BACKGROUND: Cell phones are everywhere, so scientists want to use them as tracking devices. Several state transportation agencies, including those in Maryland and Virginia, are starting to test technology that allows them to watch traffic patterns by tracking cell phone signals and comparing them to road grids. These new traffic systems can monitor several hundred thousand cell phones at once -- not private phone calls, just the radio signals emitted by the devices. The phones only need to be turned on, not necessarily in use. And advanced software now makes it possible to tell whether a signal is coming from a moving car or a pedestrian, for example.
HOW IT WORKS: Any cell phone that is turned on constantly interacts with cellular towers, which are placed every few hundred feet in a metropolitan area, or every half-mile or so in a rural area. In the new system, listening posts are placed throughout a city that can detect but not send radio signals. The listening post picks up a signal from a cell phone that's on and time-stamps the signal's arrival. By analyzing how long it takes the radio wave to reach the listening post from the cell phone, a computer can calculate almost precisely where someone is located on the highway. Three such posts are needed to determine a 2D position of a cell phone user. Radio tags, or transponders, can also be placed along highways to time when vehicles pass between those points. This data is fed into a computer system, which can then determine the car's location and speed. Collected information can be disseminated via Web sites, electronic road signs, or even registered cell phone users who sign up for customized traffic reports. By getting this information to commuters more quickly, they will have more time to react to traffic warnings and avoid congested areas.
TRAFFICKING IN PHYSICS: On a sparsely populated highway the cars are generally far apart, and can move at whatever speed they choose while freely maneuvering between lanes. A physicist would compare this to molecules in a gas, which are spaced further apart and move around randomly, only occasionally encountering other molecules. During rush hour, traffic density is much greater, so there is less room for cars to maneuver without risking collision, and the average speed is lower. Traffic is more like a liquid at that point. If the density of cars on the highway becomes too great, the flow of traffic freezes up: clusters of a "solid" can form, where cars are packed so closely together they can't move -- a traffic jam.
TRAFFIC FACTS: State and federal agencies spend $750 million a year on traffic monitoring with sensors, signal meters and other technologies. It is not yet clear how much a cell phone monitoring system would cost. The average American motorist spends 36 hours in traffic delays ever year. The cost of traffic congestion in the U.S. alone is about $78 billion, representing the 4.5 billion hours of travel time and 6.8 billion gallons of fuel wasted sitting in traffic.
The Institute of Electrical and Electronics Engineers, Inc., and the American Society of Civil Engineers contributed to the information contained in the TV portion of this report.