Nav traffic poor coverage

While spending hours yesterday stuck in traffic on the way back to LA, I was very disappointed to find the Nav traffic system only functions within metro areas. I had naively assumed that, given the monthly fees involved, it would cover freeways and other major routes nationwide, so at least giving me the same coverage I can get from Google Maps for free.

Are other GPS traffic systems similarly limited, or do some provide better coverage? I'm wondering if it's worth buying a third party unit just for longer trips like this, as it would really have been helpful to have seen the state of things up ahead this time so we could have adjusted our schedule or route accordingly. Having become accustomed to the benefits of the system, it's absence was all the more painful.

The funny thing was that it did suggest rerouting to avoid traffic, but it only changed the very end of the trip, several hours away, because that was the only part of the route for which it had traffic information.