Comcast executive vice president David Cohen. Getty Images Comcast is testing data caps for its broadband internet customers that would limit them to 300 GB of data per month, according to The Times Leader.

Comcast has started open trials of its home broadband data caps in several large markets around the country.

Here are all the regions affected:

Mobile, Alabama

Huntsville, Alabama

Tuscon, Arizona

Atlanta, Georgia

Augusta, Georgia

Savannah, Georgia

Jackson, Mississippi

Charleston, South Carolina

Knoxville, Tennessee

Memphis, Tennessee

All of central Kentucky

The entire state of Maine

Comcast will limit those residential customers using its Xfinity Internet Service to 300 GB per month. They’ll have to pay $10 for each additional block of 50 GB they use.

Depending on one’s compression stream, Comcast spokesperson Charlie Douglas told The Times Leader that 300 GB could get you “more than 230 hours of high-definition movies, or also 575 hours of standard-definition movies, 40,000 to 93,000 ebooks a month, [or] 585 million tweets.” Douglas also said people that use less than 5 GB of data a month can opt into the program and save $5 a month.

Comcast doesn’t believe many are will exceed the 300 GB limit. In an interview with Ars Technica last year, Comcast said “98% of our customers nationally don’t use 300GB/month.” And in a conference call with shareholders last month, Cohen reiterated those claims.

I would also predict that the vast majority of our customers would never be caught in the buying the additional buckets of usage, that we will always want to say the basic level of usage at a sufficiently high level that the vast majority of our customers are not implicated by the usage-based billing plan. And that number may be 350 -- that may be 350 gig a month today, it might be 500 gig a month in five years, but it will never -- I don't think we will want to be in a model where it is fully variablized and 80% of our customers are implicated by usage-based billing and are all buying different packets of usage.

During last May's call with shareholders, Cohen said the new data caps are trials that may never reach the rest of the U.S., as the company doesn't want to "turn people off to what arguably could be the most important product that we offer." But in that same conference call, Cohen also said “people who use more should pay more and people who use less should pay less.”

Comcast, as well as Time Warner Cable and Verizon, are still lobbying the FCC into creating an “internet fast lane” so web companies can pay them more for better service.