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

Abstract

Objectives: To assess the ability of patients with binocular 6/9 or 6/12 vision 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 professionals 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 general 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 professionals' advice about driving on the basis of visual acuity.

Results: 26% of patients with 6/9 vision failed the number plate 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. The level of acuity at which optometrists, opticians, and ophthalmologists would advise drivers against driving ranged 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 to meet the required visual standard for driving. Patients with 6/9 vision or 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.