I bought the GPS about one month before driving my family to Washington DC. In order to get used to it, I would use it on my work commute. I quickly discovered that it had an undesirable habit of choosing congested roads with many traffic lights.
It didn't seem to matter whether I choose Shortest Route, or Quickest Route. It was oblivious to the Avoid Highways setting. Even the intriguing Econo-Route setting made no difference. It was stuck on an undocumented mode I call "Maximize Encounters with Red Traffic Lights." And where I drive, at least every 1 of 4 traffic lights will be red on any given commute. If you know how many traffic lights (L) are on the route, you can get a realistic arrival time (if you obey the speed limit) by adding this many minutes to the arrival time the GPS claims: T = 0.5 * L. My meager 10-mile commute to work features an amazing 21 traffic lights and is ten minutes shorter at 3am when most lights are either green or disabled.
And unlike Monty Python's Flying Circus, I hate traffic lights, even when they're green. Because a green light is one that is bound to turn red just as you get to it. Of course anyone who has read my rant on traffic lights (and the follow up post) knows where I stand.