Visual acuity and driving

Vision is without a doubt the most important physical factor for driving and its lack cannot be substituted for or supplemented in any way.

More than 90% of the information we receive on the traffic comes from our vision, and most maneuvers made by the driver are based on the sense of sight.

By studying the visual capacity of drivers, we can determine the amount and quality of the information from the exterior we are able to receive.

The following conditions are necessary for good driving:

  • Accurate perception of all the different elements related to the traffic.
  • Appropriate interpretation of this information by our brain.
  • Application of a neuromuscular response to the controls of the vehicle within seconds.

This time interval will increase the safer the highway is, the better the vehicle’s performance, and depending on whether the driver has adequate visual capacity and obeys the rules for speed limits, lights and safe distances.

Visual capacity is measured in terms of visual acuity, visual field, sensitivity to light, eyelid and eyeball motility, and considering the presence of disabling progressive diseases that prevent a driver from achieving the legally required standards for each group.

However, it is very difficult to establish a link between visual capacity and the number of traffic accidents.

It has been shown that visual acuity and visual field defects for moving objects are observed more frequently in drivers who have suffered accidents than in controls. This situation is aggravated under conditions of poor illumination, in elderly drivers and in persons with associated ophthalmologic diseases.

When assessing visual capacity, it is always important to consider the personal circumstances of the driver, both medically in terms of morbidity, functional grade and associated risk factors, and in terms of the type and intensity of exposure to risk occurring during driving.

Visual acuity

Visual acuity represents the ability of the eye to recognize objects, and its measurement determines the degree of development of the visual function. It is determined by the ability of the eye to perceive the shape and details of objects.

Judgment of distances does not depend only on the sense of sight, but also on its combination with experience and training.

Binocular vision provides stereoscopic vision or a sense of depth. It is essential in traffic because it allows the driver to calculate the maneuver required depending the distance to the perceived object.

When driving at night with artificial light or during the twilight hours (mesopic vision), the driver loses nearly 6 meters of depth in his/her field of vision for every 15 km per hour increase in speed.

Although night driving only represents a fourth of total driving time, it accounts for more than half of traffic accidents.

Binocular visual acuity may sometimes be better or worse than that obtained when each eye is examined separated. Half of subjects with normal “static” visual acuity have below normal scores on “dynamic” tests, especially elderly persons.

Monocular vision is the anatomic or functional loss of an eye that reduces its visual capacity to 0.10 or less. Passing other vehicles and making maneuvers at intersections is more difficult for persons with this problem, particularly during night driving.

Therefore, it is necessary to correct any reduction in visual acuity. Nearly 30% of drivers have to wear corrective glasses for some visual deficiency.


  • A driver at risk from a visual viewpoint should be advised on the risk of driving and provided with the changes and acquisition of compensating strategies for driving, and the medical control of the existing ophthalmologic condition that enables him to develop an adequate driving attitude.
  • He should have, if needed with correcting lenses, a binocular visual acuity of at least 0.5.
  • If using corrective glasses or contact lenses is necessary for it, he will be reminded that their use is mandatory when driving, and should carry replacement glasses inside the car.
  • For people with monocular vision to be able to drive, they should have the eye healthy, with normal visual acuity and the other visual capacities, and do it as legally established for the maximum speed and months since the start of this situation. They will have an external rearview on both sides and a panoramic mirror inside.
  • The glasses should better have fine rods and hoops, with fine lenses, and the closest possible to the eye. This will increase the visual field. Contact glasses not causing limitation are preferable for this.
  • There are multiple drugs, such as some antiinflammatories, antibiotics, corticoids and other drugs, etc, that affect visual acuity during a treatment. These include: allopurinol, ibuprofen, chlorambucil, indomethacin, chloramphenicol, MAOI, chloroquine, isoniazide, corticoids, nicotinic acid, digitalis, piperazine, tetracycline, ethambutol, etc.
  • Driver patients should be warned that, if they notice any symptom with the drug prescribed, they should ask their physician.