I think that about the only way that "open air" preaching (weird term) can be effective in our society is if it is truly Good News instead of the angry God-HATES-you-and-you-are-going-to-BURN-FOREVER-in-HELL-unless-you-pray-this-prayer-and-act-like-me kind of sermon, or attacks against grave social ills like women wearing pants or attending movies.
It may just be the religious makeup of this part of the country, but I've never heard a public, street corner preacher preaching anything as inspiring, convicting and liberating as Jesus teaching the Sermon on the Mount and the parables of the Kingdom of God, Peter's sermon to the household and friends of Cornelius (Acts 10),Paul and Silas at Lystra (Acts 14), the Aeropagus/Mars Hill (Acts 17), and Paul's appeal to Festus and Agrippa (Acts 26).
What do you preach when you do "open air" preaching?