Information As A Source of Distraction

Chapter 3. Legibility Testing

In addition to the psychometric tests conducted in the laboratory, the CMS was taken outside, and the legibility distance of various messages was tested. The primary interest was the relationship between letter height and legibility distance.

Methods

Equipment

The CMS was a Daktronics® VF-2320 full-color (RGB) matrix display with 0.79-inch (20-mm) pixel pitch. The display surface was 4 by 4 ft (1.2 by 1.2 m), which accommodated a 64- by 64‑pixel display. The display is shown in figure 19.

Figure 19. Photo. Sign used for legibility testing.

Testing was done midday. The stimuli were four-letter words presented individually at a rate of 1.25 words/s with a blank interval of 0.3 s between words.

Procedure

Testing was done during daylight hours between 1 and 3 p.m. The typical ambient illuminance was 2,487.9 fc (26,780 lx). The sign face was oriented to the northwest, with the Sun to the southwest at approximately 60 degrees ±15 degrees elevation. The sign brightness was set to 100percent. The sign luminance measurement results for white and yellow stimuli are show in table 8. The luminance measures were obtained by illuminating 12-inch (30 cm) squares of the specified color at the specified locations on the display. A Konica Minolta® Chroma Meter CS‑100 was used to take the luminance readings. The readings were taken from a distance of 50ft (15.2 m) with a 1-degree aperture. The luminance of a black (all pixels off) block was 67 fc (229cd/m²⁾ which yields a mean contrast ratio of 43 for yellow characters and 46 for white characters.

Table 8. Luminance and CIE color coordinate measures for colors of fonts used in legibility testing.

The three font sizes used were 7 (height) by 5 (width) pixels, 12 by 8 pixels, and 16 by 8pixels. All fonts used a 2-pixel stroke width. The actual height of the characters, which were all displayed in uppercase, can be computed by multiplying the pixel size, 0.79 inches (20 mm), by the font height. However, it is common to assume that there will be some apparent blooming of the characters that adds to the apparent font height. Therefore, for the remainder of this section, a bit less than 0.5 inches (12.7 mm) of blooming is assumed so that the three fonts are referred to by their assumed heights: 6, 10, and 13 inches (15, 25, and 33 cm). Each font was tested once with amber letters (red and green LEDs that constitute a character pixel at 100percent) and once with white letters (all LEDs that constitute a character pixel at 100 percent).

Markers were laid out on the ground on a line perpendicular to the face of the sign at intervals that were multiples of 15 ft (4.6 m). Specific distances where markers were placed were 180, 270, 300, 315, 360, 390, 405, 450, 525, 581, and 600 ft (55, 82, 91, 96, 110, 119, 123, 137, 160, 177, and 183 m). On alternating trials, participants were tested at successively longer distances until they could identify no words, or successively shorter distances until they could identify all words. Each set of trials began with the participant standing at either the closest marker or at the 581-ft (177-m) marker. At the 581-ft (177-m) marker, a wall presented a physical obstruction that prevented testing at a precise multiple of 15 ft (4.6 m). Across participants, the beginning distances were roughly counterbalanced such that particular font sizes and colors were equally distributed between near or far markers.

Fifty-nine six-word lists were available. No words were repeated within a list of six words. Each word could appear on up to three lists but not in the same serial order across lists.

Participants

Six individuals, five males and one female, were tested. All were Government or contract personnel who worked at the FHWA Turner-Fairbank Highway Research Center. The mean visual acuity of the participants was 20/17 (range 20/13 to 20/25) as assessed with the Landolt C module of the Freiburg Visual Acuity test.⁽⁴⁸⁾

Results

The mean proportions of six words that were correctly read at each distance are shown in figure 20. The MUTCD criterion for legibility distance for letters on static signs is estimated using 30 ft (9 m) per inch (2.5 cm) of letter height.⁽²⁾ With 6-inch (15-cm) letter height, nearly all words could be read at 180 ft (55 m). With 10-inch (25-cm) letters, which the MUTCD method suggests should be legible at 300 ft (91 m), 92 percent of white lists and 100 percent of amber lists were read correctly. The MUTCD method suggests 13-inch (33-cm) letters should yield a legibility distance of 390 ft (119 m). At that distance, 100 percent of white lists and amber lists were read correctly. At 581 ft (177 m), 89 percent of 13-inch (33-cm) white character lists were read correctly.

1 inch = 2.54 cm
1 ft = 0.305 m

Figure 20. Graph. Legibility testing results.

Because the visual acuity measure used here is linear, and the relationship between letter height and legibility distance is linear, the CMS results can be easily compared with the MUTCD guidance for estimating legibility distance of standard highway signs of 30ft/inch of letter height and assumes a minimum visual acuity of 20/40.

Assuming 90-percent correct word identification as the criterion for legibility distance, table 9 shows the results for this study for participants with a mean visual acuity of 20/17 and the linear extrapolation to what the results would have been with individuals having an acuity of 20/40, which is the minimum acuity required for a driver’s license in most States.

Table 9. Observed legibility and linear transform of legibility distances for persons with 20/40 visual acuity.

Because of factors such as small sample size (n = 6) and testing a limited number of fixed differences, the results are not perfectly linear. However, they suggest that for persons with normal vision, the CMS characters yield a legibility distance of 40 to 45 ft/inch (4.8 to 5.4 m/cm) of letter height, and for persons with 20/40 vision 20 ft/inch (2.4 m/cm) would yield a reasonable estimate of legibility distance.

Discussion

These results suggest the 0.79-inch (20-mm) pixel pitch full-color display used in this test provides a longer legibility distance than the previous generation of CMS displays that used a 1.6‑inch (40-mm) pixel pitch with amber LEDs.⁽¹⁾ They also suggest that for the display type used, drivers with approximately 20/20 vision, the 90- to 100-percentile legibility distance can be conservatively estimated using a factor of 40 ft/inch of letter height (9 m/2.5 cm). It appears that amber characters may have slightly longer legibility distance than white characters, even though the amber characters use one-third fewer LEDs than the white characters.

The test reported in this chapter used participants who were standing still and not looking through a windshield. This would be expected to yield somewhat more accurate and somewhat longer legibility distances than those obtained from observers in a moving vehicle. The next chapter reports on legibility distances for stimuli viewed by drivers of a moving vehicle.