Joshua Elsom wrote a nice piece that you need to read: "Open-Air Preaching and the Missional Church." A blurb from the beginning...
The combining of the words ‘open-air’ with the word ‘preaching’ is likely to elicit a wide range of images and opinions in the mind of the person reading them. For some they bring to mind the great evangelists of the explosive revivals of the eighteenth century — Wesley, Whitefield, Tennent, and Edwards; or the prophets of the Old and New Testaments — Jeremiah, Isaiah, Peter, and Paul. While for others, these words conjure up negative images of angry street heralds, with sandwich boards strung over their shoulders, thundering down threatenings of heaven upon all who would wander unawares into their field of preaching. Whatever one happens to think about, few typically associate the practice of preaching in the public square with the missional church movement. Because the missional church places such a high priority on practicing evangelism in the context of ongoing discipleship — on mission and in community — the thought of preaching to strangers who are dissociated from church or discipling relationships may seem at first to be counterintuitive. It should not be.
Check out all my open-air preaching posts and quotes.