California just passed legislation which requires the use of hands-free devices by 2007. It seems to me more a move to satisfy device manufacturers than a call for driver safety. Sadly, it doesn't seem like we can restrain ourselves from using phones while on the road, like we can't bear driving without music, food, children, and so on.

As for detecting "road noise", humans do a much easier job of this than do machines. Half the problem is that what we consider noise is represented by a wide range of audio waveforms. On top of this, the noise sensor would have to pick this noise out of normal speech, wind noise, static, and other ambient sounds, all of which in some way resemble road noise.

I have read about studies that indicate a hands free cell phone is just as dangerous for driving as hand held. Apparently the attention drawn away from driving is about equal in either case.

Seconding what sand said. Study after study is showing that talking on a cellphone, period, impairs your ability to drive as though you were over the legal alcohol limit.

While I'm not a proponent of using phones while driving, there are some people who are capable of paying attention to the road while holding a conversation. Perhaps if these people could prove this ability in a road test, they could be granted permission to use cell phones in the car.