In order to drive safely, less than half of UK drivers questioned by the Driver and Vehicle Licencing Agency (DVLA) knew they had to be able to read a number plate from 20 metres away.
It’s a quick and simple vision test. Finding a number plate to do the vision test is not difficult, and the DVLA provides estimates of about 20 metres, or five car lengths, or the width of eight parking bays. Is it, however, a fair test of vision for driving?
When you don’t have a vision test on hand, it’s simple to administer as a rough and ready test. There are, however, a number of important factors that can affect how easy it is to read, including the amount of light, the time of day, the weather, the age of the individual, the colour and cleanliness of the number plate, and the actual letters and spacing.
Visual acuity is assessed and reported as a fraction in UK ophthalmology clinics, e.g., 6/6, 6/9, or 6/12 on the Snellen test. The eye’s distance from the Snellen chart (measured in metres) is the numerator (number above the line), and the letter chart’s line is the denominator. Typically, 6/60 Snellen acuity is represented by the largest letter on the chart. A smaller denominator indicates greater vision; a bigger denominator indicates worse vision.
The test, developed in the 1860s by Dutch physician Hermann Snellen, is gradually being supplanted by the more recent LogMAR test, which features more uniform letter spacing. However, driving regulations in the UK are still Snellen for the time being.
The phrase “20/20 vision,” which refers to having good, clear eyesight, is quite well-known. The American acuity measurement paradigm, in which a 20-foot distance is used as the numerator, is where the phrase “20/20” originates. Therefore, in the UK, 20/20 vision corresponds to 6/6 vision on the Snellen chart with a numerator distance of 6 metres.
The crowding problem
In the UK, you must be able to read a number plate at a distance of 20 metres, have a full visual field (the test of your peripheral vision), and have at least 6/12 Snellen vision. Crowding is a phenomenon that causes some people who meet the Snellen standards for visual acuity and visual field to be unable to see all of the letters on a number plate at a distance of 20 metres.
When a letter is more difficult to recognise when surrounded by other letters than when it is by itself, this is known as crowding. This explains why it’s simpler to recognise the outer letters on the licence plate than the inner ones.
Certain medical problems, such as amblyopia (lazy eye), glaucoma (increased pressure inside the eye causing nerve damage), and advanced age, are associated with greater crowding than others.
People’s perception and reaction times to cluttered items are hampered by crowding. People experience crowding even when image congestion is presented independently to each eye, which is why its exact explanation is unknown but suggests it occurs in the visual cortex, the area of the brain responsible for processing vision.
In crowding, picture segregation becomes more challenging the “busier” and more cluttered the visual information is being processed in the visual cortex. The inability to adequately separate the things is the problem, not the inability to see them.
So is it fair or important that we continue to check visual acuity, including crowding with the 20m test?
The population of the UK is getting older, desiring to stay independent, and working longer before retiring. In fact, during the past ten years, the number of drivers who are 90 years of age or older has doubled to over 150,000, according to the DVLA.
There is little to no association found between driving performance or crash statistics and visual acuity. Although it may demonstrate a stronger correlation, contrast sensitivity—the capacity to distinguish between different shades of grey—is not evaluated for driving.
According to a 2008 study, there are no discernible differences in collisions and serious incidents between Australian jurisdictions that require age-based vision testing for driving and those that do not.
Glare sensitivity, visual field loss, and useful field of view (the area you can extract visual information from in a single glance without moving your eyes) are significant predictors of driving accidents. And the useful field of view has been shown by several studies to play an important role in a person’s ability to drive safely.
Since the primary focus of current driving exams is visual acuity, some safe drivers may be disqualified, and other significant features of visual or cognitive impairment that are more closely linked to driving competence may be overlooked.
The lack of a legal vision requirement for operating a mobility scooter may worry some people, but quitting driving can also lead to social isolation and depression.
Better test predictors would suggest that our current requirements are keeping people from driving who would otherwise be safe drivers.
But rather than having nothing at all, is it better to employ a readily available number plate test, despite its shortcomings, to provide an immediate assessment in the absence of a better test at the roadside? Would it be preferable to refer drivers to a more dependable exam that is taken in a reasonable amount of time?
This may be something we need to consider in the near future in a country like the UK, where the population is ageing and retirement age is rising.