That’s the self-evident conclusion of a new study from the University of Utah’s Traffic Lab. Observing the behavior of undergraduate students parked in front of driving simulators and chatting on hands-free mobile phones, researchers found that they spent up to 31% more time tailing other slow drivers instead of passing in a faster-moving lane and drove–on average–2 mph slower than others. They took more time to regain freeway speeds after braking, too.

“At the end of the day, the average person’s commute is longer because of that person who is on the cellphone right in front of them,” said Dave Strayer, a University of Utah psychologist and leader of the research team. “That S.O.B. on the cellphone is slowing you down and making you late.”

Correction: those S.O.B.s. A January 2007 survey of 1,200 motorists by Nationwide Mutual Insurance Co. found that 73% talk on cellphones while driving. Which is lousy news for the 23% who don’t. “It’s a bit like breaking wind in the elevator,” Peter Martin of the University of Utah’s Traffic Lab told Reuters. “Everyone suffers.”