clipped from tech.yahoo.com

The latest research by Carnegie Mellon University questions hands-free laws, and whether their existence only gives drivers a false sense of safety. To test this theory, neuroscientist Marcel Just studied 29 volunteers who used a driving simulator inside an MRI brain scanner. The volunteers were tasked with driving a car along a virtual winding road, with and without distractions.

Researchers found that drivers would hit the guardrail and veer out of the center of the lane more often when they tried to answer true or false questions. Brain scans of the distracted drivers showed brain activity decreased in areas that process visual and spatial information, functions that are crucial for navigation. Once again, this proved that participating in a conversation is all it took to reduce a driver's focus on the road.

As reported in This is London, Just concluded:

Drivers need to keep not only their hands on the wheel, they  also have to keep their brains on the road.