I lived in Trappes (I guess Georges could tell you a lot about Trappes) for a few years. I saw some nasty stuff but nothing ever happened to me.
I have never lived in a town with welfare cases and where there are big estates. I also avoid by all costs towns such as Poissy, Trappes, Les Mureaux, St Denis, La Courneuve, Gennevilliers, Sarcelles, Stains, Sevran Les Baudottes, Grigny, Clichy La Garenne as well as some districts of Paris such as the 19th, 18th, 20th, 17th, 10th and same places such as La Goutte d'Or, Barbes, République, Bastille, Les Halles, La Chapelle and Chateau d'Eau. I am very very selective of the places I go and I have heard too many bad things about the above quoted places. Perhaps you like these places and these towns, but you are in the absolute minority because someone with a common sense will never recommened anybody to adventure himself alone in the dark in these places. These places are known for having been places of riots, *******, harassment and muggings. I always prefer and favor classy neighborhoods for my own safety but for the safety of the other people I meet.