It is incumbent upon motor vehicle officials to set up and maintain adequate standards which will stimulate improvement of visual efficiency rather than cause large scale rejection of applicants. Civil service commissions, insurance companies, trucking companies, large corporations require certain driver groups to demonstrate better visual efficiency than that required for a regular license in all states. Management knows that a visually efficient driver is a safe driver. Yet, little or no scientific research has been done to prove this point.

The eye practitioner, by training and experience, knows that certain symptoms, signs, and reactions are manifest with certain given ocular and visual anomalies. Hyperopia, myopia, and astigmatism are ocular anomalies, wherein the axial length of the eyeball or the curves of the various refractive media vary from normal and produce an out-of-focus image of the retina. A functional vision anomaly may be considered occuring from the retina to the perceptual center of the occipital area of the brain, i.e. the sensory level of vision.

This paper will attempt to review and interpret the research and expert opinion available on the subject of motorists' vision. Familiar to all is visual acuity.

Visual Acuity and Driving

Four states require a minimum visual acuity of 20/20 for chauffeur's license. Thirty-three states require a 20/40 minimum visual acuity. Visual acuity is a measurement which signifies the keenness with which the retinal images are perceived. Visual acuity is usually rated by the use of geometric forms, numbers or letters which subtend a 5' angle of an arc at 20 feet (Snellen). Most researchers consistently mention that if it were up to them to set a drivers' standard, it would be 20/20 visual acuity.

Hofstetter and Zerbe did a follow-up on the survey performed by Hofstetter and Bryan on the prevalent and potential visual acuity of automobile drivers. They found a close similarity in the percentage of persons who could achieve 20/20 visual acuity or better if corrected. They concluded that 94%

of all drivers could have 20/20 visual acuity, or more, in the better eye if corrected properly. Their findings indicate that 20% of the records examined showed drivers to have had substandard vision until corrected.

It is significant to consider that persons in older age brackets have a rapid diminution in keenness of vision. These individuals limit their driving or stop driving voluntarily in many cases. This has been proven in Pennsylvania where 3,035 drivers, of the 394,910 re-examined so far, have voluntarily relinquished their licenses by not completing the required physical and vision examination required by Section 608G of the Motor Vehicle Code of Pennsylvania.28

Only 2% of those in the older age brackets in Pennsylvania who were checked could not achieve 20/30 or better visual acuity. It should be mandatory, therefore, that the best visual acuity possible be enforced as a driving prerequisite. This may prove a hardship on some, but proper glasses will serve the driver for many uses other than driving a motor vehicle.

There has been much research done by the American Optical Company and by the Bausch and Lomb Optical Co. through Purdue University to evaluate industrial safety programs. It has been found that there is a general increase in efficiency, less lost time, less breakage, less waste, improvement in morale and, of course, fewer injuries in plants with industrial vision safety programs. We may transfer these results to the driver and similarly raise his driving efficiency. In a survey5,6 by Hofstetter, et. al., on the best visual acuity, we note that only 2% could not achieve at least 20/30 visual acuity in the better eye. Only 0.6% could not achieve at least 20/40 visual acuity standard recommended by most groups throughout the country.

Effect of Illumination and Speed on V.A.

Considerable thought should be given to the issuance of limited licenses for persons who cannot meet the 20/30 visual acuity standard. While this license will limit the driver to daylight driving only, it will reduce complete rejections and may prove more ac

ceptable to the public and legislators. Richards finds that a driver with 20/40 static daytime visual acuity has only 20/80 visual acuity under night illumination. Some years ago, Luckiesh and Moss measured 150 people and found average, normal vision (20/ 20) at 10 foot-lamberts and a decrease in the average visual acuity to 20/55 after adaptation to 0.01 foot-lamberts. The person with the best vision at the lower level had only half of the acuity available to him at the higher level.

For a driver to experience the same acuity at the lower level of illumination, as at the higher level, symbols on road signs would need to be increased in size five-fold. Contrast would also have to be increased six to twenty times to make the same size sign legible at the lower brightness. This assumes, of course, sufficient time to see. In other words, for a given contrast there is a minimal size that can be seen. The average sign on our highways has lettering five inches high and is equivalent to 20/200 Snellen-sized letters.

Seeing is limited by speed. Unless a minimum of light is focused on the retina for a sufficient time there is no vision, and when the image moves faster than the eye can compensate, vision is impossible. For black and white seeing 1/30 second is said to be adequate, while 1/5 second is necessary for color seeing. Fast driving enhances peripheral blurring so that progressively smaller fields in front of the driver remain clear at greater distances.

Danielson3 reports that greater comfort is experienced when one drives at rapid speeds where only the central field is seen clearly and the eyes are shielded from the blurring in the periphery. The loss of clues from the blurring at the side may lead to over-confidence and even greater speed. Roper found that distances of seeing decrease by 20 feet for each increase of 10 mph. Vibration above 205 cps causes vision to decrease. The loss of vision at high driving speed increases because of the greater vibration of the vehicle. At high speeds the estimation of movement and judgment of speed may be halved. The safe speed for a given driver is that for

which the perceptual load is not too great for proper response. At night, this load is greater and speeds should be decreased proportionately. A driver with poor night vision may have to go so slowly as to be a hazard on the turnpikes or parkways where speeding is permitted.

The visual mechanism is controlled by the nervous system. Vision efficiency depends upon the response to changes from moment to moment in the amount and distribution of light entering the eye. At night the changes in illumination can occur faster than the eye can adjust to the changes. The resulting conflict in the seeing mechanism and the effort expended in trying to get a clear and sharp view, which cannot be obtained under these conditions, are a source of strain, frustration and fatigue in the driver. A sharp retinal image is perceived more quickly and is less disturbed by glare light. A properly placed quarter diopter cylinder for the correction of astigmatism has been reported to improve night acuity by 25%!?

High acuity is unnecessary for daytime driving when the pupils are small. There are reports of accident-free driving with low acuity, even 20/400, with no great difficulty after the first five minutes of adjustment. At night this is no longer true; the weak,blurred images do not have enough contrast for seeing and the driver may not see obstacles. Motorists with less than 20/20 daylight vision should drive more slowly at night than those with better vision, and the proportionate decrease in speed should be determined for a representative section of the driving public.

Aging Decreases Vision

Aging decreases vision along with the other changes of senescence. More light is necessary for seeing with smaller pupils, crystalline lens changes, etc. One study indicates that for equal seeing the light needs to be doubled for every 13 years of age.

Figure 1 summarizes other information showing how vision decreases with age. Acuity and contrast sensitivity decrease, and the decreases may result in lower levels of visual acuity than are legal for driving in

some states. This is another reason for a reconsideration of the importance of Snellen acuity as a criterion for a driving license.

Since the available illumination at night is scarcely adequate for the best eyes, many older people should drive less rapidly. Essential signs and signals should be much larger and brighter than those required for the best young eyes. Fortunately, older people generally become more conservative and drive more carefully.

The professional responsibility of the optometrist and the ophthalmologist is more important with the older patient who needs spectacles. In addition, advice should also be given to certain older people concerning the inadvisability of driving at night. Opacities forming in the eye may reduce vision to an unsafe level. Vision specialists have the problem of determining what is a safe level of visual acuity for driving and of advising traffic engineers and lawmakers so that highways can be made better and safer.

Accident-Proneness at Acuity Levels

Research at various institutes has shown us that a car traveling at 40 mph on a good road, with good tires and brakes, can stop safely at a distance of 143 feet. We can assume that a driver with 20/20 visual acuity can react easily in this situation, for he can see clearly at 280 feet, and have sufficient time for safe stopping.

If the driver has 20/40 visual acuity, his clear vision extends to only 113 feet. If he is traveling at 40 mph under favorable conditions, he may be 30 feet beyond a sign or object before he can stop.26 Therefore, 20/30 visual acuity is a more desirable base than even the accepted 20/40 minimum.

With a 20/50 visual acuity minimum, the driver needs 90 feet to read a road sign. The implication of hazard is obvious.

Lauer's and Silver's studies both suggest a rapid transition to accident-proneness at acuity levels of above 20/40 visual acuity. A pilot survey of visual acuity by the National Home Demonstration Council in cooperation with the Women's Auxiliary to the American Optometric Association showed that 21.5% of 3,000 persons screened failed to achieve 20/40 visual acuity.26

Effect of Phorias

Little or no basic research has been accomplished on the relationship of a marked heterophoria to driving accident proneness. Again, we must rely on the empirical knowledge and experiences in daily practices.

A phoria is the measurement of the position the eyes tend to assume when fusion is absent. An exact phoria measurement is taken in an eye practitioner's office with prisms. In screening instruments two dissimilar targets, which cannot be fused, are employed to take a gross measure. An individual has a measureable phoria if he is able to fuse the retinal images of both eyes into one. If he cannot fuse because of a high phoria, he has a "tropia" or strabismus. There are many borderline cases where fusion is present occasionally, but the phoria is too high to permit constant fusion (intermittent tropia). This may result in a suppression or suspension of vision in one eye. Many times this is the cause of amblyopia.

Clues which stimulate the two eyes to produce single, binocular vision become fewer with deeper twilight and place more strain on the coordinating mechanism of the eyes. It is important to have the best possible correction for any phoria or muscular deficiencies of the motorist's eyes. Shapiro and Mastaches say that if there is a phoria of an appreciable degree, or if a tropia is present, any extended period of driving at night results in blurring of vision, diplopia, impairment of depth perception, headaches, and resulting fatigue.

Refractive errors may be a cause of heterophoria. In fact, most tonic vergence

High Uncorrected Phorais Lead to Fatigue

The general subjective symptoms associated with "eyestrain" due to the continuous effort expended to maintain fusion in all visual activities (including driving) are a feeling of tiredness and discomfort in the eyes, varying from a dull ache to actual pain; headaches of various types; and rapid fatigue and blurring of vision sometimes leading to actual confusion or temporary diplopia with giddiness and even nausea and digestive disturbances; torticollis (head tilting) which results from a vertical imbalance (hyperphoria). Most of these symptoms can be induced artificially by the wearing of prisms by a normal person.9,10

Judgment of direction also suffers in heterophoria.10 The faculty of steropsis and the judgment of distances tend to be impaired. It is generally held that an exophore underestimates distances and an esophore overestimates. This could explain some of the rear-end accidents that are so common on the road.

Corrective lenses and/or orthoptic training (vision training) to encourage habits of binocularity can compensate for, improve or correct a heterophoria. Thus, if the motor vehicle inspector finds a heterophoria, the applicant for a license should be referred in an effort to have this condition corrected. If the inspector or eye practitioner does not check or examine for this function, the driver of a car operates under a false sense of security.

What does a high, uncorrected hetero

phoria mean to the operator of a motor vehicle? The above mentioned subjective symptoms lead to fatigue and confusion. Fatigue brings on a dazed condition where diminished attention and speed of reaction (both sensory and motor) make the driver prone to accidents.

In a screening of 3,000 drivers, 16.9% failed minimum requirements for lateral phorias and 13% failed for vertical phorias.26 Effect of Fusion

Binocular fusion may be defined as the process by which the visual portion of the cerebral sensory area combines the sensory impulses initiated by the two somewhat disparate retinal images of an object of regard into a single perception, so that only one tri-dimensional object is seen.o

Obviously, fusion is a binocular function. One must have two impulses or messages going into the occipital cortex of the brain for a process of assimilation, integration and other reflex functions, with a resultant single picture to be perceived and interpreted. Fusion is absent when a driver possesses only one eye. It is also absent when a person suppresses or suspends vision in one eye.

The act of suspension of vision in one eye is quite common. It does not always occur, but may be found under conditions of stress. This is a protective mechanism of binocular situation under stress. The suspension may be in the area of the macula only, or it may be total. Suppression of vision in one eye can be found in tropias. Again, this is a protective mechanism of the brain to stop diplopia.

Even when two eyes are apparently straight but, for some reason which may be muscular or neurological, the eyes cannot coordinate or the brain cannot fuse the two images because of a gross dissimilarity, the impulse from one eye to the brain is cut off. If this situation is permitted to continue, nerve fibers from the macular area may atrophy, with a resultant amblyopia. Confusion and inefficiency in the visual process are due at times to a situation in which the basically dominant' eye is more optically deficient than the other, thus making the second eye arti

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ficially dominant. This is often correctible with glasses.

Clinically, fusion is classified in three grades: Grade I. Simultaneous Perception (superimposition); Grade II. True Fusion With Some Stereopsis; Grade III. True Fusion With Stereopsis.10

It is probable that very close to 100% of all drivers could achieve Grade III fusion, with the proper care of an eye practitioner if even some fusion is present. The visual process develops in the infant in the above successive levels. If the eye doctor finds an impairment of fusion, he traces it back to its developmental level and institutes training from that point.

Now, what does the fusion faculty have to do with the safe operation of a motor vehicle on our highways? It can be safely said that good fusion is the cumulative result of an efficient visual process. If fusion is impaired somewhere along the visual pathway, from the eyelids to the occipital area of the brain, something is wrong.

In the majority of people, faulty fusion can be improved. First and foremost, however, it must be found. Fusion ability can be discovered only by the professional practitioner who is examining for it. An impaired fusion faculty may be compared to a headache, because its presence may indicate a malfunction.

Effect of Color Vision

Most authorities state that the ability of the eyes to interpret color is dependent upon the possession of certain color receptors in the retina. In the absence of these color receptors, the individual is color blind.

Genetically, color sense has developed from black and white (and the various shades of gray in between) to blue and yellow perception and then to red and green perception. Light as we see and use it is made up of the various visible wave lengths of the spectrum. The various wave lengths can be separated and measured. The eye is most sensitive to the D line of sodium, a shade of yellow.

Lack of color perception must be considered from two aspects: 1) color blindness,

where the retinal receptors are absent; and 2) color ignorance, where the individual has not learned to interpret or discriminate col

or.

About 4% of the male and 0.4% of the female population are congenitally color blind. Color blindness may be acquired through disease or injury to the retina. Partial color blindness can be caused by the excessive use of tobacco or alcohol.11

Many color blind people learn the normal colors of objects through experience and under circumstances with which they are familiar. A color blind or color deficient driver must know that he is color blind or deficient. The color deficient may be trained to distinguish color. The color blind cannot be so trained. Color blind patients who drive, know they are color blind and compensate accordingly. We might add that a few take their clues from other drivers, especially when uncertain about the color of traffic signals.

We believe motor vehicle inspectors should continue to check for color vision simply to inform the driver if color blindness exists but not deprive a license for this visual anomaly.

Stereopsis and Depth Perception

As noted above, stereopsis, the ability to see tridimensionally, is the highest development of the binocular organism. Monocular vision or substandard vision in one or both eyes is actually a “depth conception”, whereas, good binocular vision is truly "depth perception with stereoscopic vision".12

A monocular individual, or one seeing with substandard vision, depends greatly on experience to determine distance and size. These experiences are: 1) Psychological (a. aerial perspective; b. distribution of light and shade; c. the overlapping of contours; d. geometric perspective; e. interpretation of size.) 2) Physical—parallax.

These experiences constitute a learned process of spatial localization. A monocular person actually sees his space world flat. His learning, almost from birth, to localize objects in space permits him to find these objects and to orientate to his space world.