How deep should I bury a 4×4 steel post

Years ago, growing up on a rural farm in the midwest, my father solved the drunk driver mail box problem. He did this after drivers and snowplows took out the mail box several times.

He placed the mail box on a long arm that was cantilevered over a swivel post 10 to 12 feet off the side of the roadway. The design of the swivel post was such that the mail box arm would self return to the normal position if the snow plow would go by and gently push the box out of the way. On the other hand if a driver came by and made a direct hit on the box it would swing 90 degrees and latch into that position till one came to release the catch and let it return to normal position.

The "return to normal" mechanism worked by having springs pull down hard on the swivel part which was mounted against a V-groove seat on the post.

Given that 20 feet is a common stock steel length, 6 feet sounds good - if embedding them at all. It nearly meets the 2/3-1/3 rule-of-thumb for post above ground and post below ground. On more solid soils some get away with 3/4-1/4.