Phil, yeah, what Molly's pet said.
I turn the voice off most of the time, too. I have, on several occasions, uttered something along the lines of, "You're stoopid, I'm not going that way."
When I first got mine, the Traffic thing worked like, it would find a better route, and it would pop up a note and then aurally inform me, "A better route is available. Would you like to take it?" I liked that, but apparently enough people didn't that they took it out in the next software update.
Sometimes you have to fiddled with it, set it for Car, or Truck (or Bus or Taxi), Shorter Distance then Faster Time, to see which route it takes, compare times and mileage, then decide which way you want to go, or go a different route altogether. Sometimes I'll run it in Streets and Trips and go with that one. Like they said, it's a tool, just a tool.
Just don't set it for Emergency Vehicle, like an ambulance, cause it'll take you the wrong way on One-Way streets.
Wanna see something funny, sit in Detroit, plot a route going to Windsor, only set it to Avoid Tolls. It'll get you from Detroit to Windsor, and it'll avoid all tolls roads and toll bridges. It's hilarious.