Some people say that drivers should be prohibited from talking on hand-held cellphones while driving, lest they become distracted, slow down traffic, or worse, cause an accident. But there are others finding that cellphones and driving may not be so bad together. Several state transportation agencies in the United States, such as 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 agenciesâ€”a development that deeply troubles privacy advocates. 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. Any cellphone 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. The monitoring software instantly analyzes those movements. These systems, state traffic administrators say, can notably improve traffic surveillance, and reduce costs by replacing outdated systems using video cameras and sensors embedded in roadways, which cover only limited spots. 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. Another advantage privacy advocates envisage is that the traffic monitoring could be the beginning of government use of cellphones to track someone's movements. Even if the tracking is done anonymously and in clusters, they say, it could allow federal and state officials to track where people are headed en masseâ€”to know, for instance, where protesters are gathering. Cy Smith, AirSageâ€™s chief executive, said he understood that some people were concerned about privacy, but he stressed that his company did not receive information from cellphone carriers that would allow it to pinpoint the location of an individual. â€œNo person's individual information gets out of that person's cellphone network,â€? he said, then adding, â€œNobody gets a speeding ticket.â€?