How does Google Maps know the traffic?

[Photo:Baidu.com]

Most people, before any journey, will have a brief glimpse at Google Maps for real time information. You may even use Google Maps as your satellite navigation outright.

Within a few seconds you'll know how long journey the take, what are your various route options, and where to expect heavy traffic.

But how does it know? How can it, before your local radio station dishes out the information from the traffic cameras?

Speakers: Li Ningjing and Sam Duckett

The audio clip is from Studio Plus, produced by CRI.