Relationship Between Visual Function and Driving Performance: Part I

Static Visual Acuity

Good central visual acuity and its associated clear retinal image are necessary for the early recognition and reading of road signs. It also aids the early detection of small and hazardous objects, such as pedestrians, motorcycles and other obstacles in the roadway.

Good acuity allows the driver more time to make decisions about events, obstacles, and signs and, in effects, slow down the action. A driver with poor acuity requires the obstacle to be closer before its significance can be appreciated, leaving less time to react. He or she must be more alert, will there fatigue faster and become more easily perceptually overloaded.

A recent report concluded that accidents did occur more frequently amongst drivers with considerably reduced photoptic (daylight) vision. Several summaries of papers relating to visual ability and driving performance have been compiled. They have all come to the same conclusion: in general there is only a weak statistical association between static visual acuity and poor driving ability.

Dynamic Visual Acuity

The strongest and most consistent relationship between the visual functions and the driving performance was found to be dynamic visual acuity. This positive correlation indicates that good co-ordination and freedom from confusion and dizziness are very important when driving. Medical conditions that interfere with the vestibular mechanisms, such as Meniere's disease, or where there is a limitation of head movement, such as in arthritis, can affect the dynamic visual acuity. A reduction of dynamic visual acuity will also occur if the static visual acuity is defective. It has been found that only the older age group (over 54 year) show consistent relationship between dynamic visual acuity and accident rate.

Colour Vision

various studies have been carried out to determine the relationship between the accident rate and colour vision defects. Colour vision defects may be hazardous if they cause confusion between the red, green and amber signal lights. A protanope (red colour-blind) requires about four times the normal intensity to see a red light. People with red colour vision defects may also find it difficult when driving in fog, or at night, as they are unable to fully appreciate a red tail/brake light. It would be advisable for them not to wear tinted spectacles when driving as these reduce/modify the amount of light reaching the eyes.

Peripheral Visual Fields

With such a high volume of traffic on the roads today, good peripheral vision is essential. A restriction of the visual field can never be fully overcome although increasing head and eye movements and adding extra mirrors to the car can be of help. The visual field is important for maintaining the driver's orientation and in establishing relationships between the many objects in the field of view. Central vision, important as it is, can only fixate the pavement once every 7m (22ft) at 95km/h 60mph. The remaining portions of the roadway between and to the left and right of the fixations must be taken in by peripheral vision.

Visual fields can be artificially reduced, for example by aphakic spectacle corrections, thick spectacle frames and sides, and by car design. There are obvious pathological disorders that can also cause visual defects, such as glaucoma, retinitis pigmentosa and cataracts.
Medical practitioners should advise their patients when they are considered to be unsafe to drive due to their visual field loss.

Stereopsis and Oculomotor Balance

Stereoscopic vision is the ability to appreciate depth by the superimposition of two slightly dissimilar objects. The position of objects can be located with one eye by monocular cues such as relative size, position relative to the horizontal, contrast, movement, brightness, etc. Unfortunately, under conditions of poor visibility, e.g. at night, the majority of the monocular cues are missing and stereopsis becomes the major cue in depth perception. Stereopsis is inoperative beyond 500m and therefore of little benefit in high speed driving, although it is valuable for nearer tasks, such as parking a car or locating children around cars, buses, etc.

It seems that stereopsis is not related to safe driving, as no correlation has been found between defective stereoscopic vision and increased accident frequency. The absence of stereopsis can be due to a squint. The presence of a squint without double vision is no obstacle to driving because the peripheral visual field is normal. However, the presence of double vision does constitute an obstacle to driving.

A horizontal phoria, although known to be enhanced by fatigue, low illumination and alcohol, has not been shown to correlate with crash rate. However, a vertical phoria of greater than 1prism diopter has been linked with poor accident records, especially in the over 55-year-olds.

Continue from Part II

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