International Standards:

Vision Requirements for Driving Safety

Section 2: Preamble

Section 2 – Preamble

A previous report, prepared for the International Council of Ophthalmology, discussed Visual Standards, Aspects and Ranges of Vision Loss with Emphasis on Population Surveys ^[1]. In such surveys statistical averaging smoothes out the differences between individuals.

The current report addresses Individual assessment in the context of Vision Requirements for Driving Safety. It is appropriate to start this report with several caveats.

To discuss the relations between driving performance and various parameters of visual function, it is helpful to recognize the four aspects of functioning used in ICIDH ^[2] and ICF ^[3]. Of these aspects, two describe how organs or body systems function; two describe how the person functions.

In the field of vision, we use "Visual Functions" to describe how the eye functions and "Functional Vision" to describe how the person functions in vision-related activities ^[4].

Table 1 shows various links; these links are not fixed, since for each link there are multiple causal factors and therefore multiple possible outcomes. The bottom part of the diagram shows the application of these principles to driving-related functioning.

Visual functions, such as acuity, field, contrast, color, night vision, etc. can be measured as part of an eye examination. Functional vision includes performance of daily living skills, reading ability, mobility skills, driving skills, etc.

Since such activities are not easily measured in the office, the measurement of visual functions is often used as a substitute from which to estimate functional vision; this estimate of functional vision is then used to derive a further estimate of driving safety. For this purpose, visual acuity is measured often, visual field sometimes and contrast sensitivity rarely.

When estimating predicted driving performance on the basis of a few parameters, we should realize that many other factors, such as training, experience and familiarity with the driving environment can affect the prediction. Non-visual medical conditions are important also; a recent AMA publication provides a good overview ^[5]. The combination of several minor limitations may be more important than any one limitation by itself.

Another factor to take into consideration when estimating functional vision from visual functions is the fact that visual function measurements are usually threshold measurements. Functional vision, on the other hand, requires supra-threshold performance that can be maintained over time (compare running a 100m dash with running a marathon). One would not want to read a paper with a marginal print size and marginal contrast under marginal illumination. Similarly, drivers performing near the limit of their visual capabilities are probably not safe drivers.

Therefore, driving license requirements must include a safety margin. Just as a bridge designed for 5-ton vehicles should not collapse at 5.5 tons, so a person who meets the driving license requirement under stationary conditions and good illumination in the office, should still be expected to be a safe driver in a moving vehicle, at night, in rain or fog, or under other adverse conditions.

The uncertainty in relating visual functions to functional vision may be demonstrated by the diagram in Table 2 that relates a visual function measurement (visual acuity on a logatithmic scale) to a measure of functional vision (NEI-VFQ responses, after Roth analysis) ^[6]. A similar diagram for driving is not possible, since driving data did not exist over such a wide acuity range. There is no reason, however, to expect a different relationship for driving.

We note that (a) the regression line is smooth. This means that choosing a specific "cut-off" value is a policy decision, not a scientific one. Wherever the "cut-off" is placed, average performance on the left will be better than average performance on the right.

A similar conclusion was reached in a recent extensive report on vision and disability in the USA ^[7]. The wide spread of individual points further means that (b) even if average functioning can be predicted, predicting individual functioning is not possible. For any visual acuity value, some individuals are near the top, and some near the bottom of the scale. That the regression line is straight when visual acuity is expressed on a logarithmic scale, is (c) a strong argument for the use of a logarithmic progression (as on ETDRS charts).

Ultimately, driving safety does not depend so much on what is seen, but rather on how quickly and how adequately drivers respond to what is seen. Finally one must also stress that driving is a privilege not a right and that the primary responsibility of those who assess potential drivers is to the public not to the applicant.

Next: Vision Requirements for Driving Safety: Section 3 – Vision Loss and Driving Safety

Also see: Table of Contents

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