UK drivers are being urged to test their eyesight after a DVLA survey suggested 50% of motorists were not aware of the minimum standards needed for a licence.
Drivers must be able to read a number plate from 20m (65ft) away.
But the agency's survey found only half of drivers knew about the eye test and used it to regularly self-check, as they are legally required to do.
Those who do not pass the test should visit an optician, a DVLA doctor said.
Learner drivers must pass the eye test as part of their practical exam - and are then legally obliged to ensure their sight remains good enough to drive thereafter.
The DVLA's Wyn Parry warned vision can "naturally deteriorate over time", adding that regular checks to ensure good eyesight were "essential for safe driving".
Performing the test can be done on any street and only takes "a couple of seconds", said Dr Parry.
Five car lengths is said to roughly match the 20m distance, with drivers being encouraged to use the measure to test their vision on the number plates of passing cars.
There are currently 48 million registered drivers in the UK, according to the DVLA.
Last year, the Association of Optometrists called for those behind the wheel to have compulsory eye tests every 10 years.
One in three optometrists said they had seen patients who continue to drive with vision below the legal standard, the association said.
Drivers over the age of 70 must actively make a declaration every three years that they are fit to drive - but do not actually have to pass a test.