Section 12

Vision

Drivers’ visual acuity must allow them time to detect and react to obstacles, pedestrians, other vehicles, and signs while moving at the maximum posted speed, both in daylight and in darkness. Greater levels of visual acuity are required for some classes of licence to ensure public safety. Road signs should be designed to be easily legible at a safe distance for all individuals who meet the minimum visual acuity standard.

An adequate continuous visual field is important to safe driving. Any significant scotoma or restriction in the binocular visual field can make driving dangerous. Conditions often associated with loss of visual field are described in Section 12.5, Medical conditions and vision aids for driving. If a visual field defect is suspected (on the basis of a medical condition, subjective report, or confrontation field assessment), the patient should be referred to an ophthalmologist or optometrist for further testing.

Diplopia (double vision) within the central 40° (i.e., 20° to the left, right, above, and below fixation) of primary gaze is incompatible with safe driving for all classes of licence. Individuals who have uncorrected diplopia within the central 40° of primary gaze should be referred to an ophthalmologist or optometrist for further assessment. If the diplopia can be completely corrected with a patch or prisms to meet the appropriate standards for visual acuity and visual field, the individual may be eligible to drive. Before resuming driving with a patch, there should be an adjustment period of 3 months or a period sufficient to satisfy the treating ophthalmologist, optometrist, or occupational therapist that adequate adjustment for driving has occurred. The treating specialist should be experienced in driving assessment.

Individuals should be made aware of any abnormality of colour vision to allow them to compensate for this difference in their vision. Although no formal testing standards exist for colour vision, all drivers should be able to discriminate among traffic lights.

Loss of contrast sensitivity can be associated with increased age, cataract, refractive surgery, and other ocular disorders. Individuals should be made aware of any significant reduction in contrast sensitivity. Individuals with reduced contrast sensitivity may experience difficulty with driving, especially at night or during bad weather, in spite of meeting visual acuity requirements. There are no quantitative minimum requirements for contrast sensitivity; however, drivers are required to be able to discriminate among traffic lights of different colours.

Motor vehicle crashes sometimes occur because of the driver’s inability to judge distances accurately. Monocular judgments of depth can be made on the basis of such cues as the relative size or interposition of objects, clearness of details, and analysis of shadows and contrast effects. A more refined form of distance judgment, called stereopsis, is based on information coming from both eyes.

Judging distance is a skill that can be learned, even by people with monocular vision, who would fail standard tests for stereopsis. A driver who has recently lost sight in an eye or has lost the use of stereopsis may require a few months to recover the ability to judge distance accurately.

The ability to adapt to decreased illumination and to recover rapidly from exposure to glaring headlights is of great importance for night driving.

The partial loss of these functions in elderly people, particularly those with cataracts or macular disease, may in some cases justify limiting driving to daylight hours.

Processing of visual information while driving is very complex, and the visual field test evaluates only the capacity of a non-moving eye to see a stimulus. The useful field of view test is a specialized visual field test that evaluates the processing speed of centrally presented stimuli, as well as the selective and divided attention a driver needs to identify central and peripheral stimuli presented simultaneously, while ignoring distracting stimuli. Although it is not part of the current regulations, physicians must be aware of this tool and of the importance and complexity of visual information processing for safe driving.

The definition of monocularity varies within the literature, ranging from either complete loss of vision in one eye or impairment of vision below a specific cut-off (such that the second eye is functionally noncontributory) to binocular summation. For the purposes of this guide, monocularity is defined as visual acuity of 20/200 or worse and/or a visual field of 20° or narrower in one eye (legal blindness), or removal of one eye. Patients who are newly monocular should have a full ophthalmological examination, including acuity and visual field testing, before they return to driving. The literature suggests that 50% of patients will adjust to monocularity for many daily tasks within 1 month and 93% will have adjusted by 1 year (Linberg et al., 1988; McLean, 2011). A repeat examination should be scheduled for 3 months from the initial baseline exam and a follow-up consultation (with ancillary testing if needed) for 1 year from the baseline exam. Newly monocular patients may return to driving once their treating ophthalmologist has completed baseline (and any additional) screening as outlined above and is satisfied that they have adapted well to their monocular status. Individuals who are monocular from the beginning of their driver training (e.g., were monocular as children), as well as those who have had a slow loss of vision in one eye, are able to drive, provided they meet the criteria outlined elsewhere in this guide.