IMO, only God can report accurate traffic.
For what it's worth, I had XM for 2 years. The traffic they provide is nothing short of "worthless". Living in the NY metropolitan a 2 mile backup can occur over a flat tire changer on the shoulder and takes approximately 4 minutes to build. By time XM would get around to reporting it, the car was at STS, on the lift and new slicks installed.
If you want up to date traffic, go drive around. FM radio is full of live "free" traffic reports that are semi-accurate. And when you see news choppers buzzing an area, there's a tie up, betcha!
So, save your money on XM if all you're looking for is traffic updates. Get it for the music or sports, okay.
Ed