I'm going to be slightly off-kilter here and point out the obvious that no one would be able to hear them for the noise of the cars and trucks going by!
Why speak when no one can hear you? WHy speak when noise drowns out your meaning and message?
Public forums, church pulpits, groups of 2 or 3, etc, are much better for speaking The Word to ears that can hear than shouting it out under a motorway where the words are heard by no one.