Our ability to make safe drivers of ourselves—to respond quickly and effectively to the signals we perceive—is largely dependent on our ability to perceive correctly. We can't make an appropriate response to various circumstances if we are erroneous in determining what those circumstances are. We know that vision is the sense by which we primarily take that data in, but we often forget just how vulnerable that information reception service is to compromise. Anything that interferes with our vision interferes with our ability to judge situations correctly, and thus to respond appropriately. Here area few factors that can diminish our ability to see perfectly as drivers.
Time of day. It may seem obvious, but visibility is greatly diminished in the evening and night. What is less obvious, however, is the impact of that decreased visibility. Despite that less people drive after dark than they do during daylight hours, still more than half of all fatal motor vehicle collisions happen after dark.
Speeding. You may believe that the extra 5 or 10 kilometers that you drive over the limit are harmless, simply because you haven't yet had any consequences attached to this behaviour. However, even minor speeding increases your likelihood of collision significantly. Part of this is owing to the fact that you are seeing less of your surroundings at higher speeds; there simply isn't time for you to take in the information correctly.
The sun's position. During the minutes surrounding sunrise and sunset, depending on the direction in which you are driving, the sun's rays can literally be blinding. You should always have protective eyewear in your car for such occasions. Having a dedicated pair that never leaves your vehicle will help ensure that you are never without them when you need them.
Your own prescription. Many drivers with weak prescriptions feel confident in driving without any corrective eyewear at all. This is a big mistake. When you deem that you can “still see” without your glasses or contact lenses, usually you are not doing so in blizzard or heavy rain conditions...but these extra layers of decreased visibility, in combination with your own starting disadvantage of having less than perfect vision, will have much more of a negative effect than if you were to wear your corrective lenses.
Mirror positioning. If you share a vehicle with another member of your household, particularly someone of a different height and weight than yourself, then all drivers of that vehicle need to ensure they are re-adjusting the position of the side and rearview mirrors every time they are about to drive.
Weather. Fog, heavy rain, and heavy snow are known to diminish visibility significantly. Obviously the ideal thing would be to avoid driving in such conditions altogether. However, for times when that cannot be avoided, be sure that you regularly maintain your vehicle so that it is suitable for driving in such conditions. Part of this involves ensuring that the following are in working order: all of your lights, your windshield wipers, your brakes, and your defoggers. Also ensure that you have adequate wiper fluid at all times.