I complained to Magellan about the incessant "Stay on the current road" warning it gives at every major intersection. I told them I buy a GPS to tell me when to turn, not when NOT to turn. They replied saying that warning can't be turned off.
So, I turn the volume down and watch the visual cues.
NEVER ask a GPS for the shortest route. It'll take you down every alley and cow path in town. Mine is programmed to give me the fastest route.
I use Google Maps app on my phone to watch for traffic slow downs and to find alternate routes. Sometimes to double check the GPS routing.