Many UK drivers shouldn't be on the road as they don't meet new EU vision standards, according to research by Anglia Ruskin University.
The UK came into line with the rest of the EU in 2012 by introducing the need to achieve a score of 6/12 on a Snellen scale letter chart.
The EU directive aims to standardise the level of vision required to drive across the EU so that license holders in one member state will meet the requirements to drive in others.
In the UK, the directive has been addressed by introducing an acuity standard of 6/12 in addition to the existing test of reading a number plate at 20m outdoors, which has been the UK standard since 1935.
New research, led by Dr Keziah Latham and Dr Sheila Rae of Anglia Ruskin University and published by the British Journal of Ophthalmology, shows that the two tests are not comparable.
It found that with a sample of drivers with vision around the borderline for driving, 15% of those tested could read the number plate at 20m but failed the new 6/12 standard on the Snellen chart.
Dr Latham, Senior Lecturer in Vision and Hearing Sciences at Anglia Ruskin University, said: "Given that there are now two standards, we wanted to investigate whether these two tests pass and fail the same drivers. Our research shows that drivers who pass one test do not necessarily pass the other."
Whereas the number plate check is compulsory, it is only carried out during the practical driving test. The new vision test is in effect voluntary and requires anyone failing to meet the standard during a visit to their optometrist to then notify the DVLA.
Dr Latham believes the new 6/12 test could see people avoid visiting their optometrist if they are worried there is a danger of losing their licence. She added: "Optometrists are in a very difficult position as they are the ones who have to inform people if they are not fit to drive.
"Of course, a big danger is that people who believe their eyesight is failing could simply avoid seeing an optometrist, because once the individual is told they are not fit to drive it is their duty to inform the DVLA.
"Instead many people might prefer to soldier on, which is a danger not just to themselves but to other road users. And of course, it prevents them from receiving expert help which could improve their vision.
"Optometrists can not only optimise a driver's vision by providing spectacles or contact lenses, but can also advise on managing glare and other lighting issues when driving.
"An eye examination will also detect the early signs of eye disease, possibly even before there are any effects on vision, making conditions easier to manage and less likely to result in vision that doesn't meet the driving standard.
"Our findings highlight the benefits of making eye examinations compulsory for all drivers to ensure that they meet the necessary acuity requirements to drive."