title: Distance word acuity, critical print size and driving vision standards.
abstract: BACKGROUND
  A comparison was made between letter visual acuity and word acuity at distance and between letter acuity and the minimum word size allowing maximum and fluent reading speeds.
  
  
  METHODS
  Visual acuities were measured at six metres for 120 participants. Letter acuity was assessed with a logMAR layout chart with Sloan letters displayed with Thomson XPert software. A logMAR layout lowercase word chart was created in PowerPoint using the format of the MNRead near chart, with the font and colours found on United Kingdom motorway signs. Distance letter and word acuity, maximum reading speed, critical print size (CPS) and minimum print size for fluent reading were calculated.
  
  
  RESULTS
  Word and letter acuities were significantly correlated (p < 0.001) and showed a statistically (p = 0.049) but not clinically significant mean difference of 0.02 ± 0.09 logMAR. Bland-Altman analysis showed that agreement between charts varied depending on acuity level with word acuity better than letter acuity at levels closer to the driving standard of +0.30 logMAR (6/12). Maximum reading speed was achieved from print sizes 1.5 logMAR lines larger than the letter acuity and fluent reading at 80 words per minute from print sizes 0.06 logMAR larger than the letter acuity.
  
  
  CONCLUSIONS
  These results allow equivalent lowercase print sizes supporting either functional reading or maximum reading speeds to be calculated based on letter chart acuities at distance. Minimum print sizes allowing either functional or maximum reading speeds are more appropriate than threshold word acuities, where groups of words or short phrases need to be identified, such as when driving. Where a driving vision standard of +0.30 logMAR (6/12) exists, a person just meeting the driving standard should be able to read fluently words on signage in lowercase x-heights equivalent to between +0.29 and +0.30 logMAR.
authors.affiliation: Visual Function and Physiology Research Group, Department of Vision and Hearing Science, Anglia Ruskin University, Cambridge, United Kingdom.|Vision and Eye Research Unit, Postgraduate Medical Institute, Anglia Ruskin University, Cambridge, United Kingdom.
authors.forename: Sheila
authors.initials: S
authors.lastname: Rae
authors.affiliation: Visual Function and Physiology Research Group, Department of Vision and Hearing Science, Anglia Ruskin University, Cambridge, United Kingdom.|Vision and Eye Research Unit, Postgraduate Medical Institute, Anglia Ruskin University, Cambridge, United Kingdom.
authors.forename: Keziah
authors.initials: K
authors.lastname: Latham
authors.affiliation: Visual Function and Physiology Research Group, Department of Vision and Hearing Science, Anglia Ruskin University, Cambridge, United Kingdom.
authors.forename: Maria Foteini
authors.initials: MF
authors.lastname: Katsou
mesh_terms: D000293:Adolescent; D001334:Automobile Driving; D005260:Female; D006801:Humans; D008297:Male; D011932:Reading; D012858:Size Perception / Q000502:physiology*; D014787:Vision Tests / Q000379:methods*; D015348:Vision, Binocular / Q000502:physiology*; D014792:Visual Acuity / Q000502:physiology*
keywords: driver's vision; driving; reading; visual acuity