> I'd imagine Google et. al. are collecting that data [...]

They are. All phones who have Google Maps installed, and have enabled reporting (anonymous) position data (might be default on some phones) are helping Google both see if there's a traffic jam somewhere, and see what the actual driving speeds on the roads are (by sampling at a certain interval and seeing distance traveled over time).

I happily contribute to this, I think it's a great idea to crowd-source traffic data. I do hope Google makes the data publicly available though.