Relationship Between Visual Function and Driving Performance: Part III

No driver should wear tinted spectacles at night because of the reduction in visual function, which is further reduced by tinted or dirty windscreens and by age. There is no real necessity to have a tinted windscreen.
Although drivers will argue that their vision is more comfortable with the tint during the daytime, there is a potential night-time hazard caused by loss of light transmission. Even the angle at which the windscreen is mounted can reduce the amount of light transmitted. 

A tinted windscreen is also supposed to absorb heat and reduce glare. However, it is very inefficient in performing both of these functions; only 25 percent of solar heat is absorbed. It would be better to paint the car in light colours and insulate the roof and floor, as this would reduce the heat by about 44 percent. It should also be noted that there is not enough reduction in transmission of light to help reduce glare on a bright day. In bright conditions it would be better to wear tinted spectacles.

Improving the standard of street lighting on trunk roads reduces the number of accidents and saves money, despite the cost of installation. Road lighting is designed to provide a bright visual background against which drivers see a potential hazard as a dark silhouette. On a well lit road the use of dipped headlights does not generally help drivers themselves for the following reasons:

1. Dipped headlights act as glare source to all road users;
2. Pedestrians and drivers can be dazzled by them;
3. The silhouette contrast is decreased which, in turn, reduces the visibility of hazards, etc.

Other Factors

Alcohol
Alcohol impairs mental efficiency, acts as an anaesthetic and slows the response to a hazardous situation. It also can cause diplopia and blurring of vision. Moreover, the effects of smoking are summative with the effects of alcohol.
The 20-29 age groups have the highest percentage of fatalities due to illegal alcohol levels. However, there is an encouraging downward trend in the younger age groups in drink-drive accidents. Whilst these findings are encouraging, drinking and driving is still a serious problem. It is estimated that one in six of deaths and one serious injury in eleven is associated with illegal alcohol levels.

Recommended: Spectacles For Vehicle Drivers

Age
Visual functions change with age. There are changes in visual acuity, contrast sensitivity, visual fields, glare sensitivity and other visual capabilities. Many eye disorders that cause a reduction in visual acuity or visual fields are prevalent amongst the elderly, e.g. cataracts, open angle glaucoma and macular degeneration. Therefore, one might expect to find a higher accident rate amongst the elderly. However, a higher accident rate may not be due primarily to poor vision, but to other factors.

Accidents occurring to younger drivers who had good vision may have been due to inexperience, reckless driving or alcohol abuse. Older drivers may have poorer vision but there are other factors, such as slower reaction times and duller hearing which confuse their assessment. Hence poor vision cannot be stated as being the major cause of accidents in the elderly and improving their visual performance will not necessarily improve the accident rate.

Several surveys confirm the belief that older drivers travel at slower speeds and that they try to restrict their driving to low stress environments by, for example, avoiding driving at night or on icy roads. However, despite this, accident rates for failing to give right of way, improper turning, or ignoring stop signs are higher for older drivers than for the middle-aged.

The following are difficulties encountered while trying to relate driving performance to visual capabilities:

1. Vision is only one of many factors influencing driving performance.
2. There may be disparity between the visual capabilities and the degree to which they are used when driving.
3. Tests used may measure characteristics that are not closely related to the visual requirements of driving.
4. Reliability of criteria used to measure driving performance may be low.
5. Research methods may have short-comings, such as an unrepresentative sample of the driving population.

Back to Part I and Part II

Photo Credit: telegraph.co.uk