When I'm driving, I don't use my cell phone, but many of you do (legally, of course with a hands-free device). And it's not just your imagination that the car ahead of you is moving slower when the driver is using a cell phone...it is!

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According to a study by psychology professor David Strayer of the University of Utah, people on cell phones drive about two miles an hour slower and change lanes 20 percent less often than other drivers. That goes for those on hands-free cell phones as well.

All in all, the study estimates that following drivers on cell phones can increase your commute time by five to 10 percent. That may not sound like sound like much, but that adds up to roughly 20 hours a year.

Remember to use a hands-free cell phone device while driving, and make sure your driving is more important than the phone call.