International Standards:

Vision Requirements for Driving Safety

Section 5 – Suggested Criteria and Rules

Visual acuity

A review of the tables in Appendix 1 and 2 shows that binocular (both eyes open) acuity of 20/40 (0.5, 6/12) is the most common screening criterion for an unrestricted license.  We believe that this provides a reasonable safety margin for general drivers with no other problems. Note, however, that this does not imply that a person with 20/40 acuity is necessarily a safe driver.

Requirements for professional and commercial drivers are not considered in this report.  It is common to require a wider safety margin for this group, not because their visual environment is more complex, but because they may be responsible for passengers and/or for heavier equipment that can inflict more damage in an accident.

Some jurisdictions have also defined a visual acuity level beyond which not even a restricted license should be granted.  We are comfortable with a 20/200 level, considering that on traditional, non-logarithmic charts 20/200 (0.1) actually means < 20/100 (< 0.2).

Our recommendations for visual acuity can be summarized as follows:

 

Visual field

Visual field criteria are more variable. As a screening criterion, we recommend a binocular (both eyes open) visual field of 120° in the horizontal meridian with no obvious interruptions and approximately evenly divided to the left and right of fixation. 

No consensus exists about the instrument, target size or method to be used in screening for field defects. The assumption generally is that a fairly large/strong stimulus should be used, such as III4e (Goldmann) or 10 dB (Humphrey).

A vertical criterion of 20° above and below fixation (40° total) has also been proposed, but to our knowledge it has not yet been incorporated in any requirements. We know of no rules that set an absolute minimum visual field size.

Attentional problems should be considered; hemi-neglect is probably more dangerous than hemi-anopia without neglect.

Our recommendations for visual field can be summarized as follows:

While visual acuity screening with the presenting correction can be done easily and inexpensively, visual field screening is more involved.

The cost of screening all applicants should be weighed against the number of accidents prevented.

In many situations screening and testing may only be done for selected applicants, e.g. only for those who have already been referred for an eye examination because of visual acuity loss, those in whom field loss is suspected, and/or those involved in accidents.

Contrast sensitivity

Presently no contrast sensitivity requirements are listed in any jurisdiction.  This does not mean, however, that contrast sensitivity is unimportant.

When a simple, inexpensive screening test becomes available (see Appendix 3) and is validated, screening would be worthwhile, especially among the elderly. Those who fail the screening test should be referred for professional evaluation.

Contrast problems due to optical causes are probably highly related to visual acuity loss, but contrast problems due to retinal causes (common in the elderly) can exist regardless of the visual acuity level.

Restricted licenses

As stated earlier, driving license requirements are meant to establish a safety margin between performance on a stationary letter chart in the office and performance in actual traffic situations under adverse conditions.

For professional drivers the safety margin can be improved by imposing stricter requirements. For general drivers the satety margin can also be improved by prohibiting driving under certain adverse conditions. This is the purpose of issuing restricted licenses.

We feel strongly that issuing a driving license should not be an all-or-none, black-or-white decision, but that a gray area should be considered in which a license may or may not be issued.

Factors other than vision may also affect that decision. It is not unusual that elderly drivers have multiple minor deficiencies, which in combination make driving inadvisable. The AMA report, mentioned earlier ^[5], discusses such cases.

If visual acuity cannot be brought to the 20/40 level on a screening test, or if significant field defects are suspected, additional tests are indicated. This may include professional visual field testing, and testing of contrast sensitivity, glare sensitivity and night vision.

After examination, the vision specialist can advise the licensing authority on possible license restrictions and on the re-assessment interval.

The licensing authority will consider this advice and possible additional information, such as other medical problems and past driving performance and may require an on-the-road test.

The ultimate responsibility for issuing or not issuing a driving license, with or without restrictions, should rest with the licensing authority, not with the ophthalmologist or other organ specialist.

Various restrictions could be imposed, including the following.

Renewal and Recertification

Renewal and recertification pose additional problems. In the United States all licenses have an expiration date. Restricted licenses may have a shorter renewal period, based on the prognosis for progression of any medical problems.

Applicants for a restricted license should be re-assessed for each renewal; however, this may not be feasible under all circumstances. Additional re-assessments could be imposed for persons provoking car accidents, even if they meet the visual criteria.

For older drivers, we recommend that all drivers should undergo vision screening at age 65 and 70 and every 3 years thereafter, or at the closest renewal date. The vision tests should include visual acuity, visual field, contrast sensitivity, glare sensitivity and ocular motility.

These tests should, ideally, be performed by an ophthalmologist, but in many countries this is pure utopia. There is a need for simple screening devices that can be used in the department responsible for issuing motor vehicle licenses.

Use of bioptics

Some states in the USA allow the use of bioptic telescopes (bi-optic = small telescope for occasional viewing, mounted in the top half of a regular spectacle lens). Use of bioptics is not intuitive and requires a long training period. Only when the person has become comfortable with the use of bioptics can he/she be considered for a restricted license.

Peli & Peli (2002) ^[34] established a list of recommendations for people driving with a telescope. They insist on the need to prepare a travel plan, keeping clean windshields and rear mirrors, being certain not to have to check the gas level, etc. These persons should limit their driving depending on traffic conditions and refrain from driving in poor weather conditions. 

The benefit of telescopes is primarily in reading road signs at a greater distance, thus providing more time to react; however shifting back and forth between the carrier lens and the telescope takes time and thus limits the benefits.

Driving in familiar surroundings where orientation is by landmarks rather than by road signs avoids the need for telescopes and may be a condition for a restricted license.  Some users, especially older ones with limited driving needs, may find telescopes more cumbersome than helpful in driving. 

Licensing a bioptic driver should never be based on performance on a stationary letter chart only; it should always include an extended actual driving test by a qualified examiner to demonstrate that the applicant can drive more safely with a bioptic telescope than without one.

Can training improve visual performances in driving?

Coeckelbergh (2002) ^[28] has shown that some patients with retinal scotomata can benefit from training. About half qualified for a driving license after 12 training sessions.

Tant (2002) ^[29] has shown that training can also improve the performance of patients with homonymous hemianopia, although these patients did not reach the criteria for a driving license. Studies are under way to test the use of prisms in patients with hemianopia.

Again, training may probably help in some, but only an on-the-road driving assessment will determine whether an individual patient can be allowed to drive.

Next: Vision Requirements for Driving Safety: Section 6 – Summary and Recommendations

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