Hands-free devices still cause distraction

Using hands-free devices to talk on the phone, text or send emails or give some voice-activated commands while driving is distracting, and can "overload" a driver's attention span, hurting their ability to drive safely, a new study says.

Smartphone or in-car speech-to-text systems, especially, can lead "lead to significant levels of distraction," David Strayer, the University of Utah psychology professor who led the study, told NBC News.

"There may well be some things that are reasonable to do in the car with voice commands, things like say 'turn the heat up' 'turn the windshield wipers on' — commands that are short and simple, relatively infrequent and not too distracting," he said.

Read More.