TECHBRIEF Traffic Control Device 

Conspicuity

FHWA Publication No.: FHWA-HRT-13-043

FHWA Contact: Brian Philips, HRDS-30, 202-493-3468, 

brian.philips@dot.gov

This document is a technical summary of the Federal Highway 

Administration (FHWA) report Traffic Control Device Conspicuity 

(FHWA-HRT-13-044).

Objective

The conspicuity of a traffic control device (TCD) is defined as 

the probability that the device will be noticed. However, there 

is no agreed-upon measure of what constitutes being noticed. 

Various measures such as eye fixations, recall, and other verbal 

reports have been suggested. In this TechBrief, four conspicuity 

studies are reported and recommendations for increasing the 

conspicuity of TCDs are provided.

Introduction

The studies reported examined the following main issues: 

• What factors influence TCD conspicuity? 

• How should TCD conspicuity be assessed? 

Researchers have used several methods of assessing TCD 

conspicuity. One method is to ask drivers to identify TCDs 

immediately after they are passed. Another method is to track 

drivers’ gazes and determine whether drivers have fixated on 

the TCDs. Yet another method is to determine at what angle 

observers can look away from a sign and still determine 

whether the sign is present or what message the sign displays. 

The efforts reported here utilized all of these methods. 

Glance Behavior and Sign Recall

Eye glances to and recall for 21 TCDs along a 34-mi drive 

through urban and suburban areas of northern Virginia were 

recorded for 17 drivers. Recall was solicited about 2 s after 

selected TCDs (speed limit signs, warning signs, crosswalks, 

street names, and information signs) were passed. To assess 

the effect of the recall requests on glance behavior, nine drivers’ 

glance behavior was recorded without requests for sign recall.

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.fhwa.dot.gov/research/
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Several striking trends emerged from the on-road 

tests. One was that drivers were generally aware 

of the posted speed limit regardless of whether 

they had looked at the speed limit sign. Only 

changed speed limits or the first speed limit 

sign on a road were queried; so correct recall 

of sign content could only have been the result 

of detecting the most recent posted speed or 

assuming the speed limit from other roadway 

context. Because drivers who were unfamiliar 

with the roadways had similar speed limit recall 

to that of drivers who were familiar with the 

roadway, memory from previous trips cannot 

explain this finding. Ability to recall warning sign 

content was dependent on looks to the warning 

signs. The relationship between looking at signs 

and recalling them is shown in figure 1. 

Although familiarity with a road segment was not 

related to whether drivers looked at speed limit 

signs, the same was not true for warning signs. 

Unfamiliar drivers were more likely to look at and 

recall warning signs than familiar drivers. This 

was expected because drivers who are familiar 

with a road are likely to be aware of its hazards.

Sign Detection Conspicuity

A laboratory study was conducted to determine 

what factors influence the ability of drivers to 

detect the presence of TCDs. Drivers must attend 

to the roadway and other vehicles. Signs on the 

side of the roadway need to be conspicuous to 

attract drivers’ attention. This study examined 

the relationship between sign properties and the 

properties of the surrounding environment on 

the ability to detect TCDs. 

Outdoors, participants were asked to slowly look 

away from signs until they could no longer detect 

the signs in their peripheral vision. Indoors, speed 

limit and warning signs were briefly presented on 

a screen while participants focused their gaze on 

fixation crosses that varied in angular distance 

from the signs. 

Results from the indoor portion of the study are 

shown in figure 2. These results are consistent 

with the outdoor findings. To be detected, signs 

in cluttered urban environments need to be closer 

to the direction of gaze than signs in less cluttered 

environments. Speed limit signs are particularly 

susceptible to clutter effects, probably because 

they contrast less with the surrounding environment than amber or fluorescent yellow signs. 

Sign Identification Conspicuity

In the on-road study, a substantial proportion of 

signs were correctly recalled even when glances 

toward those signs could not be detected. This 

raised the question of how far a driver’s gaze 

could be from a sign and still allow the sign’s 

message to be comprehended. In this study, 

signs with five different text-based warnings and 

signs with five different speed limits were briefly 

flashed on a screen while participants focused on 

a fixation cross at various horizontal offsets from 

the sign location.

0 9

1.0

Speed Limit Warning

0.80 0.81 0.78

0.7

0.8

0.9

0.5

0.6

0.7

Mean Probability 

Correct 

Id tifi ti

0.37

0.3

0.4

Identification

0 0

0.1

0.2

0.0

No Look Look

Figure 1. Recall of speed limit and warning signs as a function of whether the signs were looked at.
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The findings are shown in figure 3. Consistent with 

the on-road findings that speed limits could be 

identified in the absence of direct fixation, speed 

limits could be identified with about 80  percent 

accuracy when the eye was fixated about 9° to 

left of the sign. Warning sign text, which is smaller 

than speed limit numbers, could be correctly 

identified with 80 percent accuracy when the eyes 

were fixated 3° to the left of the sign. 

Recommendations 

Before drivers can behaviorally respond to signs, 

they must detect them. The sign detection angle 

appears to be a good measure of detectability. 

In these studies, the sign background affected 

the detection angle, but it was not clear whether 

the effects of background were the result of 

the general background or that part of the 

S d Li it Std W FL W i

60

Speed Limit Std. Warn FL Warning

40

50

30 Mean Detection 

Angle

20

10

0

Urban Suburban Parking Lot Trees

Background Scene Error bars represent 95 percent confidence limits.

Figure 2. Angle of fixation at which the sign could be correctly detected 50 percent of the time. 

Speed Limits Warnings

0.9

1.0

0.7

0.8

0.5

0.6

Proportion

Correct

0.3

0.4

0.1

0.2

0.0

15 12 9 6 3 0 -3 -6 -9

Fixation Point Offset (Degrees)

Figure 3. Proportion of signs correctly identified as a function of sign type and fixation point offset.
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Researchers—This research was performed under contracts DTFH61-08-C-00006 and DTFH61-07-C-00042 

in support of the FHWA Research, Development, and Technology Human Factors Team.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct distribution 

is being made to the Divisions and Resource Center.

Availability—The report may be obtained from the FHWA Product Distribution Center by email to report.

center@dot.gov, fax to 814-239-2156, phone to 814-239-1160, or online at http://www.fhwa.dot.gov/research.

Key Words—Conspicuity, Traffic control devices, Detection, Eye glances, Recall.

Notice—This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for the use of 

the information contained in this document. The U.S. Government does not endorse products or 

manufacturers. Trademarks or manufacturers’ names appear in this report only because they are 

considered essential to the objective of the document.

Quality Assurance Statement—The Federal Highway Administration (FHWA) provides high-quality

information to serve Government, industry, and the public in a manner that promotes public understanding. 

Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of 

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ensure continuous quality improvement.

SEPTEMBER 2013 FHWA-HRT-13-043

HRDS-30/09-13(75)E

background that was immediately around the 

signs (i.e., within 2 ° of visual angle). Further 

studies could clarify this issue. 

The methods used in the current study should 

also be applied to assess the effectiveness of 

various conspicuity enhancements in mitigating 

the degrading effects of cluttered background 

scenes. Mitigation strategies that should be 

evaluated include increasing sign size and adding 

yellow plaques to regulatory signs.

The Manual on Uniform Traffic Control Devices 

suggests that TCDs should be in a road user’s view 

and that location and legibility should provide 

adequate time for response.(1) The present results 

suggest that the field of view for sign detection 

exceeds 60 ° under favorable conditions (lowclutter background that contrasts with sign color). 

In high-clutter environments, attention should be 

given to ensuring that signs stand out from their 

backgrounds. The present study used the speed 

limit sign as an exemplar of regulatory signs. 

Black-on-white regulatory signs are particularly 

susceptible to the effects of clutter. 

Intersections are a common location for a variety 

of black-on-white regulatory signs such as lane 

and turn restrictions. The need to place these 

signs on mast arms or posts in the immediate 

intersection environment often dictates the use 

of smaller signs and limits the ability to control 

sign background and proximity to other signs. 

In these cases, strong consideration should be 

given to increasing the conspicuity of safetycritical signs (e.g., no U-turn and no turn on red). 

Many intersections present drivers with challenging visual environments. Unlike the observers in 

the sign detection experiment, drivers in the real 

world have multiple visual tasks to perform and 

will often lack the spare capacity to detect or read 

small signs in their peripheral vision. To ensure 

that drivers look at and read safety-critical signs, 

every effort should be made to make the signs as 

large as possible and, if necessary, add conspicuity enhancements such as yellow notice plaques. 

The full report contains additional guidance to 

practitioners on the placement of regulatory 

and warning signs to better ensure the signs are 

detected and read.

Reference

1. Federal Highway Administration. (2009). 

Manual on uniform traffic control devices for 

streets and highways (2009 ed.). Washington, 

DC: U.S. Government Printing Office.