how exactly does google map traffic work??

At first i don't know about how google is able to get traffic updates so exactly. I was wondering at that time how it get traffic info of every street & corner of every country. It is not possible to the team of google to see every corner of the country with camera from satellite. Then i thinking but i'm not able to understand it.

One day i am going to my university for the first time, so I used google navigation to reach my destination. At that time I again started thinking the same 'how exactly does google map traffic work?'. Then i got the answer, How google is able to doing that.

The major access to the google for information by any means is digital devices only, in them they used mobiles to track the traffic by Global Positioning System(GPS). They track the GPS of the phones on the road. The points they take into consideration is the density of the phones on a road and the speed of the phones. They will take the average speed of phones on a road.

To explain this
1.let us consider a obstacle(large rock) on a road

while crossing it everyone drive slowly at that region. when many people did the same google maps says that you have to slow down at that region in other words there is little traffic at that region.

2.let us consider a like this

Here every person (Mobile) moves very slow which implies a heavy traffic