The city did the same thing to my household. They went around placing notices on doors with trees that had branches stretching out over the street. You shouldn't need to have an arborist come in and look at your tree. The city provided us with guidelines on how to trim the tree and how it should look. They gave us the option of going to a professional, maybe things have changed now. We trimmed the tree up pretty well and haven't been bothered since, and it's been a few years now.