Did you subscribe to the NavTraffic before trying it? It doesn't work off the start, only the radio subscription is included in the free trial. If you visit
XM Satellite Radio - NavTraffic Market Coverage and click on your city it will show you which roads will have traffic information.
You wouldn't be paying for something you didn't use either, the basic radio package does not include NavWeather or NavTraffic, you must add this on if you want the service. (NavTraffic only, as I was not able to add NavWeather)
I just think it would be cool to change my morning iPhone weather check to looking at it in my car on the way to work. I'm kinda a features snob though, if it's possible to have it, I want it even if I won't use it a lot!