The Internet “is how we deliver our shows,” said Jim Louderback, chief executive of Revision3, a three-year-old media company that runs what it calls a television network on the Web. “If all of a sudden our viewers are worried about some sort of a broadband cap, they may think twice about downloading or watching our shows.”

Even if the caps are far above the average users’ consumption, their mere existence could cause users to reduce their time online. Just ask people who carefully monitor their monthly allotments of cellphone minutes and text messages.

“As soon as you put serious uncertainty as to cost on the table, people’s feeling of freedom to predict cost dries up and so does innovation and trying new applications,” Vint Cerf, the chief Internet evangelist for Google who is often called the “father of the Internet,” said in an e-mail message.

But the companies imposing the caps say that their actions are only fair. People who use more network capacity should pay more, Time Warner argues. And Comcast says that people who use too much — like those who engage in file-sharing — should be forced to slow down.

Time Warner also frames the issue in financial terms: the broadband infrastructure needs to be improved, it says, and maybe metering could pay for the upgrades. So far its trial is limited to new subscribers in Beaumont, Tex., a city of roughly 110,000.

In that trial, new customers can buy plans with a 5-gigabyte cap, a 20-gigabyte cap or a 40-gigabyte cap. Prices for those plans range from $30 to $50. Above the cap, customers pay $1 a gigabyte. Plans with higher caps come with faster service.

“Average customers are way below the caps,” said Kevin Leddy, executive vice president for advanced technology at Time Warner Cable. “These caps give them years’ worth of growth before they’d ever pay any surcharges.”