Imagine you are driving down the road, and, suddenly, a child darts out in front of your car. A simple model of how people act describes three stages: first, you perceive the child running, then you think about what to do–perhaps decide to swerve–and then you act, swerving the car out of the way. Each of these stages require a certain amount of your attention; each stage–perception, cognition, action–takes an increasing amount of your attention.
Turning your attention from one thing to another is called “task-switching.” The more tasks a person has to undertake at the same time, the more difficult it is for a person to achieve the tasks, not only because there are more tasks to do, but also because task-switching takes a certain amount of attention.
In some cases, people can do more than one thing at the same time. For example, they can keep a reasonably steady speed by applying pressure to the accelerator while scanning for oncoming traffic. This can only occur if different resources (your foot, your vision) are available for the tasks. It’s much more complicated than this, of course, because tasks are often comprised of other tasks, and if any subtask needs to share a resource, then task-switching has to occur. Further, some tasks are so well learned that very little cognition is required. For example, shifting from one gear to another becomes such a routine act that we find it difficult to describe.
The main task of driving a car is to arrive at a location safely. Other tasks, such as eating a pizza, listening to our e-mail, turning off the radio, or talking to the kids in the back seat, are potential distractions from this. In the simple model of human attention processing we have given, we want to minimize the number of tasks and task switches that occur. Of course, people will do other things than drive, but it’s worth noting that when car radios were first being installed in automobiles, there was a big debate–reminiscent of today’s debates about cell phone use–as to whether they ought to be banned as a dangerous distraction.