When navigation sends you on an alternate route than what you are expecting, give some kind of pop up that warns of significant traffic jam. I write this as I have been sitting in traffic for 90 minutes now. When I was navigating along it changed my route but I hadn't noticed it but if it showed me the traffic jam bar associated with the change and sounded a bell or something, I would have certainly taken the alternate route.
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An audible signal should accompany every popup.