The study, conducted for the AAA Foundation for Traffic Safety, confirmed previous research suggesting talking on a hands-free cellphone was just as distracting as using a hand-held phone while driving, a university release said.
"Our research shows that hands-free is not risk-free," lead study author David Strayer, a University of Utah psychology professor, said.
"These new, speech-based technologies in the car can overload the driver's attention and impair their ability to drive safely," he said. "An unintended consequence of trying to make driving safer -- by moving to speech-to-text, in-vehicle systems -- may actually overload the driver and make them less safe."
The potential for dangerous mental distractions exist even when drivers keep their hands on the wheel and their eyes on the road, the researchers said.
As mental workload and distractions increase, reaction time can slow, brain function can be compromised, and drivers tend to scan the road less and miss visual cues including stop signs and pedestrians, they said.
"There is a looming public safety crisis ahead with the future proliferation of these in-vehicle technologies," AAA President Robert L. Darbelnet said. "It's time to consider limiting new and potentially dangerous mental distractions built into cars, particularly with the common public misperception that hands-free means risk-free."