With experience, some driving tasks don’t require every eye fixation to be directed only toward traffic and the path of the car.

In this situation, for example, a driver is concerned with avoiding a pedestrian, but is still able to maintain control of the vehicle by using only very brief glances at the intended path of travel.
Here, a driver has developed the ability to use side vision to judge distance.
Here a driver was told to fixate on the car on the right while maintaining proper distance behind the car directly ahead. Her central vision remains on the car to her right but she‘s still able to follow the car in front without looking directly at it.

Let’s look at two examples in which drivers are functioning at less than top efficiency.

This driver is tired. Notice how his central vision tends to fixate lower and to the right, often dipping all the way down to the road surface.
This driver is mildly under the influence of alcohol. His example allows us to answer our third question – What is the effect of alcohol on visual perception?

Even though he is not legally drunk, after only one drink the effects of alcohol on his eye habits are readily noticeable. His concentration is failing, his eyes are staring straight ahead, and his central vision is stuck squarely on the center of the road. He notices nothing on either side, which creates a very dangerous situation, both for himself and every other driver on the road.