Hands-free is not risk-free

Just shut up, will you | The Economist

Good explanation.
"Dr Kunar reckons that having to think about responses during a phone conversation competes for the brain's resources..."

Back in my other life, when I worked, I had a hands free phone and would sometimes end the conversation and then look around in bewilderment; how did I get here?
A few minutes of conversation had erased all memory of the drive. Did I go through any stop signs? How fast was I going?