For the unconscious part of the brain, it can multitask highly effectively. For the conscious part of the brain, your ability to multitask is similar to that of a single core CPU, just not nearly as efficient. You rapidly switch between devoting your attention to multiple things.
Depending on how engaged you are, you can effectively do it by devoting more resources to more important tasks.
Eg how do people play very complex games while at the same time in a voice chat planning things with the team.
Based on the hands free ban, you would think that voice chat will make you less effective as a gamer.
Also how is hands free different from the driver talking to the passenger in the car? are they going to ban that also?
just about all of the clear cut crashes involving anything relating to a communication device is based on people texting while driving or hoolding the phone up to their ear while driving.
The motor function in the brain is not a very effective multitasker. To test this, try this, with your left leg, make counterclockwise circles and with your left arm, make clockwise circles. (I bet you cant)
when you are preoccupying your motor functions, you are reducing your physical ability to quickly interact with the car.
With hands free calling, you are for the most part using 2 different parts of the brain with only a small amount of overlap in the prefrontal cortex (which is designed to provide some multitasking abilities)
Multitasking breaks down when you try to make certain parts of the brain multitask, eg listen to speeches at the same time and write a summary of both speeches,
you wont be able to effectively do that, but at the same time, you can listen to someone say something to you and clearly understand it in a room with 100 people talking at the same time.
Now lets test the visual part of your brain. take 2 computers, use the mouse from computer 2 and the keyboard from computer 1.
Put up a simple racing game designed to work with the WASD keys, and on the second PC, launch audio surf. (you will do poorly in both games)
The visual part of the brain can only focus on a small area of any scene. so attempting to multitask will require you to rapidly change focus. If you cant do that test, try this. look at the first word in a sentence and try to read the rest with out moving the center focus of your eye from the first work, (you may make it to 4 words before you cant read further without moving your eyes no matter how close or far you are from the text).
With the significant limitations on the visual functions of the brain, you can easily see why texting and driving is dangerous.
talking while driving is no more dangerous than listening to the radio while driving as for the most part with both task, you are adding a processing load to different parts of the brain (provided you use hands free so you are not adding additional load to the motor functions of the brain)