Vision and Driving Safety

Published on
December 4, 2023
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Driving is a task that most people perform every day. This involves judging distances, reading street signs, using our visual field and processing dynamic information. The majority of decisions we make behind the wheel are influenced by what our eyes can see on the road, and this depends on a number of factors. Book an eye test with an optometrist at our Adelaide or Henley Beach branch to ensure your vision can accommodate safe and comfortable driving.

Vision and Driving Safety

The Facts
  • Around 90% of the decisions we make while driving is based on visual information.1
  • Poor eyesight is the main risk factor for road crashes.1
  • 20% of drivers can’t see the road clearly due to uncorrected poor vision and have a 30% higher crash involvement rate.2
  • Importantly, 80% of all vision impairments can be prevented or cured.3

By age 50, risk of road accidents increases due to:

  • Increased light sensitivity to glare created by headlights.
  • Impaired depth perception and judgement of distances.
  • Difficulty seeing clearly due to uncorrected eye prescription.

How Can Vision Affect Driving Ability

Distance acuity

This is probably the most important visual skill for driving. Distance acuity is the ability see clearly at far distances. Even the simplest reactions in driving take at least 0.4 seconds. If your distance acuity is poor, you might not see a stop sign until you are almost on it—and you may not have 0.4 seconds to react. Seeing debris on the road surface or spotting someone planning to cross the road in front of you is also dependent on your distance acuity. The faster you travel, the less time you have available to react to what you see.

Snellen visual acuity chart. This chart features many different sized letters, used to measure ones ability to see
Figure 2. Snellen visual acuity chart

Using glasses or contact lenses will give you the best chance of seeing everything clearly on the roads. Drivers with anormal license need to see the 6/12 line of letters with both eyes open, meaning that if you have a weaker eye, you may still pass the standard. Commercial drivers have a stricter standard and need to see well to the 6/9line with both eyes individually.

Drivers who use overnight orthokeratology contact lenses to see clearly the next day should wear their lenses every night to ensure their vision is sharp enough when behind the wheel. Your optometrist can give you a letter that can be left in your glovebox that explains your form of vision correction to anyone concerned.

Nighttime Driving

At night, drivers notice the following:

  • Driving is riskier and more challenging.
  • Visibility is limited.
  • The ability to detect and response to dangers is reduced.
Colour Vision Deficiencies
  • Colour vision deficiencies (CVD) affect approximately 1 in 12 men and 1 in 200 women in the world.
  • CVD can be due to genetics or can develop over time due to ocular distance.
  • The visibility of road signs for people with colour vision deficiencies is often reduced.6
Two images of traffic lights, one with normal colour vision, the other with colour vision deficiency
Figure 4. Traffic light colours normally VS with colour vision deficiency
Depth perception

Passing and changing lanes in busy traffic require accurate judgement of distances between moving objects. Both eyes need to function properly as a team for reliable depth perception. Depth perception deficiencies are common in drivers and get worse as speed increases. Even if you meet the driving standard without glasses or contact lenses, having these on when driving may make you safer on the road because they improve depth perception.

Field of vision

The ability to ‘see out of the corner of your eye’—to see over a large area in your periphery without moving your eyes or head— is an important part of safe driving. It enables a driver to see cross-road traffic and pedestrians at the roadside without looking away from the road ahead. Normally, the field of vision is about 180 degrees, reduced with increasing speed, and is only 40 degrees for distant objects at speeds of100 kilometres an hour.

  • Around 50% of people with some form of visual field loss are unaware of the problem.5
  • Studies have shown that people with visual field defects have double the risk of road crashes and trafficviolations.5

To meet the driving standard, you need to see with both eyes at least 110 degrees horizontally, with no visual field problems within 20 degrees of where your eyes point. This means patients with vision in only one eye may still meet the driving standard but with require a period of time (usually 3 months) to adapt to only using one eye first if it isa new condition. Patients with conditions like glaucoma may not meet this standard due to loss of their peripheral vision. At Innovative Eye Care we have special visual field-testing machines that can check if you meet this driving standard.

Driving view demonstrating the field of view with restriction visual field
Figure 5. Impact of restricted field on driving.
Muscle balance

Good muscle balance means that both eyes can be pointed easily and simultaneously at a given object. It is essential for good two-eyed vision, depth perception and field of vision. Although drivers usually can compensate for muscle imbalance under favourable driving conditions, the effort involved may take its toll in fatigue and discomfort. Alcohol, tiredness and drugs can upset muscle balance so that a slight imbalance becomes unmanageable. Visual training may improve the muscle balance of someone with a condition that affects their muscle balance.

You are not allowed to drive if you see double. This may happen if you suffer a nerve palsy or diabetes, for example. Temporary prisms attached to your glasses may alleviate the double vision while your condition improves, allowing you to continue driving.

Accommodation

A driver has to change focus quickly and easily from the road to the dashboard and back again. This ability to change focus from a far object to a near object, and vice versa, is called accommodation. As we get older presbyopia sets in and we cannot change our focus so easily. Spectacle lenses such as progressive multifocal lenses can give focus for both distance and intermediate areas such as the dashboard.

Glare

As we get older the tissues of our eyes become more opaque, causing conditions such as cataract. A cloudy lens will cause glare as the light from sun and headlights will be scattered rather than focused well. Some younger patients who are fair will also have issues with glare due to the lack of light-absorbing tissues in their eyes.

If glare becomes more problematic, then shielding your eyes from light during the day with quality sunglasses can be helpful. The anti-reflective coating on the modern lenses that we use at Innovative Eye Care will also reduce the reflections of light within your lens, especially from behind you, which makes driving easier.

Glare is the most complained about visual discomfort by drivers. It can occur during daytime and nighttime and is caused by excessive or uncontrolled brightness. It is caused by various different ocular diseases including but not limited to cataracts, uncorrected refractive error (short-sightedness, long-sightedness or astigmatism.) When driving, glare slows detection and reaction time.4

Figure 7. An illustration to show how a spectacle lens with an anti-reflective coating(right) can reduce glare at night.

Recommendations

In order to stay safe on the road and maintain good vision, we recommend the following:

  • Have your vision examined regularly, with special attention to the needs of driving.
  • If spectacles have been prescribed for driving, make sure you wear them.
  • If you are troubled by glare, minimise your night driving and look into sunglass options for the daytime (preferably polarised sunglasses).  Polarised lenses have been shown to improve driver reaction times during the day by eliminating reflectedglare.4
  • Compensate for poor field of vision by making good use of offside and rear-view mirrors and by turning your head to see objects at the side.
  • If your depth perception is inadequate, take extra care when passing other cars.
  • Remove objects from your rear-view mirror and ensure that it is correctly adjusted.
  • Ensure that your windows are clean and free from scratches or pitting that can increase glare.
  • Check headlights periodically, so that the provide maximum light, with each beam properly positioned.
  • Slow down. Most vision problems are accentuated by high speed.
  • Protect your eyes from glare on sunny days

Contact us to see one of our friendly optometrists regarding vision and driving safety.

References

1) World Health Organization. Road traffic injury prevention: training manual. 2006

2) Vision Impact Institute. The Social and Economic Impact of Poor Vision. 2012.

3) World Health Organization. Fact Sheet Number 282. 2014.

4) Zikos G, Nason R, Rivukitti S, Ali S,Selenow A. CSF and Reaction Time Differences in Young and Elderly Subjects with Polarized and Tinted Lenses in a Driving Environment. Assoc Res Vis Ophthalmol.2009; 7. Cole BL. Protan colour vision deficiency and road accidents. Clin ExpOptom. 2002 Jul;85(4):246–53.

5) Johnson CA, Keltner JL. Incidence of VisualField Loss in 20,000 Eyes and Its Relationship to Driving Performance. ArchOphthalmol. 1983 Mar 1;101(3):371–5

6) Cole BL. Protan colour vision deficiency and road accidents. Clin Exp Optom. 2002 Jul;85(4):246–53.

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