It should take no more than a story like the one about a 13-year-old Massachusetts boy killed by a hit-and-run driver whom police suspect was text-messaging while driving to convince us all to not mix cell phones with driving.

But if you think that's too tragically unusual an event that would never happen to you, maybe this will have some sway: Drivers on cell phones slow down commuting traffic, according to a study by a University of Utah professor.

Drivers talking on cell phones, even with hands-free devices, travel at speeds about 2mph slower than drivers not on phones, according to observations of three dozen students driving in simulators. David Strayer, the professor who will present his findings to the Transportation Research Board of the National Academy of Sciences, figures being in traffic with cell phone chatters can add 20 hours a year to an hour-a-day commute.

Much as we're sure we're so good at multitasking, doing a few things at once has to take a toll. The good news, I suppose, is drivers slow down rather than speed up while talking.

LINK: Study: Drivers on cells clogging traffic [AP]