title: legibility distances of fluorescent traffic...

Paper Number: 01-2417

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Transportation Research Board

Title: LEGIBILITY DISTANCES OF FLUORESCENT TRAFFIC SIGNS AND

THEIR NORMAL COLOR COUNTERPARTS

Authors: Thomas Schnell, Keith Bentley, Elizabeth Hayes, and Martin

Rick

Transportation Research Board 80th Annual Meeting January 7-11, 2001

Washington, DC

Schnell, Bentley, Hayes, and Rick 2

LEGIBILITY DISTANCES OF FLUORESCENT TRAFFIC SIGNS AND THEIR NORMAL

COLOR COUNTERPARTS

Thomas Schnell, Ph.D.

Assistant Professor and Director

Operator Performance Laboratory (OPL)

Department of Industrial Engineering

4135 Seamans Center

The University of Iowa

Iowa City, Iowa 52242-2527

(319) 384 0811

[email protected]

and

Keith Bentley, Elizabeth Hayes, and Martin Rick

Undergraduate Students

Department of Industrial Engineering

The University of Iowa

Iowa City, Iowa 52242-1527

Transportation Research Board 80th Annual Meeting January 7-11, 2001

Washington, DC


ABSTRACT

It is well known that fluorescent traffic sign have a daytime conspicuity that is far superior to that of normal

color signs. This study was conducted in order to test the hypothesis if adding the property of fluorescence

while leaving all (or in practical terms most) other sign features the same, does in fact increase the

legibility distance. Six diamond shaped signs were fabricated and laminated with the normal colors on one

side and their fluorescent color counterparts on the other side. The investigated sign background colors

included yellow, fluorescent yellow, fluorescent yellow-green, pink, fluorescent pink, purple, and

fluorescent purple. Four of the six signs used Landolt rings to avoid word familiarity issues. A pedestrian

crossing symbol sign and a PLANT ENTRANCE warning sign were used in addition to the Landolt ring

signs. It was found that fluorescence actually did statistically significantly increase the legibility distances.

However, the actual increases were fairly small from a practical point of view, ranging from 5.3% to 15.9%

when adding the property of fluorescence. In general, for negative contrast signs and for positive contrast

signs where both the legend and the background are fluorescent, a slight increase in the legibility distance

can be expected. It should be noted, that the illumination conditions that were present in this study (fair

day) represent somewhat of a worst-case scenario for fluorescent colors. Larger percent increases in

legibility distance may be expected when using fluorescent colors during the time of twilight, during

overcast days, and during daytime in inclement weather.


INTRODUCTION

Fluorescent colors in traffic sign sheeting materials represent a fairly new method for attracting driver

attention and for enhancing legibility of text or recognition of symbols. Substantial previous research (see

below in literature review) indicated that fluorescent colors are far more attention getting than their normal

color counterparts. The attention getting quality of fluorescent colors is especially pronounced during

overcast or inclement weather daytime hours, and during the time of twilight. Also, fluorescent colors are

far better detected and recognized (color classified) than normal colors at large angles of eccentricity in

the visual periphery. In 1998, the Federal Highway Administration (FHWA) amended The Manual on

Uniform Traffic Control Devices (MUTCD) [1] to approve the optional use of the color fluorescent yellow

green (FYG) for pedestrian, school, and bicycle crossing signs.

The present study was conducted to determine the legibility distances of traffic signs laminated with

selected fluorescent color sheeting materials and their normal color counterparts. The goal was to

determine the legibility distance of traffic signs with and without adding the property of fluorescence. The

investigated sign background colors included yellow, fluorescent yellow, fluorescent yellow-green, pink,

fluorescent pink, purple, and fluorescent purple. Fluorescent yellow-green was investigated because of its

importance in school zones, pedestrian zones, and bicycle lanes. Fluorescent yellow was investigated

because this color gains importance in high hazard warning sign applications. Pink and purple as well as

their fluorescent counterparts were investigated, because these unassigned MUTCD colors may become

important in the near future for emergency vehicle routing and other special purpose signing.

The study results indicate a small increase in the legibility distance when using selected fluorescent

color combinations on traffic signs. We hypothesize that the increase in legibility distance primarily comes

from the increase in luminance contrast when fluorescence is added. This increase in luminance contrast

is particularly notable for negative contrast signs where the legend consists of black paint and the

background is a light fluorescent color. Care must be taken to avoid contrast reduction when adding

fluorescence to the background of positive contrast signs, especially if the legend is non-fluorescent. In

such cases it is actually possible that a reduction in the legibility distances may be observed. In summary,

it is concluded that fluorescent colors outperform their normal color counterparts both in terms of detection

and in terms of recognition.


REVIEW OF THE TECHNICAL LITERATURE

Studies involving Fluorescent Colors

There are references to fluorescent colors that are technically incorrect. Wortham [2] described

fluorescent colors as reflecting ultraviolet (UV) light, leading to sign reading distances that are 3 to 5 times

greater than what is found with normal colors. We certainly question that one can increase the legibility of

a sign by this stated amount simply by adding fluorescence. Detection distances are likely to be much

longer when fluorescence is used but legibility distances appear to be much less affected by the presence

of fluorescence. Secondly, humans cannot see UV light and thus one would not perceive any visual

stimulation if UV light was actually reflected from a sign. Durable fluorescent traffic sign sheeting materials

do not make use of any UV light, much rather, short wavelength light (in the visible blue range) is

absorbed by the fluorescent pigment, and re-emitted at a longer wavelength, thus giving the overall

appearance of the material to be brighter than would be expected based on reflectance alone.

A sound technical description of the workings of durable fluorescent traffic sign materials is given by

Burns and Pavelka [3]. Zwahlen and Schnell [4] investigated daytime conspicuity of fluorescent and non-

fluorescent background color targets in the field against a green background as a function of the

peripheral angle and the target size. Zwahlen and Vel [5] conducted a daytime field study to determine the

conspicuity in terms of peripheral visual detection and recognition of different fluorescent and non-

fluorescent color targets against different backgrounds using only one target size (0.152m x 0.305m, 6" x

12"). Burns and Johnson [6] conducted chromaticity measurements and subjective rating experiments in

the field using fluorescent and non-fluorescent color targets. Brekke and Jenssen [7] conducted a study

investigating the visibility distances of fluorescent and non-fluorescent signs.

Studies on Legibility

Schnell and Zwahlen [8][9] hypothesized that for a given available (rectangular) display width and height, it

is possible to enhance the standard highway alphabets (FHWA, 1977) with regard to legibility by reducing

the absolute value of the legibility threshold contrast. Information collected by Hummer and Scheffler [10]

indicated that fluorescent traffic signs affected driver behavior in a positive way. Zwahlen and Schnell [11]

described an exploratory daytime/nighttime sign recognition/legibility field driving experiment involving 11


new reflectorized right shoulder mounted traffic signs and 10 young, healthy subjects (3 replications). The

experimental procedure used in the present research followed exactly the experimental method used by

Zwahlen and Schnell [11]. Table 1 shows the daytime results of the legibility study performed by Zwahlen

and Schnell [11]. In our present research, we used a PLANT ENTRANCE warning sign with the exact

same dimensions as the one used by Zwahlen and Schnell. Their PLANT ENTRANCE sign provided an

average daytime legibility distance of 125.5m while our non-fluorescent PLANT ENTRANCE sign provided

113.1m. This appears to represent a fairly good correlation on that base line. Sanders [12] provides a

simple mathematical relationship for converting the Snellen visual acuity score of a person into a detection

distance for reading text. Zwahlen, Sunkara, and Schnell [13] conducted a thorough review of the legibility

literature and categorized the research findings of the reviewed studies into three major groups, effects of

width to height ratio (W/H), effects of stroke-width to height ratio (SW/H), and effects of inter character

spacing to height ratio (S/H) on legibility. Their paper represents an extensive collection of relevant

legibility literature.


METHOD

This study involved daytime viewing, and it was therefore decided that a field setting was the most

appropriate method for the collection of the legibility distances. There are established laboratory methods

[14] to determine the legibility of textual messages, however, to fully capture the potential of fluorescent

color signs, it was felt that a dynamic driver observation task in the field was more appropriate. A total of

six diamond signs were fabricated with one side being the fluorescent color and the other side being the

non-fluorescent color. Landolt Rings were used as legend. Landolt rings (or often called Landolt Cs) are

internationally standardized optotypes established at the 11th International Congress of Ophthalmology,

which was held in Naples, Italy, in 1909, and the inventor was Mr. Landolt, a French ophthalmologist. The

height of a Landolt ring must be exactly five times its stroke with, and the gap opening must be exactly

equal to the stroke width. Like the Snellen E optotypes often used by ophthalmologists, the subject’s job is

to detect the orientation of the Landolt ring by identifying the direction in which the opening is facing (either

in 90o increments, up, down, left, right, or in 45o increments). A PLANT ENTRANCE warning sign was also

used for base line comparison purposes with previous work performed by Zwahlen and Schnell [11]. A

pedestrian symbol sign was used because of ongoing pedestrian safety research at the Operator

Performance Laboratory (OPL) at the University of Iowa.

Subjects

Nine young and healthy subjects (6 males, 3 females) were recruited for this experiment. All subjects had

a valid US driver’s license, normal or better visual acuity, normal or better contrast sensitivity, and showed

no color perception deficiencies. All subjects signed consent forms and instruction forms prior to

beginning their experimental runs. The subjects were instructed to verbally indicate the first point in time

the orientation of the Landolt ring opening was clearly discernible, the PLANT ENTRANCE legend was

clearly legible, or the pedestrian symbol was clearly recognizable during the approach. Guessing was

discouraged. Resulting from this method are functional legibility distances. A discussion as to the

adequacy of our subject sample is given in the discussion section of this paper.


Apparatus

Experimental Traffic Signs

The six diamond shaped experimental signs are shown in Figure 1 and photometric details are shown in

Table 2a. All Landolt rings had a diameter of 200mm and a stroke width of 40mm (being equivalent to the

gap opening). From Figure 1 and Table 2a, it can be seen that sign 1 had a non-reflective black paint

Landolt ring and a microprismatic normal yellow background on side 1, and a microprismatic fluorescent

yellow-green background on side 2. Sign 2 had a non-reflective black Landolt ring, a microprismatic

normal yellow background on side 1, and a microprismatic fluorescent yellow background on side 2. The

reader may note that both sign 1 and sign 2 had one side that was normal yellow. This duplication by

introduction of the dummy level of normal yellow was necessary to keep the experiment fully balanced, as

we were evaluating three types of yellow on four sign faces. Sign 3 had a non-reflective black Landolt ring,

a non-reflective normal pink background on side 1, and a microprismatic fluorescent pink background on

side 2. Sign 4 had a microprismatic normal yellow Landolt ring on a non-reflective normal purple

background on side 1, and a microprismatic fluorescent yellow Landolt ring on a microprismatic

fluorescent purple background on side 2. Sign 5 was a pedestrian symbol sign with a black non-reflective

paint symbol, a normal yellow background on side 1, and a microprismatic fluorescent yellow background

on side 2. Sign 6 was a PLANT ENTRANCE warning sign with a black non-reflective paint legend (0.15m,

6” series C), a normal yellow background on side 1, and a microprismatic fluorescent yellow background

on side 2.

The color coordinates that were measured for a D65 illuminant are listed in Table 2a. Using the CIE

1976 (Luv) colorimetric system it is possible to determine the color difference between the backgrounds

and their corresponding legends. The rightmost column in Table 2a lists the color difference ∆E*uv (for

formulae see [15]) of the individual color backgrounds against their respective legends. From this table, it

can be seen that the added property of fluorescence did in fact increase the color difference ∆E*uv,

indicating that the fluorescent color signs have the potential for being more legible in the configuration

used in this study. The yellow, fluorescent yellow, and fluorescent yellow-green sheeting materials were

commercially available microprismatic (type VII) materials. The microprismatic fluorescent pink and

microprismatic fluorescent purple sheeting materials were prototype materials provided by a sign sheeting

manufacturer. An attempt was made to locate a close color match in the corresponding non-reflective

normal pink and purple counterparts. The signs were equipped with a quick mount mechanism for easy


posting and removal when changing from the fluorescent condition to the non-fluorescent condition was in

order. The sign posts were standard 76mm square posts inserted into sleeves that were driven into the

ground. The bottom tip of each sign was 2.4m (8 ft) above the road surface. The lateral offset from the

right road edge to the center of the sign was 4.2m (wide shoulders are common in Iowa for agricultural

transportation).

Experimental Vehicle

The experimental vehicle was a 1996 Ford Taurus LX Sedan (Figure 2a). The vehicle had a windshield

transmission of 0.71, measured at the observer line of sight. All runs were made with the engine idling and

the vehicle coasting at about 10 – 15 km/h. The following average dimensions apply: Ground to Eye height:

1.14m, lateral distance from longitudinal car axis to the driver's sagital plane, 0.31m. The vehicle was

equipped with a calibrated Distance Measuring Instrument (DMI).

Test Site and Layout

The test site was located on Washington County highway G26, 6 miles north of Washington, Iowa. The

test site map is shown in Figure 2b. The G26 site is ideal for such experiments because of its extremely

low traffic volume, and its straight and level alignment. The highway runs almost exactly in the East-West

direction. During the time of the experiment (8:00am to 4:30pm, April 22, 2000) the sun was always high in

the southern hemisphere, providing a fairly uniform illumination. It should be noted that this experiment

was performed on a fair day with only few clouds. Figure 2c shows the layout of the signs on the test site.

A total of six square posts were driven. Three posts were staggered in the Eastbound direction, and three

posts were staggered in the Westbound direction. Two starting positions were marked on the ground

400m in advance of the first sign in the Eastbound direction and 400m in advance of the Westbound

direction. The pedestrian sign was posted as the first sign in the cluster of signs in the Westbound

direction, so that there was enough approach distance to ensure that no subject could recognize the

symbol from the start. The signs within the test cluster were separated by 121m (with the exception of the

separation between signs 1 and 6, which was 243m). This separation was determined with a small pilot

experiment and turned out to be ideal from an efficiency point of view. A tighter spacing might have led to

a high workload condition for the subject, who would have had to keep track of the signs as they became

legible in rapid succession, a longer spacing would have wasted run time.


Experimental Design and Procedure

The subjects were screened, instructed, and briefed in the laboratory about one day prior to arriving at the

test site. Two subjects were scheduled to arrive in pairs at the test site in intervals of about 1 hour and 30

minutes. Prior to beginning the experimental runs, the subjects familiarized themselves with the

experimental vehicle and the test course. Each subject drove the instrumented Ford Taurus provided by

the Operator Performance Laboratory (OPL) at approximately 10 - 15 km/h down the course. One practice

loop was allowed, and the subjects were given a chance to look at all traffic signs and post installations

prior to the experiment. Care was taken to ensure that the subjects understood that they were to verbally

indicate the first point in time at which they could clearly discern the orientation of the Landhold rings,

clearly recognize the pedestrian symbol, or clearly read the PLANT ENTRANCE legend. A two-factor

within-subjects repeated measures design was used for the Landolt ring signs. The following independent

within subjects variables were used: 1. Fluorescence (present, absent), 2. Sign color (black on yellow-

green, black on yellow, black on pink, yellow on purple). For clarification, the experimental design used in

this study is called a within-subjects design, because the levels of all factors (fluorescence and color) are

presented to all subjects, thus subjects served as their own control, allowing us to determine the effect of

the factors within the subjects.

The dependent variable was the legibility distance obtained with the experimental signs. Three

replications were performed, resulting in a total of 24 Landolt ring observations per subject. The additional

two signs (pedestrian and PLANT ENTRANCE) increased the total number of observations per subject to

36. Presentation order of the fluorescent vs. normal colors and the approach direction were balanced to

avoid any sequence effects. The time for each subject to complete the total number of 36 observations (in

6 round robin passes through the test site) was approximately 45 minutes. One experimenter was riding

along in the passenger seat. This experimenter instructed the driver subject during the experiment. Prior

to each run, the experimenter in the experimental vehicle instructed the subject to pull the car up to the

start mark so that the longitudinal eye location was exactly abeam the start mark. The experimenter then

reset the DMI, and asked the subject to start the run. The DMI display was paused (but the DMI kept

counting distance in the background) as soon as the subject indicated legibility of the sign closest to

him/her on the right road shoulder. The distance traveled was read off the DMI and logged into a data

collection sheet. The subject immediately concentrated on the next sign ahead as soon as the present

sign was readable. After one pass through the test site in one direction, the vehicle was lined up at the


start mark on the other end of the test site and the process repeated. The signs were rotated (factor of

fluorescence was switched) after the first three half-replications were complete. The remaining half of the

readings was then completed.


ANALYSIS AND RESULTS

The first step in the data analysis was the offline calculation of the legibility distances. This was

accomplished by subtracting the travel distance from the overall start-to-sign distance (see Figure 2c) for

each sign. The detection distances reported herein represent the longitudinal distance from the observer’s

eye location to the vertical sign face plane. Figure 3 shows the cumulative percentages as a function of

the legibility distance [m] for each of the six signs. Initial visual inspection of these cumulative graphs

immediately reveals that the increase in legibility distance due to fluorescence is not very strong. However,

there is a consistent increase in all cases tested in this experiment. The strongest increase effect is found

in sign 3 (black on pink). Table 2b shows the average legibility or recognition distances for each of the six

signs along with information about the legend dimensions, visual angle subtended by the legend height,

coefficient of variation, the average legibility indices, and the percent increase in legibility when adding the

property of fluorescence. From Table 2b it can be seen that switching from a normal yellow background to

a fluorescent yellow-green background (both with a black legend) increases the average legibility distance

by 5.3%. When switching from a normal yellow background to a fluorescent yellow background (both with

a black legend), the increase is 11.8%. This finding was somewhat surprising as we expected the

fluorescent yellow-green to provide the greater increase of the two. Switching from a normal pink

background to a fluorescent pink background (both with a black legend), increased the legibility distance

by 15.9%. The yellow on purple sign provided an increase in legibility distance of 9.2% when switching

both the legend and the background from non-fluorescent to fluorescent. The fluorescent yellow-green

pedestrian sign showed a recognition distance that was 7.9% longer than that of its normal yellow

counterpart. Finally, a 5.3% increase in the legibility distance was observed for the PLANT ENTRANCE

sign when switching from normal yellow to fluorescent yellow.

Table 3a shows the individual subject legibility distance responses for the six signs. It should be

noted, that the three replications were lumped together, as no significant learning was found. An initial

Analysis of Variance (ANOVA) was performed on the individual replication data, and it was found that

learning was not significant with FREP(2,16)=1.919, p =0.17. Table 3b shows the Analysis of Variance

(ANOVA) table for the Landolt ring signs with all replication data lumped together. The repeated measures

ANOVA was performed only on the Landolt ring signs, because these signs differed only in the within-

subjects independent variables (presence or absence of fluorescence, and color). From the ANOVA table,

it can be seen that the factor fluorescence statistically significantly increased the legibility distances with


FFLUOR(1,26)=17.778, p = 0.0002. However, the practical gain in legibility distance appears to be only

within the range of 5.3% to 15.9% when adding fluorescence. Box plots for the six signs (12 sign faces)

are shown in Figure 4. The boxes are arranged so that for each color the fluorescent member is always

immediately followed by its non-fluorescent counterpart. Similarly to the cumulative plots in Figure 3, it is

seen from Figure 4 that the property of fluorescence works in the right direction for the signs used in this

study. However, the overall increase in legibility distance is smaller than we expected.


DISCUSSION AND CONCLUSIONS

This study was conducted in order to test the hypothesis if adding the property of fluorescence while

leaving all (or in practical terms most) other sign features the same, does in fact increase the legibility

distance. Some concerns were voiced in the literature [7], that the increased daytime brightness provided

by fluorescent signs may actually reduce legibility. This field study used six diamond shaped signs that

were fabricated with one side being the fluorescent color and the other side being the non-fluorescent

color. The fluorescent colors were fluorescent yellow-green, fluorescent yellow, fluorescent pink, and

fluorescent purple. The normal colors were yellow, pink, and purple. It was found that adding the property

of fluorescence to the colors used in this study did in fact statistically significantly increase the legibility

distances. However, the actual increases were quite small from a practical point of view, ranging from

5.3% to 15.9% when adding the property of fluorescence. However, all signs showed a consistent

increase in the legibility distance, which is good news for traffic sign designers. We hypothesize that the

small increase in the legibility distance may be attributed to the increase in the color difference ∆E*uv

between the legend and the sign background (see Table 2a), and the increased background luminance,

leading to an increased luminance contrast.

The reader is made aware at this point that our sample of nine subjects consisted only of young,

healthy subjects with normal or better visual acuity. As such, the sample is quite homogenous and with

three replications, we are confident that the sample is very adequate from a statistical point of view, as

evidenced by the high statistical power in the Analysis of Variance (ANOVA) of 0.99 and 1.0 for

fluorescence and color, respectively (see Table 3). These high power values indicate that our statistical

test based on our subject sample was so powerful, that the shift in the mean detection distance due to the

factors of fluorescence and color would be detected virtually at all times. In other words, we are very

confident, that the increases in legibility distance are in fact due to fluorescence and not due to chance

and variation. However, we are aware that our subject sample does not fully represent the licensed driver

population as a whole, since we did not include any older subjects.


A word of caution may be in place here. The property of fluorescence may potentially reduce

legibility distance in fluorescent positive contrast signs, if the legend is non-fluorescent. In this case it is

well possible that the increased background luminance actually reduces the luminance contrast, because

the non-fluorescent legend will not change in terms of luminance. In such cases, sign designers may have

to trade off the increased detection potential of the fluorescent sign with a slight reduction in legibility. In

most cases, this slight reduction can be expected to be in the range of a few percent. Sign designers may

want to use a larger legend in positive contrast signs with a fluorescent background and a non-fluorescent

legend. It may also be possible to use a thin black border around the outline of the non-reflective positive

contrast legend to somewhat reduce the above-mentioned effect.

In general, for negative contrast signs and for positive contrast signs where both the legend and the

background are fluorescent, a slight increase in the legibility distance can be expected. It should be noted,

that the illumination conditions that were present in this study (fair day) represent somewhat of a worst-

case scenario for fluorescent colors. Larger percent increases in legibility distance may be expected when

using fluorescent colors during the time of twilight, during overcast days, and during daytime in inclement

weather.


REFERENCES

[1] US Department of Transportation, Federal Highway Administration, Manual of Uniform Traffic Control

Devices for Streets and Highways, Part VIII, 1988

[2] Wortham, S., “Do Fluorescent Crossing Signs Have a Bright Future ?…”, Traffic Safety, Volume 95,

Pp. 20-3, July-August, 1995

[3] Burns, D. and L. Pavelka. Visibility of Fluorescent Materials for Signing. Color Research and

Application. Vol. 20, No.2, Apr 1995, pp. 108-116.

[4] Zwahlen, H. T., and Schnell, T., “Visual Detection and Recognition of Fluorescent Color Targets

Versus Non-fluorescent Color Targets as a Function of Peripheral Viewing Angle and Target

Size”, Transportation Research Record, Number 1605 , 1997.

[5] Zwahlen H.T., U.D. Vel, Conspicuity in Terms of Peripheral Visual Detection and Recognition of

Fluorescent Color Targets Versus Non-fluorescent Color Targets Against Different Backgrounds

in Daytime, Transportation Research Record 1456, pp. 125-38, 1994.

[6] Burns D.M., Johnson N.L., The Correlation of Measured Spectral Radiance of Fluorescent and Non-

Fluorescent Materials to Perceived Conspicuity under Natural Lighting, Paper Presented at the AIC

Interm Meeting on Colorimetry, Berlin, September 3-6, 1995.

[7] Brekke B and GD Jenssen. A Comparison of the Visibility of Fluorescent and Standard Retroreflective

Traffic Signs Day and Night, in Proceedings of the 1997 Progress in Automobile Lighting (PAL)

Symposium, Darmstadt, Germany, September 23-24

[8] Schnell, T., Zwahlen, H.T., “A Numerical Method For Optimizing Uppercase Alphanumeric Text In

Traffic Sign Applications”, Paper presented at the 15th Biennial Symposium on Visibility,

Transportation Research Board, May 15-16, Washington, DC, 2000

[9] Schnell, T., and Zwahlen, H.T., “Legibility Threshold Contrast of Uppercase Text Seen Against a Dark

Background”, in Proceedings of the Human Factors and Ergonomics Society 43rd Annual

Meeting, September 27 – October 1, Houston, TX, 1999,

[10] Hummer, J. and Scheffler, C., “Driver Performance Comparison of Fluorescent Orange to Standard

Orange Work Zone Traffic Signs”, Transportation Research Record 1657, TRB, National

Research Council, pp. 55-62.

[11] Zwahlen, H.T., and Schnell, T., “Legibility of Traffic Sign Text and Symbols”, Transportation Research

Record 1692, National Academy of Sciences, Washington, DC., 1999


[12] Sanders M.S., McCormick E.J., Human Factors Engineering and Design, 7th edition, ISBN 0-07-

054901-X, Library of Congress TA 166.S33, McGraw Hill, Publisher, 1993

[13] Zwahlen, Helmut T., Sunkara, Murali, and Schnell, Thomas, "A Review of Legibility Relationships

within the Context of Textual Information Presentation," paper presented at the 74th Annual

Meeting of the Transportation Research Board, Paper No. 950888, January 22-26, 1995,

Washington, DC. Published in Transportation Research Record No. 1485, 1995, pp. 61-70.

[14] Schnell, T., "Legibility Optimization of Uppercase Alphanumeric Text for Displaying Messages in

Traffic Applications", 1998, Ph.D. Dissertation, Department of Industrial and Manufacturing

Systems Engineering, Ohio University, Athens, Ohio 45701-2979, 471 pages.

[15] Wyszecki G., Stiles W.S. Clor Science: Concepts and Methods, Quantitative Data and Formulae, 2nd

edition, John Wiley and Sons, Publisher, New York, 1982.


a. Sign 1: Black Paint Landolt C on Yellow Micro prismatic, Side 1 Normal Yellow, Side 2 Fluorescent Yellow Green (Figure shows Fluorescent Yellow Green as Example)

b. Sign 2: Black Paint Landolt C on Yellow Micro prismatic, Side 1 Normal Yellow, Side 2 Fluorescent Yellow (Figure shows Normal Yellow as Example)

c. Sign 3: Black Paint Landolt C on Pink, Side 1 Non-Reflective Pink Film, Side 2 Microprismatic Fluorescent Pink, (Figure shows Fluorescent Pink as Example)

d. Sign 4: Yellow Microprismatic Landolt C on Purple, Side 1 Background is Non-Reflective Purple Film, Legend is Microprismatic Normal Yellow, Side 2 Background is Microprismatic Fluorescent Purple, Legend is Fluorescent Yellow, (Figure shows Fluorescent Purple as Example)

0.76

0.6

0.30.05

e. Sign 5: Black Paint Pedestrian on Yellow Micro prismatic, Side 1 Normal Yellow, Side 2 Fluorescent Yellow Green (Figure shows Fluorescent Yellow Green as Example)

e. Sign 6: Black Paint PLANT ENTRANCE (Series C) on Yellow Microprismatic, Side 1 Normal Yellow, Side 2 Fluorescent Yellow (Figure shows Normal Yellow as Example)

Figure 1. Experimental Signs Used in the Present Study


Test Section

a. Rear View of the Experimental Vehicle Performing a b. Test Site used in Present Study is Located Run in the Westbound Direction, Signposts Staggered on Washington County Highway G26, 6 Miles along Shoulder North of Washington, Iowa, Sign Approaches

Exactly Along East West Axis

c. Layout of the Six Experimental Signs used in the Present Study Figure 2. Experimental Test Site and Layout


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LC, Fl. Yellow Green Rep 1to 3, Average= 122.8 m,Std.dev= 22.4 m, N= 27

LC, Yellow Rep 1 to 3,Average= 116.6 m, Std.dev=25.7 m, N= 27

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LC, Fl. Yellow Rep 1 to 3,Average= 125.3 m, Std.dev=23.8 m, N= 27

LC, Yellow Rep 1 to 3,Average= 112.1 m, Std.dev=24.7 m, N= 27

a. Sign 1: Black LC, Yellow or Fluorescent b. Sign 2: Black LC, Yellow or Fluorescent Yellow Green Background Yellow Background

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LC, Fl Pink Rep 1 to 3,Average= 109.9 m, Std.dev=28.2 m, N= 27

LC, Pink Rep 1 to 3,Average= 94.8 m, Std.dev=20.7 m, N= 27

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LC, Fl. Purple, Rep 1 to 3,Average= 119.0 m, Std.dev=20.7 m, N= 27

LC, Purple, Rep 1 to 3,Average= 109.0 m, Std.dev=18.5 m, N= 27

c. Sign 3: Black LC, Pink or Fluorescent d. Sign 4: Yellow/Fluorescent Yellow LC, Purple Pink Background or Fluorescent Purple Background

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cent

age

PED, Fluorescent YellowGreen Rep 1 to 3, Average=194.7 m, Std.dev= 21.1 m,N= 27PED, Yellow Rep 1 to 3,Average= 180.5 m, Std.dev=25.2 m, N= 27

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 50 100 150 200 250


Cum

ulat

ive

Per

cent

age

PE Fluorescent Yellow GreenRep 1 to 3, Average= 119.1m, Std.dev= 18.8 m, N= 27

PE Yellow Rep 1 to 3,Average= 113.1 m, Std.dev=20.3 m, N= 27

e. Sign 5: Black Pedestrian Symbol, Yellow f. Sign 6: Black PLANT ENTRANCE, Yellow or Fluorescent Yellow Green Background or Fluorescent Yellow Background

FL=Fluorescent, LC=Landolt C, PED=Pedestrian Symbol on diamond shaped sign, PE=Plant Entrance legend on diamond shaped sign

Figure 3. Cumulative Percentage as a Function of Recognition Distance for Traffic Signs used in the

Current Study


0

50

100

150

200

250

Legi

bilit

y D

ista

nce

[m]

LC F

L P

urpl

e

LC P

urpl

e

LC F

luor

. Yel

low

LC F

luor

. Yel

low

Gre

en

LC Y

ello

w

LC Y

ello

w.2

LC F

luor

Pin

k

LC P

ink

Flu

or. Y

ello

w P

ed

Yel

low

Ped

Flu

or. Y

ello

w P

LAN

T E

NT

RA

NC

E

Yel

low

PLA

NT

EN

TR

AN

CE

Note: LC = Landolt C, Ped = Pedestrian Symbol Sign, FL=Fluorescent, LC Yellow and Yellow 2 are both ordinary yellow but were presented on two separate signs (signs 1 and 2, see Figure 1) at separate locations (see Figure 2c)

Figure 4. Boxplot of Legibility Distances [m] Obtained in Present Study


Table 1. Daytime Legibility Results of the Study Performed by Zwahlen and Schnell [11]

Table reproduced from [11]

Stimulus Daytime

Sign

Symbol/ Letter

Height H [m]

Symbol Width,

Average Letter Width W [m]

Typical Symbol Stroke

Width or Letter StrokeWidth

SW [m]

Contrast Polarity

Average Legibility/

Recognition Distance D

[m]

Visual Angle [min]

subtended by H at D

StdDev. [m]

Coeff. Of Variation

COV

Average Legibility

Index [m]/[cm ]

Average Legibility

Index [ft]/[in]

N

Sign1, Curve Arrow (W182) 0.46 0.46 0.1 Negative 318.9 4.96 62.2 0.20 6.9 57.8 30Sign 2, Large Arrow (IM24) 0.56 0.3 0.11 Positive 291.2 6.61 59.4 0.20 5.2 43.3 30Sign 3, Right Angle (IM19) 0.21 0.33 0.06 Positive 182.5 3.95 48.9 0.27 8.7 72.4 30Sign 4, Small Arrow (IM26) 0.25 0.17 0.07 Positive 149.6 5.74 36.4 0.24 6.0 49.9 30Sign 5, DO NOT PASS (R33) 0.15 0.0825 0.02 Negative 125.7 4.10 28.5 0.23 8.4 69.9 30Sign 6, NO EDGE LINES (OW 167) 0.2 0.134 0.03 Negative 165.7 4.15 45.8 0.28 8.3 69.1 30Sign 7, RUN CREEK (D4-A) 0.33 0.264 0.06 Positive 259.9 4.37 54.7 0.21 7.9 65.6 60Sign 8, PLANT ENTRANCE (W75) 0.15 0.0825 0.022 Negative 125.5 4.11 28.6 0.23 8.4 69.7 30Sign 9, Landholt C Blue, Day 0.2 0.2 0.04 Positive 157.1 4.38 45.8 0.29 7.9 65.5 30Sign 10, Landholt C Green, Day 0.2 0.2 0.04 Positive 141.1 4.87 40.4 0.29 7.1 58.8 30Sign 11, Landholt C White, Day 0.2 0.2 0.04 Negative 148.0 4.64 49.9 0.34 7.4 61.7 30Average 0.25


Table 2. Characteristics and Legibility Results of the Experimental Traffic Signs

a. Measured CIE 1931 (x,y) and CIE(Luv) 1976 Chromaticity Coordinates (2 degrees, D65 Illuminant) Note: MP = Microprismatic, PT = Nonreflective Paint, NRF, Nonreflective Film

Chromaticity Coordinates CIE 1931 Chromaticity Coordinates CIELUV 1976

Color Difference Legend to

Backgroundx y Y X Z u' v' L* u* v* ∆E*uv

Yellow Background MP 0.529 0.461 20.270 23.260 0.440 0.283 0.555 52.141 57.835 58.803Black Landholt C PT 0.302 0.326 4.500 4.169 4.757 0.194 0.471 25.260 -1.235 0.949

Fl. Yellow.Green Background MP 0.412 0.573 83.980 60.384 2.198 0.182 0.570 93.442 -19.111 123.096Black Landholt C PT 0.302 0.326 4.500 4.169 4.757 0.194 0.471 25.260 -1.235 0.949Yellow Background MP 0.529 0.461 20.270 23.260 0.440 0.283 0.555 52.141 57.835 58.803Black Landholt C PT 0.302 0.326 4.500 4.169 4.757 0.194 0.471 25.260 -1.235 0.949

Fl. Yellow Background MP 0.533 0.460 45.860 53.138 0.698 0.286 0.555 73.455 84.251 83.109Black Landholt C PT 0.302 0.326 4.500 4.169 4.757 0.194 0.471 25.260 -1.235 0.949Pink Background NRF 0.510 0.280 10.750 19.580 8.063 0.382 0.472 39.156 93.779 1.800Black Landholt C PT 0.302 0.326 4.500 4.169 4.757 0.194 0.471 25.260 -1.235 0.949

Fl. Pink Background MP 0.538 0.274 19.690 38.661 13.510 0.413 0.473 51.485 143.968 3.190Black Landholt C PT 0.302 0.326 4.500 4.169 4.757 0.194 0.471 25.260 -1.235 0.949Purple Background NRF 0.380 0.170 5.200 11.624 13.765 0.355 0.357 27.297 55.837 -39.353Yellow Landholt C MP 0.529 0.461 20.270 23.260 0.505 0.283 0.555 52.141 57.721 58.580

Fl. Purple Background MP 0.432 0.185 6.180 14.431 12.794 0.397 0.382 29.862 77.215 -33.441Fl. Yellow Landholt C MP 0.533 0.460 45.860 53.138 0.809 0.286 0.555 73.455 84.129 82.872

Yellow Background MP 0.529 0.461 20.270 23.260 0.440 0.283 0.555 52.141 57.835 58.803Black Pedestrian PT 0.302 0.326 4.500 4.169 4.757 0.194 0.471 25.260 -1.235 0.949

Fl. Yellow.Green Background MP 0.412 0.573 83.980 60.384 2.198 0.182 0.570 93.442 -19.111 123.096Black Pedestrian PT 0.302 0.326 4.500 4.169 4.757 0.194 0.471 25.260 -1.235 0.949Yellow Background MP 0.529 0.461 20.270 23.260 0.440 0.283 0.555 52.141 57.835 58.803

BlackPLANT

ENTRANCE PT 0.302 0.326 4.500 4.169 4.757 0.194 0.471 25.260 -1.235 0.949

Fl. Yellow Background MP 0.533 0.460 45.860 53.138 0.698 0.286 0.555 73.455 84.251 83.109

BlackPLANT

ENTRANCEPT 0.302 0.326 4.500 4.169 4.757 0.194 0.471 25.260 -1.235 0.949

6 Side 1 86.943

6, Side 2 127.988

Sign Number Color Item Optics

5, Side 1 86.943

5, Side 2 141.026

4, Side 1 101.053

4, Side 2 124.406

3, Side 1 96.029

3, Side 2 147.570

2, Side 1 86.943

2, Side 2 127.988

86.9431, Side 1

141.0261, Side 2

b. Average Legibility/Recognition Distance, Visual Angle Subtended by Symbol/Letter Height, Standard

Deviation, Legibility Index, Number of Observations, and Percent Increase in Legibility Distance When Using Fluorescence

Stimulus

Symbol/ Letter

Height H [m]

Symbol Width,

Average Letter

Width W [m]

Typical Symbol Stroke Width

or Letter StrokeWidth

SW [m]

Contrast Polarity

Average Legibility/ Recogniti

on Distance

D [m]

Visual Angle [min]

subtended by H at

D

StdDev. [m]

Coeff. Of Variation

COV

Average Legibility

Index [m]/[cm]

Average Legibility

Index [ft]/[in]

N

% Increase in Legibility Distance

When Fluorescent

1, Side 1 Yellow Background MPBlack Landholt C PT

1, Side 2 Fl. Yellow.Green Background MPBlack Landholt C PT

2, Side 1 Yellow Background MPBlack Landholt C PT

2, Side 2 Fl. Yellow Background MPBlack Landholt C PT

3, Side 1 Pink Background NRFBlack Landholt C PT

3, Side 2 Fl. Pink Background MPBlack Landholt C PT

4, Side 1 Purple Background NRFYellow Landholt C MP

4, Side 2 Fl. Purple Background MPFl. Yellow Landholt C MP

5, Side 1 Yellow Background MPBlack Pedestrian PT

5, Side 2 Fl. Yellow.Green Background MPBlack Pedestrian PT

6 Side 1 Yellow Background MP

BlackPLANT

ENTRANCEPT

6, Side 2 Fl. Yellow Background MP

BlackPLANT

ENTRANCEPT

7.9

5.3

5.3

11.8

15.9

9.2

6.0 49.6 27

Sign Number Color Item Optics

119.1 5.77 18.8 0.16

0.18 5.7 47.1 27

0.11 9.7 81.1 27

0.14 9.0 75.2 27

0.022

180.5 3.81 25.2

194.7 3.53 21.1

113.1 6.08 20.3Negative

0.6 0.3 0.05

0.15 0.0825

5.5 45.4 27

119.0 5.78 20.7 0.17 6.0 49.6 27

109.0 6.31 18.5 0.17

4.7 39.5 27

109.9 6.26 28.2 0.26 5.5 45.8 27

5.6 46.7 27

125.3 5.49 23.8 0.19 6.3 52.2 27

112.1 6.13 24.7 0.22

0.2 0.2 0.04

Negative

Positive

5.8 48.6 27

122.8 5.60 22.4 0.18 6.1 51.2 27

116.6 5.90 25.7 0.22

94.8 7.25 20.7 0.22


Table 3. Individual Legibility Distance Data and ANOVA Table

a. Individual Subject Responses, 9 Subjects x 3 Replications = 27 Data Points Per Sign Condition

Note: There were three types of yellow: 1. Fluorescent Yellow-Green, 2. Fluorescent Yellow, 3 Normal Yellow. In order to completely remove the effect of sign post location, we used signs that were florescent on one side and normal on the other side. The signs were simply turned over on the same post to switch from fluorescent to normal. However, since there were three yellow colors, we had to use normal yellow as a dummy variable twice (see also Table 2a). Therefore, in the table below, there are two non-fluorescent (normal) yellow conditions. The signs were identical and the variation between the two signs is small.

PurpleFluor

Yellow

Fluor. Yellow Green

PinkFluor. Yellow

Ped

Fluor. Yellow PLANT

ENTRANCE

Purple Yellow Yellow PinkYellow

Ped

Yellow PLANT

ENTRANCE

101.8 108.2 99.1 64.3 175.3 123.4 107.6 80.2 93.6 80.5 207.6 87.888.1 134.1 158.5 121.9 199.3 89.9 87.8 118.3 128.3 112.5 166.7 94.889.6 97.8 138.1 95.1 187.1 107.0 91.7 108.8 124.1 84.7 195.1 68.9

146.9 133.2 134.7 156.1 202.7 84.1 127.7 130.5 126.8 112.5 197.5 85.6120.7 105.2 103.6 110.0 184.7 115.5 109.7 92.0 73.5 74.7 166.7 93.6122.5 124.4 128.0 112.8 178.6 104.6 136.6 142.6 87.5 64.3 122.8 89.0141.1 183.2 122.5 162.5 195.7 110.3 127.4 169.8 150.3 120.7 170.7 107.688.4 109.7 78.0 84.1 197.8 86.9 110.3 107.9 150.9 93.0 209.4 101.8

134.7 133.2 137.8 139.9 202.7 100.0 134.4 141.1 146.6 110.9 153.3 99.4104.9 93.6 96.0 49.7 224.6 132.0 83.8 69.2 76.8 66.8 215.2 118.3129.5 152.4 150.0 86.0 173.1 124.1 90.2 117.0 118.6 97.8 178.0 115.894.2 110.0 113.1 91.1 158.2 123.1 93.0 78.3 112.5 103.3 190.5 119.2

124.1 150.6 143.0 129.5 194.5 99.1 143.0 113.4 128.9 91.4 179.2 90.2133.5 112.2 100.0 110.3 131.7 123.4 121.9 94.5 73.2 74.4 178.3 103.6121.6 119.5 141.4 110.0 180.4 126.8 114.0 120.1 108.5 76.5 167.6 104.2133.5 167.0 149.7 148.4 199.9 118.3 129.2 143.9 121.9 142.3 219.2 119.5121.6 136.2 78.9 83.5 175.3 111.9 89.0 109.4 130.1 96.0 191.4 114.9133.8 124.1 129.2 136.9 195.1 111.3 113.1 125.9 146.3 117.0 208.5 117.097.8 75.3 99.7 78.9 211.5 145.4 74.7 75.9 79.2 69.5 226.5 131.1

103.6 119.8 133.8 109.7 193.2 143.9 85.0 118.9 115.8 104.5 182.3 134.7108.2 111.6 124.7 100.0 205.1 135.9 100.0 100.9 107.6 84.7 176.2 136.6134.7 153.9 141.4 146.6 201.2 146.3 92.7 79.9 121.9 87.8 153.3 141.7131.1 102.4 104.9 86.9 227.4 153.3 112.5 95.4 92.0 78.9 187.5 145.1117.7 140.8 120.1 111.6 222.8 119.5 112.8 102.7 99.1 65.8 142.3 143.0171.9 138.1 147.8 137.2 216.4 105.2 131.7 145.1 166.4 126.5 141.7 130.189.6 110.3 102.4 93.0 208.8 128.6 110.6 111.3 131.7 111.3 175.3 132.3

128.3 137.5 140.5 112.2 214.0 145.4 113.1 132.6 137.5 112.5 170.4 127.7Average [m] 119.0 125.3 122.8 109.9 194.7 119.1 109.0 112.1 116.6 94.8 180.5 113.1Std.Dev [m] 20.7 23.8 22.4 28.2 21.1 18.8 18.5 24.7 25.7 20.7 25.2 20.3LC Average [m]LC Stdev. [m]

119.324.3

108.123.7

Landholt C

Fluorescent Non Fluorescent

Other Landholt C Other

b. Landolt C ANOVA Table for Replication 1 – 3 Data Lumped together

26 66046.147 2540.236

1 6703.945 6703.945 17.778 .0003 17.778 .990

26 9804.135 377.082

3 10235.510 3411.837 12.637 <.0001 37.912 1.000

78 21058.693 269.983

3 619.261 206.420 1.016 .3904 3.047 .259

78 15853.795 203.254

DF Sum of Squares Mean Square F-Value P-Value Lambda Pow er

Subject

Fluor

Fluor * Subject

Color

Color * Subject

Fluor * Color

Fluor * Color * Subject

ANOVA Table for Legi Dist [m]

title: legibility distances of fluorescent traffic...

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