Most serious traffic accidents occur when drivers are making a left-hand turn at a busy intersection. When those drivers are also talking on a hands-free cell phone, "that could be the most dangerous thing they ever do on the road," said Dr. Tom Schweizer, a researcher at St. Michael's Hospital.
The researchers were able to show for the first time that making a left-hand turn requires a huge amount of brain activation and involves far more areas of the brain than driving on a straight road or other maneuvers.
When the drivers were also involved in a conversation, the part of the brain that controls vision significantly reduced its activity as the part that controls monitoring a conversation and attention was activated.
"Brain activity shifted dramatically from the posterior, visual and spatial areas [of the brain] to the prefrontal cortex,"...
... multitasking while driving may compromise vision and alertness. 'Hands free' not does mean 'brains free.'"
It seems that a cellphone conversation is significantly different from talking with passengers in the car:
[in the driving simulator] Drivers talking by cell phone drove significantly worse than drivers talking to passengers. The cell-phone users were more likely to drift in their lane, kept a greater distance between their car and the car in front, and were four times more likely to miss pulling off the highway at the rest area. [...]
When driving tasks got more complicated, drivers appeared to modulate the complexity of their speech, as measured by syllables-per-word. Drivers also talked more when using cell phones, perhaps, the authors speculated, because they were trying to control the conversation to avoid using the mental resources required to really listen to the other person.
Meanwhile, passengers took an active role in supporting the driver, often talking about surrounding traffic. That shared situational awareness could be helpful to the driver.