I think people are willing to trust the GPS precisely because because they don't understand that the routing is based on a cost function that was ultimately determined by humans with imperfect knowledge about the conditions on every road in the country (not to mention variable conditions that can't be known precisely in advance, like weather and traffic). When you understand that the route is simply the output of an algorithm whose input may or may not be accurate, you're a lot more willing to disregard the route if other information contradicts it than if you believe that the GPS is somehow telling you the "best" route.