Relationship between Visual Functions & Driving Performance

Good Vision → Safe Drive → Save Life

Driving Performance ∝ Visual Functions

80% of the information we received from the environment is visual and rest 20% through non-visual such as ears, skin etc. The commonest visual function tests that are performed at clinics and hospitals are: Visual Acuity, Visual Field, Colour Vision and Contrast Sensitivity.

Different testing procedures and standard is followed by different countries at the time of issuing a Driving Licence. In India, visual acuity and peripheral field of vision(side vision) is tested for driving licence. In other countries more rigorous examinations are used to assess visual functions, such as measurements of visual field, dark adaptation, stereopsis and glare recovery.

There are several factors affecting driver behaviour such as-

• Vision
• Fatigue
• Alcohol
• Vehicle Visibility
• Road Lighting
• Motor Co-ordination
• Accident Proneness
• Attention

Good visual acuity allows the driver more time to react against obstacles and signs, where as a driver with poor visual acuity requires the obstacle to be closer before its significance can be appreciated, leaving less time to react. He or she must be more alert, and will therefore fatigue faster.

Colour vision defects may be hazardous if they cause confusion between red, green and amber colour signal lights. According to Coles and Brown, a red colourblind person requires about 4times the normal intensity to see a red light.

Good peripheral vision is essential as significant increase of traffic day by day.

Stereopsis is important for judging the nearby vehicles.

Many drivers complain about glare during night driving. This is mainly due to dirt & scratches on the windscreen causing scattering of light.

Glare recovery time(the recovery time of retinal sensitivity) not only increases with age but also with the use of tinted spectacles or windscreen.

Alcohol impairs mental efficiency and slows the response to a hazardous situation. It also can cause diplopia(double vision) and blurring of vision.

Precautions to be taken by the Driver: 

1. First time spectacles or contact lens user should be well adjusted before start driving
2. Thick rim frames should be avoided as it may restrict peripheral vision(rimless frame is ideal)
3. Lenses of the spectacles should be high impact resistant(eg- polycarbonate and MR8)
4. If bifocals are required  they should be dispensed with the smallest possible bifocal segment. Progressive lenses are ideal for driving
5. Tinted spectacles should not be worn for night driving