First people started to realize that drivers were distracted while using cell phones. Then everyone thought hands-free sets were the answer. Now the psychologists are saying that it isn’t that your hands are busy, it’s that when you’re on the phone you can actually fail to register something that you’re looking right at.
Why do phone conversations have such a distracting effect, but this doesn’t happen when you’re talking to someone sitting with you in the car?
I would think that it has something to do with the mind being occupied with sub-conscious visualization of the person to whom you’re speaking. When you speak to a passenger, your brain doesn’t have to add anything. But when you converse with someone not present, your mind wants to fill in the missing visual information.
This is similar to the odd behavior of turning down the radio when looking for a parking place. I do it, lots of people do it, but it doesn’t seem to make much sense.
Cell phones are a deadly distraction because our brains are circuits and we can’t have too many things going on at the same time because it ties up the “lines”; and they aren’t the only distractions. When you are on the phone talking you don’t have all your attention, “Sights, sounds, and mind, and reflexes” on the road incase you need to whip them out fast. The phone has these responses being used. Now before phone use in cars, people were having enough trouble with being sleepy, reading papers, putting on makeup, watching tv, eating, shaving, you name it.
Cell phones in cars will be cooler if we get to the point to having our cars drive for us. 
In addition to the visualization problem, I think that if you observed the dynamics of people converfsing in person in a car, you would find that the conversation level adjusts to the situation. When I am talking in a car and have to react to an upcoming situation, I stop talking and listening automatically. In the few instaces in which I have used my phone while driving, I noticed that his didn’t happen - I felt as though I had to either keep talking/listening or explain why I had stopped.
Also you might hear something nasty on the phone & take it out on your fellow drivers.
there are much more deadly distractions acording to AA like eating and changing a cd. That said I think the hands free adapter (HFA) is more distracting that just usinghte phone because:
1 for short trips you have 2 options (you are not going to hook up your HFA for a short trip- unless you expect a call)
_a pull over quickly and talk - not safe to pull over in the time it takes to answer the phone.
_b fumble around trying to get the HFA connected
_c Hi Opal
2 for longer trips where you attached a HFA - you have to fumble around looking for that cord thingie end so you can put the ear bud in and find the phone
3 Hi Opal
Hey a double Hi Opal!