International Standards:

Vision Requirements for Driving Safety

Section 3: Vision Loss and Driving Safety

General considerations

Driving is a demanding activity. The various stages can be summarized as follows:

Drivers must see obstacles or hazards, straight ahead or in their peripheral visual field (e.g.; a person crossing the road), consider the possible reactions of this person, estimate the distance, decide whether they have to slow down or stop and then react accordingly.

Thus, driving safety depends on many external factors as well as on the condition of the driver.

External factors	Driver related factors
Visual characteristics of the obstacle (size, color, contrast, etc.)	Visual performance: Visual acuity Visual field Contrast sensitivity
State of the vehicle (windshield,rear view mirrors, etc.)	Age, Experience, Risk assessment
Irrelevant information (publicity along the roads, etc.)	Motivation, divided attention (cell phone use, talking to passengers, anxiety)
Reduced visibility (night, fog, rain, etc.)	Fatigue, Alcohol, drugs and medication
Condition of the road	Note that the first three are probably constants, and therefore testable; the last ones may be just as important, yet, are not testable in advance, since they may vary from trip to trip.

Driving license requirements

In our society losing one’s driving license means a major limitation of one’s independence and, especially for older persons, a marked restriction in social contacts.  Depending on the development and accessibility of public transport the consequences may vary tremendously from country to country, but in every case the decision to deny any person the right to drive must not be taken lightly. 

Notwithstanding the caveats mentioned in the preamble, one must realize that motor vehicle accidents are the leading cause of injury related deaths among 65 to 75-year olds. Vision is the most important source of information during driving and many driving related injuries have been associated with visual problems. Visual assessment for driving is thus a major health issue.

This paper was developed to explore ways in which the ICO could contribute towards global harmonization in this important area.

Questions

There are a large number of questions.

These are very sensitive issues and we will not be able to provide an answer that will be valid for every single country. However some guidelines could be deducted from scientific studies and proposed to the official authorities responsible for traffic safety.

Age

There is a relationship between age and driving safety.  According to Keltner & Johnson (1987) ^[8] the California Department of Motor Vehicles Driver Record Study reported in the period 1972-1974 an incidence of 2 accidents /100.000 miles in 20 year old drivers.  This number regressed to 1/100.000 for the age group 30-60 years and rose again after that age to reach 2/100.000 at the age of 70 years.

As regards to traffic violations speeding is most common at the age of 20 (3,5/100.000 miles) and regresses gradually to 0,5/100.000 miles at age 70.

Disregard of traffic lights, which is found in 1,25/100.000 miles at 20 years, diminishes to 0,5 at 35 years and increases again from the age of 60 years to reach 1,3 at 70 years. 

In the age group 60+ one notices a gradual increase of failure to yield the right-of-way.  Younger drivers take more risks especially in speeding, whereas older persons are probably more rapidly distracted or fail to appreciate a potentially dangerous situation. 

These are all non-visual factors. The person’s physical condition, hearing and slowing of reactions with age also play a role ^[9].

Visual impairment

In most published studies the statistical correlations between vision and traffic accidents are weak. The recent AMA publication "Physician’s Guide to Assessing and Counseling Older Drivers" (2003) ^[5] points explicitly to the interactions of various deficits, where vision is only one of the relevant parameters, while visual acuity is only one parameter of vision. It must be remembered that the decisive factor is not what drivers see, but how they react to what is seen.

Still, after (momentarily) divided attention, visual problems, which are permanent and therefore easily tested, are probably important among the reasons for accidents and traffic violations. They may be particularly important in those older persons who are not aware or do not want to admit that they do not fulfill the visual requirements anymore or who do not wish to abandon driving.

In a study published in 1988 Paetkau et al ^[10] studied the prevalence of illegal motor driving among visually impaired elderly in Alberta.  Of 491 patients aged 65 or older 22 (4%) with vision below legal limits persisted in driving a car.  Significantly more men (11%) than women (1%) were driving illegally and among the men a higher proportion of those aged 65 to 79 years (17%) than those aged 80 years or older (6%).

Visual acuity

Fonda (1989) in a paper with the provocative title "Legal blindness can be compatible with safe driving" ^{[11, 12]} states that some people with a stable ocular condition, a visual acuity of 20/200 (0.1) (which means < 20/100 (<0.2) on most traditional charts) and a visual field of 120° are capable of safe daytime driving at a speed restricted to 40 mph.  He tested 8 legally blind persons. The subjects were instructed to approach a particular traffic sign on foot and to stop as soon as they could identify the sign correctly. The minimum distance was 115 feet for one subject and the average distance was 221 feet.  This distance allows drivers with normal reaction time and good motor ability to safely stop their vehicle when traveling at 40 mph as determined by the US Bureau of Public Roads. This is an argument for allowing restricted licenses.

Szlyk et al (1993) ^[13] compared the driving performances of 20 patients with juvenile macular dystrophy (Stargardt disease or cone-rod dystrophy) and visual acuity between 20/40 and 20/70 with 29 control subjects with normal vision.  Driving performance was defined by accident involvement based on self-report and state records and by evaluating performance on a driving simulator.

The proportion of individuals involved in accidents in the central vision loss group was comparable to that of the control group. However on an interactive driving simulator the macular dystrophy group showed longer braking response times and a greater number of lane boundary crossings than the control group. Also for 12 of 20 subjects with central vision loss who did not restrict their driving to daylight hours, there was a greater likelihood of involvement in nighttime accidents.

Szlyk et al (1995) ^[14] also studied a group of 10 persons with age-related macular degeneration and average visual acuity of 20/70 and compared their driving skills with those of 11 age-matched persons with normal vision. The rate of accidents in a 5-year period was obtained from a self-report questionnaire and from the state records of accidents. The subjects underwent a road test and a test on a driving simulator.

The state records indicated no accidents in either group; however 4 of 11 control subjects had one or more traffic violations, most commonly speeding, whereas none of the ARMD patients had state convictions. Six of the eleven controls also reported accidents as compared to only one ARMD patient.

As could be expected higher cognitive abilities were related to better performances on the simulator and on the road test. In the simulator the performances of the ARMD group were poorer with delayed braking response to stop signs, slower speed, more lane boundary crossings and more simulator accidents.  In the road test this group drove more slowly and had significantly more points deducted for not maintaining proper lane position.

The contradiction between the situation in a test environment and the number of accidents and traffic violation in the ARMD group is explained by compensatory attitudes in this group: 1. not driving in unfamiliar areas. 2. driving at slow speeds.  3. limitation of driving to daylight conditions.  4. taking fewer risks such as not changing lanes. These findings point to the importance of making subjects fully aware of their limitations and advising them about ways for compensation.

The same group also studied the driving performance of 21 retinitis pigmentosa patients with visual acuity of 20/40 or better as compared to 31 normals with comparable age, years of driving experience and miles driven per year (1992) ^[15].

A significantly greater proportion of RP patients reported accidents within a 5-year period and performed less well on the driving simulator. There was a statistically significant correlation between the severity of the field loss and the number of self-reported accidents. In the driving simulator a stop sign was initially presented at a location of 30° in the visual field to the right of the center of the screen in a curved portion of the road. This probably corresponded to a scotomatous area for most RP patients.  A significantly greater proportion of RP patients had accidents on the driving simulator. They also showed an increase of braking response time and of brake pedal pressure.

Interestingly 67% of the RP patients did not restrict their driving to daylight.  In this subgroup the proportion of nighttime vs. daytime accidents was comparable with that of the control group.  The authors concluded that visual field loss is a primary correlate of car accidents in RP patients.

Visual fields

Patients with visual field defects, whether they are due to glaucoma, a retinal disease or cataract, have double the incidence of road accidents or traffic violations as compared to persons with a full visual field according to a study of 10.000 persons (Johnson & Keltner, 1983) ^[16]. The incidence of visual field loss was 3,0 to 3,5 % in the age group 16-60 years and 13 % for the subjects older than 65 years. Even more important almost half of the persons with visual field loss were unaware of any problem with peripheral vision.

Monocular vision

It is important to be able to correctly judge distances. At the distances and speeds involved in automobile traffic, binocular stereopsis is not the most important depth cue. Persons who lost one eye will regain adequate distance judgements after an adaptation period, taking into account the size or the interposition of objects, and noticing the shadows or the precision of details.

The majority of patients who were enucleated for a malignant melanoma regain their ability to drive safely (Edwards & Schachat, 1991) ^[17]. Usually it took about 6 months.

Cataracts

In patients with lens opacities, the problems are not only the reduction of central vision and the visual field restrictions. Poor contrast sensitivity and glare play an important role.

Owsley et al. (2001) ^[18] studied the impact of cataract on driving in an older population (274 with cataract and 103 cataract free drivers). Drivers with a history of crash involvement were 8 times more likely to have a serious contrast sensitivity deficit in the worse eye (defined as a Pelli-Robson score of 1.25 or less) than those who were crash free.  They concluded that severe contrast sensitivity impairment played a major role in car accidents even when it was present in only one eye.

Wood et al (1993) ^[19] simulated three conditions of visual impairment in 14 young, visually normal, individuals: monocular vision, cataract and peripheral field restriction.  Using modified swimming goggles the extent of visual fields and low contrast visual acuity were significantly decreased.

In this study simulated cataract caused the greatest detriment to driving performance followed by binocular visual field restriction even though the drivers still satisfied the visual requirements for driving licensure. On the other hand monocular vision did not significantly affect the driving performance.

Next: Vision Requirements for Driving Safety: Section 4 – Which Visual Functions Might Be Tested?

Also see: Table of Contents

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