because the said it could swing around and do more damage on the trip back around. They had no problem with the mailbox, other than the breakaway design. I put in a steel bolt, but they didn't like that either. I finally ended up welding the flange to the vertical pipe.
I thought it was a neat idea, but it sure got shot down fast.