Passing and changing lanes in busy traffic requires accurate judgement of distances between moving objects. Both eyes need to function properly as a team for reliable depth perception. Depth perception deficiencies are common in drivers and the problem gets worse as speed increases.

The ability to see out of the corner of your eye to see over a large area without moving your eyes or head is an important part of safe driving. It enables a driver to see cross-road traffic and pedestrians at the roadside without looking away from the road ahead. Normally the field of vision is about 180 degrees. It is reduced with increasing speed and is only 40 degrees for distant objects at a speed of 100 kilometres an hour.

Good muscle means that both eyes can be easily pointed simultaneously at a given object. It is essential for good two-eyed vision, depth perception and field of vision. Although drivers usually can compensate for muscle imbalance under favourable driving conditions, the effort involved may take its toll on fatigue and discomfort. Alcohol, tiredness and drugs can upset muscle balance so that a slight imbalance can become unmanageable.

A driver needs to be able to change focus quickly and easily from the road to the dashboard and back again. This ability to change focus from a far object to a near object and vice versa is called accommodation.

Safe driving at night requires the ability to see in low light beyond the range of the car headlights. It also requires the ability to recover quickly from the glare of oncoming headlights. Night vision deteriorates rapidly after 40 years of age. The best glare resistance is in 20 to 30 years old drivers. Older drivers can compensate to some extent for the reduction in the quality of their night vision by driving more slowly.

Sunglasses should never be worn while driving at night because they dangerously reduce the ability to discern cars, pedestrians and road hazards.

Colour plays an important part in road safety. Drivers need to recognise various colours and distinguish them from others. Drivers who are aware of colour vision deficiencies can learn to distinguish traffic signal lights by their position and differences in brightness. Changes in vision too often go unnoticed by drivers. Most drivers have defects that once detected can be corrected or for which they can compensate.