How does traffic data work?

It's probably looking for larger correlations, like multiple phones on the same stretch of highway all reporting similar speeds.

A single phone that's reporting that it's stopped near a road probably isn't going to affect things.

 

On freeways they have sensors embedded into the road way. Most surface streets have sensors that tell the stop lights that there are cars waiting to go. I imagine they can determine traffic speed/congestion, but have no immediate answer.

Apps that use crowd sourcing don't rely on public/city/state/national provided traffic data, or use it in conjunction with that data. I don't imagine your phone, sitting at home and browsing the maps app would alter traffic data, there's likely an algorithm that sorts the outliers (phones that are too far from a main road or haven't moved in x minutes) and ignores them.