I was passenger today and got to finally play with this app. While I know the route by heart, I was amazed how accurate the GPS locator was. Not down to within 20-25 meters, but right on. It also established the rate of travel. Just plug in the destination, mount the BB to a dash holder and follow the directions.
I got this app from AppWorld for my wife for her travels required by her work. I am VERY impressed.

It's free and you can get it from AppWorld by searching for Blackberry traffic.