I see a lot of gard rails on hiways these days use wood posts, with Armco atached to it. Now the armco[that sideways W shaped stuff] is not that easy to find, but for your use, channel iron should get the job done. Just sink some 6x6 post, lag a lenght of chanel on, and your set. Spaceing would depend on the lenght of the steel. If you can get 10 footers, I would put a post on each end, and one in the middle. That should do it. Now this isnt a high speed road is it? If so all bets are off.