Paddy,
Also look into the local regulations at the highway dept. There is laws on the strength of the post, it varies from state to state. I got tired of replacing my mail box, so I sunk a 12" pipe, filled it with concrete, welded another section of 12" horizontal on top for the box... a couple of nights later "somebody lost an aluminum baseball bat not too far from my mail box" their hands are probably still stinging. Ok back to the regs, the highway dept said it was a hazard to motorists if they happened to wreck into it, and I could be held liable for their injuries, so I removed it.