Scientists have long known that regardless of what we want to believe about multi-tasking, our brains are generally not capable of focusing on more than two things at any given time. This is the way most meditation works—you focus on your breathing and some other task, such as counting backward, and your brain becomes emptied of all other stressful thoughts or ideas.
If you are driving and talking, you have no room left for any other activity. It could be something as simple as looking for the landmark or street where you want to turn. How many times have you driven past your exit while talking on the phone? That’s because your brain is already at its threshold. Missing your exit may not be a big deal. You can turn around and get to it. But what if there’s an unexpected act by another driver or another person. Someone walks out in front of you or another driver fails to stop at a light or traffic sign. Before you can take appropriate measures, you have to stop talking on your phone. By then it may be too late.