The biggest problem is that our leadership style is unnecessarily too confrontational. What is so difficult with calling these informal traders to a meeting, listening to them and coming to a mutually agreed solution (reminds me of the late Karisa Maitha)?
IMO, whenever you disregard an adult and show him he doesn't matter, (s)he will most certainly always fight back in an attempt to prove that, indeed, they matter.
Telling informal traders to vacate the streets without offering viable alternatives in my opinion only exacerbates the problem.
Life is like playing a violin solo in public and learning the instrument as one goes on.