You’ve seen them puttering along in the passing lane (if you don’t know what the passing lane is, it’s the left lane, but that’s a whole other post) talking on their cellphones while cars line up behind them waiting for them to move. Cell phone drivers seem to slow things down, but here’s actual proof:

All those drivers you see chatting on cell phones are slowing down traffic, even if they’re using hands-free devices, according to a new study from University of Utah psychology professor David Strayer. The research, based on simulator driving, found that motorists on cell phones go about two miles per hour slower on clogged roads and don’t keep up with the flow of traffic. They are far more likely to stay behind a slow car and they change lanes about 20 percent less often. Overall, cell phone drivers took about three percent longer to travel the same highly-congested road and about two percent longer on a medium-congested road. Strayer said that, with about one in 10 drivers on the phone, that can really add up.