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Unlimited data plans are slowing down mobile speeds for Verizon and AT&T customers, according to data released today by mobile network measurement company OpenSignal.

Verizon and AT&T reinstated their unlimited plans in February to compete with T-Mobile and Sprint, which have long offered unlimited data plans, and have since seen a deluge of demand.

Greater data demand — either more data usage or more customers — means slower speeds. Think of it as increased traffic on a highway. Verizon and AT&T also have nearly double the subscribers of T-Mobile and Sprint, so changes in their offerings hit their networks harder.

Carriers have long supported greater leeway to manage their networks as part of the U.S. government’s fierce debate over net neutrality. T-Mobile’s unlimited plan often limits video streaming quality in a bid to ease the burden on its network; others like Verizon recently have tested similar tools to improve speeds. To staunch advocates of open internet rules, however, these techniques violate the spirit of federal safeguards meant to ensure all web traffic is treated equally.

Both Verizon and AT&T saw a notable decline in speeds after introducing unlimited plans. T-Mobile and Sprint have been able to gradually account for the increase in data demand, so their speeds weren’t negatively affected this year — indeed, they both got faster since OpenSignal’s February report.

Update: A Verizon spokesperson contested the findings in an email, writing, “We constantly monitor our network performance. The launch of our unlimited data plans has not impacted wireless speeds on our network.”

U.S. mobile speeds are generally slower than other developed countries.

Verizon and T-Mobile were basically tied for speeds at the beginning of this year. Now, T-Mobile has taken the lead with an average LTE download speed of 17.5 Mbps, compared with Verizon’s 14.9 Mbps.

Unsurprisingly, unlimited data plans are very popular, as consumers spend more time and complete more activities on their phones. Ericsson expects typical North American smartphone users to blow through 25 GB of data a month in 2022, up from 5 GB in 2016, making unlimited data plans even more appealing. See how data traffic has already eclipsed voice:

For this report, OpenSignal collected over five billion measurements from 172,919 U.S. smartphones during the month of June with its signal mapping app.

Correction: This story has been corrected to show that North American data usage will increase from 5 GB a month to 25 GB, not Mpbs.

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