Relationship Between Visual Function and Driving Performance: Part II

Night Vision

It is often found that the accident rate relative to distance traveled during the hours of darkness, is greater than that during the hours of daylight. Accidents occurring at night are also more likely to result in fatalities or severe injuries, and the majority of accidents to pedestrians occur at night.
The increase of potential accidents during the hours of darkness is partly due to poor street lighting and to the glare from oncoming headlights reducing visibility.

A driver with 6/12 day acuity will require 5 to 100 times more illumination to detect a given target than he would have required if he had day time acuity of 6/6. This means that 6/12 by day cuts the critical perception distance in half compared with 6/6. At night it may cut it to one-tenth or less. Therefore, drivers with a daytime visual acuity of 6/12, should not be permitted to drive at night at all.

Night Myopia

A phenomenon known as night myopia may also influence night driving. People become relatively more short-sighted at night. This myopic shift can be as much as 4.00D, although it varies from person to person and with the level of lighting. There may be quite marked short-term variations and the dark focus can also be affected by non-visual factors such as psychological stress. Hence, there are problems in prescribing spectacles for night driving:

• Several refractions on different days may be required to determine the representative dark focus to take into account patient variability; and
• The prescription found will be appropriate only for a limited set of conditions.

The Association of Optometrists has made a statement about night myopia and prescribing a correction. It suggests that the variable nature of the myopia induced at low levels of illumination creates a major problem in prescribing a correction. When driving at night there is never total darkness, as there is light from car headlights, street lamps, etc. Under conditions of low illumination (mesopic level) the degree of myopia induced will be reduced and, as stated, it will vary according to the lighting level so that a single prescription will not be suitable. 

Also, if a correction is provided there may be problems for the driver when traveling into a well lit area. The extra negative power of the correction may become a hazard because it may induce confusion, misjudgment of distances and speed, etc. The difficulties encountered by a driver at night are not just due to myopia; there are other factors, such as increased reaction time and increased glare recovery time. Therefore, the provision of a correction is not a simple answer to the difficulties experienced by many when driving at night.

Glare

Many drivers complain about glare during night driving. It is due mainly to dirt and scratches on the windscreen causing scattering of light. Smoke from cigarettes can cause an increase in glare from oncoming headlights at night and will cause veiling glare. Windscreens should be replaced every 50,000 miles due to wear and tear. Tinted windscreens and the use of tinted spectacles are detrimental to those visual functions that are important for night driving:

1. Glare recovery, i.e. speed of retinal recovery after being dazzled by glare;
2. Glare resistance, i.e. the ability to see against glare;
3. Visual acuity, i.e. the ability to see efficiently in low illumination.

Recommended: Glare and Its Control

Tinted spectacles reduce the visual acuity during day and night driving conditions. Visual acuity is found to decrease in proportion to the reduction in transmission of light and is not affected by the colour of the tint.
Yellow night driving glasses are popular because they reduce glare. However, although the tint does diminish the amount of light entering the eye, and so reduces glare, other areas of the visual field can be covered with a dangerous level of darkness; their use is not recommended. They give the subjective impression of brightening the overall scene whilst, in fact, reducing the contrast of objects against an already dark background.
Similarly, tinted windscreens are also detrimental to visual functions and are not recommended for night driving.

Continue from Part III
Back to Part I 

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