Your phone company already limits your data. Your cable company could be next.

After years of quiet experimentation on Internet pricing, Comcast rolled out its latest trial in Atlanta (GA): an unlimited data plan that lets heavy users surf as much Internet as they want for an extra $35 a month. The voluntary add-on waives the 300-gigabyte monthly limit on data consumption that Atlanta residents have been subject to since 2013. It's a limited trial, but the economics of Comcast's unlimited plan make it a potentially dramatic shift in the way we'll buy broadband in the future. If Comcast decides it's working, other markets where the 300 GB data cap is in place could start seeing similar offers, and it's not crazy to think the plan might someday roll out nationwide. Here's why.

When Comcast surveyed these folks, it found that 60 percent were willing to pay a flat $30 to $40 a month extra to be freed from overage payments. Which is how we arrived at the $35 unlimited plan in Atlanta; also, a separate experiment launched this fall in parts of Florida that charges slightly less -- $30 -- for unlimited service. This is why the Atlanta trial is so significant: It costs $5 more. And this is how Comcast could turn the unlimited plan into a revenue source in its own right. Even if there are only 100,000 people in the entire country who behave this way, that's an extra half-million in free money for Comcast every month if they all signed up for the plan. More, if it can entice non-heavy users to buy into it, too.