That post technically contains a misplaced modifier so punctuation is not really relevant. In that sentence, walking behind people functions as an adjective and modifies I. Therefore, it should be placed closer to I to clarify that it modifies I.
As written, walking behind people technically is describing rage. Note that it does not make sense to describe rage as walking so English speakers will assume walking behind people modifies I and not misunderstand the meaning. However, consider the following sentence:
I saw a girl walking down the street.
In this sentence, both girl and I can potentially be walking so you have to rely on grammar rather than the meaning of the words to figure out who is walking. Girl is being modified because is it closer to walking down the street.
If you wanted to clarify that I was walking, you have to invert the word order:
Walking down the street, I saw a girl.
So in proper English, the post above should be written:
Walking behind people, I get road rage.
Now just as a final note, the sentence I wrote above is completely correct and the one in the post is not. However, the correct sentence differs from the incorrect one in that it emphasizes walking behind people. The incorrect one is more of a simple statement with walking behind people thrown in as an afterthought.