If you are going to write a letter about traffic congestion, then here are a few considerations.
First, you should come off as understanding and kind no matter what you write. If you are writing to an editor, then you want to be heard, and one of the best ways to be heard is to come off as reasonable and rational.
Second, you should have a thesis, that is, something you want to get across to the editor. If you want to write about traffic problems, then this will be your thesis.
Third, you should have your facts in order. You want to make your point clear and convincing.
Finally, make sure that you thank the editor for his or her time.