I think people who test vision should put a licence plate outside so that individuals can test themselves

Dr Zanna Currie, Royal Hallamshire Hospital, Sheffield

Legally, anyone who drives is required to be able to read a car licence plate from a distance of 20.5 metres.

According to the Driver and Vehicle Licensing Agency, this means scoring between 6/9 and 6/12 on a standard eye chart.

Perfect vision is defined as 6/6 - where one can read a certain line of letters from a distance of six feet.

People with 6/9 vision can only read a line at six feet that a normal individual could read at nine feet.

Those with 6/12 vision can read a line at six feet that people with 6/6 sight can read at 12 feet.

The Royal College of Ophthalmologists recommends that a score of 6/10 should be the limit.

However, this score does not exist on the chart - it is between two lines of letters.

This, according to the researchers, leaves many health professionals confused and means some approve those who shouldn't be allowed to drive.

Study

Dr Zanna Currie, a specialist registrar at Sheffield's Royal Hallamshire Hospital who carried out the research, studied 100 people with relatively poor vision who had had an eye test.

Standard eye charts are used to test vision

She found that one in four people with 6/9 vision and three out of four with 6/12 vision were unable to read a licence plate at 20.5 metres.

She also found that health professionals differed when it came to deciding whether people with these levels of sight could drive.

For instance, three out of four GPs said they would tell someone with 6/9 vision that they could drive while one in five would tell someone with 6/12 vision they could get behind the wheel. Opticians and ophthalmologists were stricter.

Dr Currie said the study showed that using a standard eye test to determine if someone could drive was inadequate.

Speaking to BBC News Online she said the only way of making sure an individual had sufficient eye sight to be able to drive was to see if they could read a licence plate.

"As the law stands at the moment it is the driver's responsibility to make sure they come up to scratch.

"I think people who test vision should put a licence plate outside so that individuals can test themselves."

She said that being able to read an eye chart indoors did not mean an individual would be able to read a licence plate.

She said differences in light meant that the results of an indoor test could be skewed.

A spokesman from the DVLA which is responsible for issuing licences said its recommendations were only meant to guide health professionals.

"This guide is as it suggests purely for guidance to the medical profession and DVLA are aware that there will be certain people who will be unable to meet the legal requirement despite apparently satisfactory visual acuities.

"This guide is not intended to be prescriptive and the final legal requirement is the ability to read the number plate."

The study is published in the British Medical Journal.