We’ve all had to swerve out of the way of someone who’s paying more attention to their cell phone than to the road. Presumably the problem stems from the fact that the person is driving one handed, dialing, looking at the phone, etc. So, on July 1, drivers in California will only be able to use ‘hands-free’ cellular devices. Makes sense, right?
Actually, tests of hands-free devices show that people are distracted by the actual act of talking on the phone, which people spend much more time on than dialing. Somehow this particular kind of multi-tasking causes people to miss important information in their visual field, such as roadsigns or unexpected cars.

But even if ‘hands free’ still causes problems, its better than not having any law, right? Maybe. But, if people believe that the only problem w/cell phones come from dialing and holding them, then they might abandon any extra care or attention they took before.

Strayer, D., Drews, F., and Johnston, W. (2003). Cell-Phone induced failures of visual attention during simulated driving. Journal of Experimental Psychology: Applied, 9, 23-32.