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Publication Number:  FHWA-HRT-13-044    Date:  August 2013
Publication Number: FHWA-HRT-13-044
Date: August 2013


Traffic Control Device Conspicuity

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This report may be of use to traffic engineers and researchers who are concerned with the conspicuity of traffic signs.

The Manual on Uniform Traffic Control Devices advises that, “Signs should be placed on the right side of the roadway where they are easily recognized and understood by road users.” Guidance is provided on the spacing and prioritization of signs, and in some conditions, additional steps may be needed to ensure that signs are conspicuous. Engineering judgment is required when locating signs. However, little additional information is available to engineers to assist in making such judgments. The research described in this report was intended to develop scientific support for additional guidance on traffic control device conspicuity. The report concludes with guidance on sign conspicuity enhancement for practitioners and provides suggestions for additional research to advance the overall state of knowledge on sign conspicuity.

Monique R. Evans
Director, Office of Safety
Research and Development


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. This report does not constitute a standard, specification, or regulation.

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.


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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 its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.

Technical Report Documentation Page

1. Report No.
2. Government Accession No.
3. Recipient's Catalog No.
4. Title and Subtitle
Traffic Control Device Conspicuity
5. Report Date
August 2013
6. Performing Organization Code
7. Author(s)
Vaughan W. Inman, Stacy A. Balk, William A. Perez
8. Performing Organization Report No.
9. Performing Organization Name and Address
6300 Georgetown Pike
McLean, VA 22101-2296
10. Work Unit No. (TRAIS)
11. Contract or Grant No.
12. Sponsoring Agency Name and Address
Federal Highway Administration
1200 New Jersey Ave. S.E.
Washington, DC 20590-9898
13. Type of Report and Period Covered
Final Report. 10/1/2008-9/30/2011

14. Sponsoring Agency Code


15. Supplementary Notes
FHWA Contracting Officer's Technical Representative (COTR) was Chris Monk (HRDS-30).
16. Abstract

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 have been suggested, including eye fixations, recall, and verbal reports. Four conspicuity studies are discussed in this report.

It has been observed that conspicuity is not solely a property of a TCD but must include consideration of the surrounding environment. The first of the studies described in this report used multidimensional scaling (MDS) to identify factors that characterize drivers' perceptions of TCD environments. The MDS study revealed that two dimensions, clutter and predictability, characterized the roadway environments included in the study.

In the second study, drivers' eye glances to TCDs were recorded on a 34-mi (55-km) drive. After passing selected TCDs, drivers' recall of the TCD was assessed by asking them to identify it. That study showed that warning signs are seldom glanced at and only about half of them are recalled just 2 s after they are passed. About 20 percent of speed limit signs received glances, but drivers were aware of the posted speed limit about 80 percent of the time.

The third study examined drivers' ability to detect speed limit and warning signs. The ability to detect speed limit signs, as measured by conspicuity angle, was degraded by cluttered backgrounds. However, the detectability of fluorescent yellow-green warning signs was not affected by background clutter.

The fourth study examined the effect of background environment on drivers' ability to read TCDs. Background had no effect on speed limit sign readability and had a small effect on warning sign readability. Recommendations for enhancing the conspicuity of regulatory signs are proposed.

17. Key Words
Conspicuity, Speed-limit sign, Warning sign, Sign detection, Sign recall, Eye tracking
18. Distribution Statement
No restrictions. This document is available through the National Technical Information Service, Springfield, VA 22161.
19. Security Classification (of this report)
20. Security Classification (of this page)
21. No. of Pages

22. Price


Form DOT F 1700.7 Reproduction of completed page authorized

SI* (Modern Metric) Conversion Factors


List of Figures

Figure 1. Photo. Panorama 1
Figure 2. Photo. Panorama 2
Figure 3. Photo. Panorama 3
Figure 4. Photo. Panorama 4
Figure 5. Photo. Panorama 5
Figure 6. Photo. Panorama 6
Figure 7. Photo. Panorama 7
Figure 8. Photo. Panorama 8
Figure 9. Photo. Panorama 9
Figure 10. Photo. Panorama 10
Figure 11. Photo. Panorama 11
Figure 12. Photo. Panorama 12
Figure 13. Photo. Panorama 13
Figure 14. Photo. Panorama 14
Figure 15. Graph. MDS solution for 14 panoramas along field study route
Figure 16. Photo. Eye-tracking system with three dash-mounted cameras and two infrared flashers
Figure 17. Photo. Analysis software ROI around a speed limit sign
Figure 18. Photo. ROI shift from the first to the second sign in a sequence of speed limit signs
Figure 19. Graph. Probability of identification as a function of sign type and receiving a look
Figure 20. Graph. Probability of identification as a function of sign type and rated familiarity
Figure 21. Illustration. Slippery when wet warning sign
Figure 22. Graph. Probability of a look to a sign as a function of familiarity, sign type, and whether the driver was asked to identify signs
Figure 23. Photo. Copse background without speed limit sign
Figure 24. Photo. Copse background with speed limit sign
Figure 25. Photo. Parking lot background without speed limit sign
Figure 26. Photo. Parking lot background with speed limit sign
Figure 27. Graph. Mean detection offset angles measured outdoors
Figure 28. Graph. Mean critical detection conspicuity angles measured in the laboratory
Figure 29. Photo. Illustration of the locations of fixation crosses relative to a target sign
Figure 30. Graph. Proportion of correct sign identification responses as a function of fixation point offset and sign type
Figure 31. Graph. Proportion of correct sign identification responses as a function of background scene and sign type
Figure 32. Illustration. Relationship of viewing distance and visual angle offset from the driver's forward view of the roadway

List of Tables

Table 1. MDS coordinates of a two-dimensional solution for the panorama similarity ratings
Table 2. Matrix of correlations between each descriptor rating and each MDS dimension
Table 3. Percent fixated on and recalled for selected objects in Luoma study
Table 4. Percent correct recall of signs from Johansson and Backlund
Table 5. TCDs for which recall was requested and glances were tabulated
Table 6. Percent of TCDs of each type correctly and incorrectly identified as a function of whether drivers looked at them
Table 7. Identification responses to the slippery when wet warning sign
Table 8. Example critical detection conspicuity angle computation for one participant

FHWA Federal Highway Administration
GEE Generalized estimating equations
MDS Multidimensional scaling
MUTCD Manual on Uniform Traffic Control Devices
ROI Region of interest
ROW Right-of-way

Traffic control device