CarPlay Maps - awesome but traffic comes and goes?

Running CarPlay in a 2016 VW Golf - amazing functionality, love the integration with the car, messages, etc.

I use Apple Maps every day instead of car-based navigation. The views and directions work very well. But even though I have traffic display 'on' it only shows up in the map about 40% of the time.

Some days I get into the car and plug in the iPhone (5s with iOS9.2.1 - AT&T network) and maps comes up with traffic right away. It's a beautiful thing, I can see which of the three routes to work I should take immediately... but most days, no traffic indicators show up, even when I'm sitting in bumper-to-bumper traffic. Signal is good, coverage is good, clearly there are a lot of other drivers with devices to generate data (Raleigh-Durham area, NC)...

Does anyone have ideas why is the traffic data so inconsistent?