Several state transportation agencies, including those in Maryland and Virginia, are starting to test technology that allows them to monitor traffic by tracking cellphone signals and mapping them against road grids.

The technology underlines how readily cellphones can become tracking devices for private companies, law enforcement and government agencies – a development that deeply troubles privacy advocates.

These new traffic systems can monitor several hundred thousand cellphones at once. The phones need only be turned on, not necessarily be in use. And advanced software now makes it possible to discern whether a signal is coming from, say, a moving car or a pedestrian.

State officials say that the systems will monitor large clusters of phones, not individual ones, and that the benefits could be substantial. By providing a constantly updated picture of traffic flow across thousands of miles of highways, they maintain, cellphone tracking can help transportation agencies spot congestion and divert drivers with radio alerts or updated electronic road signs.

Next month, Maryland, with the help of the University of Baltimore, plans to begin tests for a cellular tracking system in the Baltimore area. Virginia also plans to test a system around the Norfolk beltway. Missouri says it is about to sign a deal that will allow it to monitor traffic movements over 5,500 square miles of state roadways. Similar mapping technology is in use in London, Tel Aviv and Antwerp, Belgium.

“The potential is incredible,” said Phil Tarnoff, director of the Center for Advanced Transportation Technology at the University of Maryland. He said the monitoring technology could possibly help reduce congestion in some areas by 50 percent.

But he and others involved in the emerging technology said there were critical hurdles. Chief among them, Mr. Tarnoff said, is getting the cellular carriers, which have been distracted with mergers and customer service problems, to collect and share the cellphone data.

The carriers already collect an enormous amount of data; they can tell, for example, whether a cellphone user is roaming out of their network. But separating the data to show the speed at which cellphones are being passed from one cell tower to another is still a challenge.

To get the data, the monitoring companies have to reach agreements with cellular carriers – presumably, they would pay a cut of their revenue to the providers. But whether they can be profitable or make being involved worth the while of providers is an open question.

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