Shields said the woman told him, "My husband was saying he was working late and it turned out he was going to the Holiday Inn. Now he's living at the Holiday Inn."
In 2002, Wherify Wireless Inc. of Redwood Shores, Calif., debuted a GPS wrist device -- which looked like a gaudy digital watch -- for tracking children. The company declined to say how many it had sold, but one was bought by Zittrain.
"My dog had gotten lost a couple times," he said. "I put it on her collar."
Cellphones entered the picture in 2001, when the Federal Communications Commission ordered mobile telephone carriers to add technology to handsets that pinpoint their location. The idea was to make it easier to track 911 emergency calls, which increasingly come from cellphones.
Some carriers adopted technology that used signals from cellphone towers to determine location. Others, including national carriers Verizon Wireless, Sprint and Nextel, went with GPS.
Although Nextel is the only national carrier to offer GPS services, all new phones sold by these carriers are GPS- equipped. By the end of 2005, companies that chose GPS are supposed to have converted at least 95% of their subscribers to the phones, although some carriers have indicated they will ask the FCC for an extension.
Even without the government regulations, GPS probably would have made its way into cellphone handsets eventually, said James Dempsey, executive director of the Center for Democracy and Technology in Washington.
"The commercial value of location services is so valuable, we would probably still be seeing a proliferation of them anyway," Dempsey said. In addition to locator services, Nextel offers a function that gives driving directions. A Sprint spokeswoman suggested that one day users could buy a movie ticket and then automatically get directions to the theater.
Joe Betar just wanted to know where his 13-year-old daughter was.
The owner of a Utah car dealership had already raised two teenagers. "There were numerous nights when they were not home when they were supposed to be," he said. "We would lie awake worrying about them. I ended up driving around, looking for them."
So when his daughter wanted a cellphone, Betar picked one out -- with a subscription for GPS tracking. He didn't tell her about it.
"If she knew, she might be tempted to just leave it in some location," Betar said.
For Mark Frankel of the American Assn. for the Advancement of Science in Washington, that crosses a line. "If a parent gives a teenager one of these phones and tells them, 'It has the ability to track you,' it can carry the message 'We are concerned about your safety,' " said Frankel, who is director of the group's Scientific Freedom, Responsibility and Law Program.
"But it troubles me that someone would be tracked without their knowledge, outside of a criminal situation. When the child finds out about it, and there's a good chance they will, it's a betrayal. It carries the message 'I have no trust in you.' "
Frankel said that part of being a teenager "is to develop an independent personality. And part of that is privacy."
Tom Pratt has no such qualms. He told his two children about the GPS units in their mobile phones. But he said being a kid today is far more dangerous than when he grew up on Long Island in New York.
"Back when I was a kid, on a Saturday you left home when the sun came up and then came back home when it was time for dinner," he said. Now he worries about his 12- and 13-year-olds, and he pitched the GPS unit to them as a way to give them more freedom.
"We told our son, 'You don't have to call home every hour anymore,' " Pratt said.
Danielle Butler, whose road trip was interrupted with the warning about speeding, is practically an adult. But she said she hardly thinks about the phone that allows her to be tracked. "I don't mind," she said. "I have nothing to hide."