Let us draw a parallelism and look at the brain as if it were a CPU (in computing; central processing unit) of a computer, in which tasks are allotted percentages according to their priority and resources available, so the CPU needs to target those tasks and assign them different percentages and prioritize those that are more important.
The activity of a human brain can be compared to that of a computer in the way that handles and processes data. The human brain allocates percentages of memory assigned to each task just as a computer does, i.e., clusters or packets of information that need to be handled all at the same time. So if the owner of the mentioned brain performs too many or bigger tasks that exceed its capability of processing data, it is likely to overload (overheat) and might not be able to perform as intended anymore.
The brain will often resort to lowering the percentages of resources allocated to a particular cluster (borrowing) from it and redirecting them to another cluster that requires more horsepower. Complex tasks such as driving and conversing, particularly on the phone, are fine examples of actions that demand higher levels of concentration.
People can rarely decide or control their emotions and the way their unconscious mind will relate to some subjects that might be discussed during a conversation, which may very well turn out to be interesting, boring, upsetting, exciting, saddening etc. Emotions, for instance, may come between rationality and effective decision making. Naturally, this is involuntary, and the so-called unconscious mind might interpret certain topics as having greater relevance than others, ergo telling the brain to allocate far more resources and focus on certain activities than is advisable while executing something as risky as being in command of a 5000-pound steel monster on the road.
To conclude, for your own safety and that of your family’s, it is advisable to switch off cellular phones and not drink a drop of alcohol before going for a drive.