Hundreds of plowers protested by sending the phones back. If they made mistakes logging their time on the phones, they worried, they'd lose out on pay. And they resented the implication that they waste tax dollars. People were saying, " 'It's time that these guys ... got out of the coffee shops,' " complains Fred Nava, a veteran plow driver from Kingston, Mass. "That is a slap in anybody's face."

Just before the snow fell on Dec. 5, contractors agreed to use the phones on a trial basis. But from the highway department's vantage point - one shared by a growing cadre of employers - there's no turning back from GPS.

Whether they're dispatching a fleet of garbage trucks or a team of bail-bond collectors, managers love the idea of knowing where their "assets" are. They can coordinate routes that arebased upon traffic data, offer customers a narrower time frame for delivery, and, yes, make sure mobile employees are actually on the job when they're supposed to be. The popularity of GPS in the workplace parallels its growth in the consumer realm: By 2006, according to some estimates, 4 out of 5 new vehicles will come equipped with the technology.

For workers, GPS can add convenience and security. But as the plowers' protest shows, close tracking can put a dent in morale. Privacy experts warn that employees may not realize how much they can be tracked, especially if they use a company cellphone or a vehicle with GPS during personal time.

"You can start with the assumption that a certain amount of surveillance is necessary ... but the question really becomes, when does it go too far?" says Frederick Lane, author of "The Naked Employee: How Technology Is Compromising Workplace Privacy." Public discussions of GPS and other monitoring tools aren't keeping pace, he says, "and in the hands of some managers - who are very concerned about efficiencies and profits and so forth - it becomes more punitive than helpful."

Mr. Lane suggests that a boss might jump to conclusions if employees stop at a hospital or a family-planning clinic during lunch hour. Or that workers will face pressure to take fewer bathroom breaks. If GPS is misused, mobile employees may feel trapped in a type of high-tech sweatshop.

UPS workers in the Teamsters union negotiated last year to make sure GPS data wouldn't be used in evaluations. But such protective efforts are rare. "Unions have not felt they've been in a position to fight this ... when they're really fighting over jobs and working conditions," Lane says. "But stuff is creeping into the workplace that unions really would object to if they had time."

Companies that sell mobile-resource management (MRM) technology counter that education and privacy policies would put workers at ease. "When sales groups were [first] deployed with pagers [and later, cellphones], they were out in the field thinking ... 'My boss can call me at any time and he expects me to call him back!' Now we wouldn't dream of working without them," says J.D. Fay, vice president of corporate affairs for At Road Inc. in Fremont, Calif.

Cable installers, construction crews, and school-bus fleets are among the 117,000 subscribers of At Road's MRM services. For about $40 a month, At Road provides a real-time view of vehicle locations and other data over a secure Internet site. In client testimonials, a landscaping company reported saving $300 a week, and a bail-bond collector said his employees feel better being tracked because they carry cash.

Right now, restrictions on the uses of GPS technology are up to each company. Despite some congressional attempts in recent years, no federal laws protect workers' privacy in this realm.

"I regularly get e-mails from employees concerned about their employers tracking their whereabouts after hours with company cars or GPS cellphones," says Daniel Sovocool, a partner at Thelen Reid & Priest LLP in San Francisco. The legal advice he gives depends on several factors, including the company's privacy policy and whether the worker has the ability to turn off the location tracker. In some cases, even when the devices appear to be turned off, they still emit signals that can be detected.

Mr. Sovocool also urges employers to inform their staffs how they are being monitored and for what purpose, and then to stay within those boundaries.

GPS is spreading fast and is being heavily marketed to businesses, experts say. Lane cites estimates that by 2006, it will be at least a $34 billion industry. Like most technology, GPS has been getting smaller and cheaper. Units that are mounted in trucks can cost more than $2,000. But a 1999 federal law required cellphonemakers to phase in GPS to help emergency crews respond to 911 calls. The phones given to Massachusetts snowplowers - built for extreme weather conditions - cost the state just $90 each.

Privacy advocates hint at what's to come by pointing to cases in the consumer and workplace realms. A rental-car company in Connecticut, for instance, automatically withdrew money from customers' credit cards or bank accounts whenever its GPS-equipped cars exceeded 79 miles per hour. Eventually it was forced to stop because the practice violated state trade regulations.

In another case, cited in Lane's book, a transit-system employee had an ID card that doubled as a transit pass - and the locations and times of his rides were used against him in a murder trial.

Other experts argue that laws will catch up: "[As] these technologies become more widespread and more powerful, there will be people who will [push for] regulation to limit the power of the employer to use the technology in a way that's inappropriate," Sovocool says.

Technology itself may provide a faster fix than would government. Entrepreneurs have already developed a sleeve that fits over various devices to block wireless signals.

For municipalities that use GPS to improve public services, the privacy concerns aren't persuasive. Charleston, S.C., reportedly saw a 20 percent decline in overtime after installing GPS devices on its garbage trucks this summer.

Aurora, Colo., has a longer track record, using GPS to track street sweepers since 1998. "Within the first six months, we saw a 15 percent increase in productivity," says project engineer Lynne Center. The main goal had been to measure how many more workers were needed to keep up with population growth, but Ms. Center found that once they knew they were being monitored, they spent more time cleaning streets and less time at their base facility at the beginning and end of shifts.

The value of the GPS system was driven home to employees, she says, when a resident called to say a street sweeper had sideswiped his car. "We were able to prove that we didn't have street sweepers on that street that week," she says, so they easily avoided litigation.

Doug McKowen, who has operated street sweepers for 13 years, says the benefits of proving to complaining residents that work has been done outweigh having a boss look over his shoulder. Once, his manager questioned why the GPS system showed he was 15 minutes late after lunch. Mr. McKowen explained that he and his partner take walks during their breaks, and that day, they just had mistakenly walked too far.

"As far as Big Brother looking over you, of course it smacks of that," he says. "But if you're doing your job, you don't worry about that."

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