Originally Posted by eaglesrest

Your best bet when 'Dynamic Route' is ticked, is to leave only 'fastest' ticked. You won't get prompted to re-route, it will just calculate a new route automatically and make an audible warning "traffic congestion ahead.... the route is being calculated."

If it cannot find a better route without congenstion, it will make an audible and visual warning to this fact and the existing route will persist.