Directions on how to do this were posted on a few major sites over two years ago, and I'd be surprised if they weren't floating around underground before that.
http://jalopnik.com/5141430/how-to-h...onic-road-sign
http://www.i-hacked.com/content/view/274/48/
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Addendum: This post is not a response to Tyler's post directly above. Just a general comment that I started writing before Tyler's post showed up in the thread.