Twitter, smartphones can help determine crowd size
 

Data from smartphones and Twitter can accurately show the size of a crowd, helping first responders in an emergency, a study on Wednesday said. Scientists at England's University of Warwick devised a computer model based on data from Twitter and from phone companies in Milan, northern Italy.

The mobile phone system is cellular, meaning that it is a grid comprising pockets, or cells, where users are connected via a relay antenna.
When there are more users in the cell, this shows up in a spike in the volume of phone calls, SMS messages and tweets written on mobiles.

Amazing combination of smartphone and twitter very helpful to know about the traffic easily.