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••JUIY. (.CI_'IU'4 1•ellr-I'I LtrGVT DahlIJ nv.moo •`+' 11 I TABLE 8 MINIMUM RESOLVABLE SNELLEN FAND20/60 TER SIZE OF lEYES WITH 20/20, 20/40, EN 100 F ACUITY AT ND 1/4'MILE CES LETTER SIZE ANCE MINIMUM (RESOLVABLE (Feet) (Inchon) 20/20 20/40 20/60 100 1.8 3.3 5.2 200 3.5 7.0 10.5 400 7.0 14.0 20.9 600 10.5 21.0 31.4 800 .14.0 28.0 41.9 1000 17,5 35.0 52.4 1320 23.1 46.1 69.1 Visual acuity is usually meast,rj'ed under optimal conditions in a doctor's office where the viewer is not required to attend to anY other tasks. Two important influences, the fac, of motion arld the influ- once of color, are not taken into account in tables of normal visual acuity. 3'his implies that letter sizes suggested by the statistics on visual acuity may not be adequate for a sign in the environment. One other factor needs to be considered: letters on a sign form words and words are more immediately recognizable than separate, unrelated letters. Words are easier for the brain the process than letters because they are meaningful chunks of information. Even if we assume normal visual acuity to. be a constant factor, we still must make adjustment for changes in the sign and the angle of the sign in relation to traffic. Roughly speaking, one inch of letter size to twenty-fiVe feet of viewing distance has been considered about average. No one would argue that the ratios,given in. Table 8, which describe viewing requirements in a doctor's ofice, can be directly applied to the motorist attending to the driving task and extracting visual information from a diverse visual environment. Table 8 figures are meant to show that there is not a universal norm for visual acuity and that sign size must be related to actual Visual capacities of the sign viewing public at large. 153 pozerrnining The Size Of A Sign