By Eyecare NG
Safety on the roads does not depend only on road and vehicle conditions. How well drivers see objects in front of them and their ability to respond quickly and adequately to what they see are also very important for road safety.
Driving is a highly visual task. Most of the information required to drive safely and accurately is obtained from one’s vision. This is why it is necessary for everyone to undergo an eye check before driving
In most countries including Nigeria, to obtain a driving licence, some tests are done to determine whether it is suitable for the person requesting for the licence to drive or not. Most times, the only visual function tested is visual acuity (a test that determines how well an individual can see an object in front of him).
Multiple visual skills come into play during driving. In addition to visual acuity, many other visual skills as outlined below are important for safe driving. Ensuring optimum vision function in drivers is necessary for avoiding accidents.
This is a measure of how well the eye sees. It is the clarity of the central vision. Drivers need to see obstacles in front of them, road signs and directions to drive effectively. Poor central vision reduces the reaction time of drivers and this can cause accidents.
Visual acuity is the visual function that is usually tested before driving licences are issued. Normal visual acuity is 6/6. This means that the individual was able to see at 6 metres the size of letters that are meant to be seen at that distance.
The minimum standard visual acuity for drivers is 6/12 with or with optical correction (glasses or contact lenses) in the better seeing eye. For drivers of articulated vehicles like trucks, the minimum standard is 6/9 visual acuity.
Peripheral vision (visual field)
This is the extent one can see objects at the sides while looking at objects straight ahead. Peripheral vision is very important for driving because people need to see objects at their sides, process them and decide how to respond to them while still looking ahead. Someone with poor peripheral vision is at risk of brushing objects by his sides while driving.
Peripheral vision is often not tested before driving licence is issued in Nigeria. However, it is a requirement in USA and Canada. The recommended minimum is 120 degrees binocular visual field. This means that 60 degrees monocular visual field in each eye is required for driving.
This is the ability to see things in three dimension and also judge the distance between objects accurately. When you have good depth perception, you can see the length, width and depth of objects well and also judge how far or near the objects are. While an individual may be able to judge the depth of an object fairly with one eye, good depth perception can only be achieved when the two eyes work together properly.
Good depth perception is important for locating and avoiding bad spots on the road and for changing lanes while driving. It is also required for accurately judging the distance of other vehicles and persons away from your car.
Poor depth perception makes it difficult to locate and avoid bad spots or objects quickly and this can cause accidents on the road.
Eye movements such as fixating on objects, scanning objects in the side vision, and following objects (tracking) as they move in the visual field are important for driving. Defective eye movements due to binocular vision problems, fatigue or use of substances such as drugs and alcohol can affect drivers’ performance and this is a risk factor for accidents.
Accommodation is the ability of the eyes to change focus from a far object to a near one and vice versa. This helps maintain the clarity of objects seen by the eye at different distances. Accommodation is essential for safe driving as a driver needs to be able to change focus quickly and easily from the dashboard or other items in the car to the road and vice versa.
This is the ability to see in conditions where lighting is poor. Night vision tends to decrease with age. To drive safely in low light conditions such as at night, a driver needs to have good night vision. Driving with conditions that interfere with or deteriorate night vision unaided is a risk for accidents.
This is the ability to coordinate visual information with information from other parts of the body. It includes skills such as eye-hand coordination, multitasking, visual-auditory compatibility and other skills in which the brain processes information perceived by the eyes and uses it to initiate reactions by other parts of the body.
Colour vision is the ability of the eyes to distinguish objects based on the wavelength of light they reflect. It is important for driving because drivers need to be able to identify and distinguish traffic signs, reflectors and other objects on the road based on their colours.
Driving with a colour deficiency disease can be risky especially at night. This is because of the difficulty people with this condition experience with identifying reflectors on the road and the rear signal lights of cars ahead.