Probably uses the google maps info for live traffic updates.

These days almost everyone has a smart phone and in this country by and large it's android so I suspect that android phones report back their position (not sure how often) and this can determine if your sitting still in traffic or travelling along.

Now upscale that by a few million phones and now you have up to date traffic conditions on the roads

I use TomTom MyDrive on my PC. As it maps congestion on even minor side-roads I assume it's using mobile phone data.

It's accurate provided you take into account that it always lags behind the current situation by several minutes at least, which is worth bearing in mind when checking what parts of your journey will be like by the time you actually get there.

In other words, you have to know the current trend - e.g. is it towards the end of the rush hour? Or is it early rush-hour and therefore likely to get worse? If your route passes a school, how close is it now to pick-up/drop-off times?

Little infra-red sensors fitted on bridges over traffic lanes.