According to the latest study, drivers talking on cell phones drive slower on busy commuter roads than people not on phones. David Strayer, a psychology professor at the University of Utah, wrote the report and found that on average drivers on cell phones, even those using hands-free devices, go about 2 mph slower than people not on phones, failing to keep up with the flow of traffic.

Strayer’s study, which will be presented to the Transportation Research Board of the National Academy of Sciences, is based on 36 students driving in simulators.

In the study, he found that drivers on cell phones took about 3% longer to complete the same highly traffic-clogged route (and about 2% longer to drive a medium congested route) than people who were not on the phone. Strayer claims that with one in 10 drivers on the phone, cell phones could be adding an extra 5% to 10% to any commute.