Safe Driving Requires Good Vision

Vision is the one human sense that is absolutely essential for safe driving.  Although other senses relay important, but somewhat less essential, information to a driver, so that the appreciation of a situation is much improved, it is estimated that up to 90 percent of the information received is visual.

Clearly a blind person cannot be permitted to drive. However, as a person’s vision can vary from blindness to 6/5, there must be a minimum standard below which it would be inadvisable for anyone to drive.

There has been a marked increase in the volume of traffic on the roads in recent years and the advances in car engine design mean that the traffic is now traveling at much higher speeds than before.

Surprisingly, there is very little evidence that relates the standard of vision to a driver’s performance. Hence it is not easy to determine the minimum level of vision for safe driving. One of the simpler vision tests in use involves only the reading of a number plate at a prescribed distance, although more rigorous examinations are used to assess visual functions, such as measurements of visual field, dark adaptation, stereopsis and glare recovery.

Drivers need to cope with range of confusing visual situation. For example, conditions when visibility is poor due to rain, mist or fog, demand a good standard of vision for safe driving. Some studies have failed to relate poor vision to accident rate. This may be due to the fact that accidents do not generally have one single cause but often result from a combination of events. 

There are believed to be 1000 independent factors affecting driver behavior. These relate to the road conditions, the vehicle and the driver. It is well understood that vision, fatigue, alcohol, vehicle visibility, road lighting, motor co-ordination, accident proneness and attention affect a driver’s behavior. Fortunately, there is a certain amount of self-selection. Some people give up driving when their vision deteriorates to a level they consider unsafe. This is particularly evident amongst the elderly who ‘do not like to drive at night’ as they have reduced dark adaption. They often restrict their driving to the daylight hours.

Recommended:  Relationship between Visual Function and Driving Performance

Road user error is responsible for at least 95 percent of accidents and 44 percent of drivers at fault is judged o have made perceptual errors. Vehicles and road/environment is responsible for only 9 and 28 percent of accidents respectively. 

The perceptual error made is mainly due to distraction and misjudgment of speed and distance. The latter is often due to a lack of visual adaptation to speed. After driving at 70 mph, 30 mph seems very slow, but to travel 30 mph after being in a slow moving traffic jam seems relatively fast. The visual adaptation is believed to occur due to the streaming of the visual field across the periphery of the retina. A method of reducing accidents at roundabouts, where many accidents are due to drivers entering too fast, is to paint transverse bas across the road with decreasing separation. The lines are intended to create the illusion of speeding and hence the driver will reduce speed. 

They also act as a visual hazard warning. These transverse bars have reduced accidents due to cars entering a roundabout too fast by about 60 percent at 50 trial sites.

Photo Credit: theorydrivingtest.co.uk