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Microsoft hold a patent on an "Avoid Ghetto" feature (search "ghetto" in the upper right), so Waze can not implement this.
Also, Custom Avoidance is NOT supported by Waze at this time. This is mainly due to to the fact that routing calculations are done server side, while with traditional GPS units they are done client/device side. Routing is done on the server side to support the real-time nature of routing decisions as well as the ability of Waze to re-route due to changing traffic conditions. When done on a traditional GPS Device, the routing system is only worried about ONE user, but with Waze, the routing system is concerned with routing thousands, if not millions, of users - and if each of them had custom avoidance, the calculations would get much (exponentially) more difficult.