Just a few years ago, GPS's were the hot thing. Over time, some providers started to combine the GPS software with traffic information...
And then, smartphone providers started to provide GPS services for a fee. This had the added benefit of up-to-date road maps.
The Google Maps app available on many smartphones now provides these features for free (with your smartphone's data-plan + the smartphone's GPS). We've tested the "Google Maps app" -- with traffic enabled -- and the results have been very accurate (especially in metro areas).
Here's an interesting article on how traffic data is collected.