How does Google Map Detect Traffic

How does Google Map Detect Traffic
Google Map predict traffic based on two information:

1. Historical data
Information about how much average time the user is taking to travel from one location to another on the road at a particular time. It also processes specific date like holiday etc.

2. Crowdsourced traffic data
Crowdsourcing means when Real-time data is sent by sensors and smartphones that report how fast cars are moving. Beside this, Crowdsourced Data also include data collecting by Cellular telephone companies while monitoring the locations of user phones. Trilateration is a tracking method whereby the distance to three or more surrounding cell phone towers is measured.

So what really Googe Map does?
It processes all the raw data about mobile phone and user locations. But wait a minute what if a postal vehicle sending its data. Then Google will exclude these kind of data because these type of vehicle makes frequent stops.
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