Reliability of Snellen charts for testing visual acuity for driving: prospective study and postal questionnaire

Objectives To assess the ability of patients with binocular 6/9 or 6/12 vis ion on the Snellen chart (Snellen acuity) to read a number plate at 20.5 m (the required standard for driving) and to determine how health professiona ls advise such patients about driving. Design Prospective study of patients and postal questionnaire to healthcare professionals. Subjects 50 patients with 6/9 vision and 50 with 6/12 vision and 100 genera l practitioners, 100 optometrists or opticians, and 100 ophthalmologists. Setting Ophthalmology outpatient clinics in Sheffield. Main outcome measures Ability to read a number plate at 20.5 m and health p rofessionals' advice about driving on the basis of visual acuity. Results 26% of patients with 6/9 vision failed the number plate test, and 3 4% with 6/12 vision passed it Of the general practitioners advising patient s 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 patie nts with 6/12 vision, 21% said the patients could drive, 54% said they shou ld not drive, and 25% were unsure. The level of acuity at which optometrist s, opticians, and ophthalmologists would advise drivers against driving ran ged from 6/9(-2) (ability to read all except two letters on the 6/9 line of the Snellen chart) to less than 6/18. Conclusions Snellen acuity is a poor predictor of an individual's ability t o meet the required visual standard for driving. Patients with 6/9 vision o r less should be warned that they may fail to meet this standard, but those with 6/12 vision should not be assumed to be below the standard.