Reliability of Snellen charts for testing visual acuity for driving: prospective study and postal questionnaire
Another look at visual standards and driving
Guidelines to assess whether a person's vision is good enough to drive are a poor predictor of an individual's chance of meeting the legal visual standard for driving and need clarification, finds a study in this week's BMJ.
The legal standard required for driving a private car or motorbike is to be able to read a number plate at 20.5 metres. Guidelines issued by the Driver and Vehicle Licensing Authority suggest that this corresponds to a score of between 6/9 and 6/12 on a standard visual acuity chart (known as the Snellen chart). Researchers at the Royal Hallamshire Hospital in Sheffield tested 50 patients with 6/9 vision and 50 with 6/12 vision on their ability to read a number plate from a distance of 20.5 metres. They also assessed the advice given to these patients by healthcare professionals using a postal questionnaire.
They found that 26% of patients with 6/9 vision failed the test, and 34% with 6/12 vision passed it. Of the general practitioners advising patients with 6/9 vision, 76% said the patients could drive, 13 said they should not drive and 11% were unsure. Of the general practitioners advising patients with 6/12 vision, 21% said the patients could drive, 54% said they should not drive, and 25% were unsure.
It cannot be assumed that a driver with a visual acuity of 6/9 will meet the standard for driving, say the authors. Conversely, it should not be assumed that a driver with a visual acuity of 6/12 is below the standard for driving. All drivers with 6/9 vision or less should be encouraged to self assess their vision, they conclude.
In an accompanying editorial, William Westlake at The Lions Eye Institute in Australia argues that more sophisticated tests are required to help determine the driving ability of people who do not meet the current standards and, when appropriate, allow them to retain their licenses. He stresses the need for the Driver and Vehicle Licensing Authority in the UK to monitor the results of the current visual requirements to "confirm that there is at least some benefit to be gained for society from the devastating effect that removal of a driving licence can have upon a visually impaired individual."
Contacts:
Zanna Currie, Specialist Registrar, Royal Hallamshire Hospital, Sheffield, UK
Email: zanna@zcurrie.freeserve.co.uk
William Westlake, Visiting Research Fellow, The Lions Eye Institute, Western Australia
Email: westlake@networx.net.au