Most U.S. states ban hand-held cellphone use while driving but allow hands-free devices. However, hands-free devices are still distracting. Talking on a hands-free phone and driving is multitasking, and humans are not good at doing two cognitively demanding tasks at the same time.
For example, having a phone conversation in hands-free mode while driving causes you to stop looking out for hazards on the road and gets you into more close calls where you slam on the brakes than if you were not on the phone.
These detrimental effects last even after you end your call. There is a hangover effect: You can remain mentally distracted nearly 27 seconds after you finish using your cellphone. At 65 miles per hour, you’ve traveled nearly half a mile in 27 seconds.