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REPORT
This report is an archived publication and may contain dated technical, contact, and link information
Publication Number:  FHWA-HRT-16-061     Date:  November 2016
Publication Number: FHWA-HRT-16-061
Date: November 2016

 

Intersection Conflict Warning System Human Factors: Final Report

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FOREWORD

Intersection conflict warning systems (ICWSs) have been implemented across several States to warn drivers on the through lanes of the presence of traffic at stop-controlled cross streets and to warn drivers at stop-controlled approaches of the presence of traffic on the through lanes. Before-after studies have shown crash reductions ranging from 3.5 to more than 19 percent at installation sites at rural two-way stop-controlled intersections. However, there has been a lack of standardization of the wording placed on the ICWS message signs and the placement of the signs.

This study was performed to provide empirical evidence to support standardization of ICWS messaging and sign placement. Data were obtained from 189 licensed drivers in a four-part laboratory study. The wordings that best conveyed the intent of the ICWSs’ message and were most preferred by the participants were “CROSS TRAFFIC AHEAD” on the major approach and “CROSS TRAFFIC” or “EXPECT CROSS TRAFFIC” on the minor, stop-controlled approach.

This report should be of interest to State and local transportation agencies concerned with rural intersections, high-speed rural roadways, and intersections with visual obstructions to cross traffic.

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

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 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.

FHWA-HRT-16-061

2. Government Accession No. 3 Recipient's Catalog No.
4. Title and Subtitle

Intersection Conflict Warning System Human Factors: Final Report

5. Report Date

November 2016

6. Performing Organization Code
7. Author(s)

Vaughan W. Inman and Steven Jackson

8. Performing Organization Report No.

 

9. Performing Organization Name and Address

Leidos, Inc.
6300 Georgetown Pike
McLean, VA 22101-2296

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

DTFH61-13-D-00024

12. Sponsoring Agency Name and Address

Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101-2296

13. Type of Report and Period Covered

 

14. Sponsoring Agency Code

HRDS-30

15. Supplementary Notes

The Government’s Task Manager was Jim Shurbutt, and the Contract Officer’s Representative was David Yang.

16. Abstract

Intersection conflict warning systems (ICWSs) are used to warn drivers on the through road of the presence of traffic at stop-controlled cross streets and to warn drivers at stop-controlled approaches of the presence of traffic on the through lanes. ICWSs have been shown to reduce crashes at rural two-way stop-controlled intersections; however, there is no standard wording for ICWS messages and no standard for the placement of these signs on the stop-controlled approaches. This study was performed to provide empirical evidence to support standardization of ICWS messaging and sign placement. Data were obtained from 189 licensed drivers in a 4-part laboratory study. In part 1, participants were shown video animations of approaches with ICWS signs, and comprehension of the signs was assessed. In part 2, participants indicated their level of agreement with 21 statements concerning various aspects of ICWS messages. These ratings were used to assess participants’ mental model of ICWS messaging. Three dimensions of this model were identified. From most influential to least influential, these dimensions were comprehension, safety, and affinity or likeability. In part 3, participants rated the wording of ICWS messaging alternatives. The preferred wording included the words “CROSS TRAFFIC.” On the major road, “CROSS TRAFFIC AHEAD” was preferred. For the stop-controlled minor road, “CROSS TRAFFIC” or “EXPECT CROSS TRAFFIC” options were preferred, depending on whether the “WHEN FLASHING” placard accompanied the sign. Part 4 explored how comprehension varied between when the ICWS beacons were active and inactive, when the “WHEN FLASHING” placard was present or absent, and whether blank-out signs improved comprehension over static signs. A worrisome 28 percent of participants agreed that inactive ICWS beacons meant that it was not necessary to watch for cross traffic. Blank-out signs did not improve comprehension, and blank-out signs were often misinterpreted when blank. The “WHEN FLASHING” placard did not greatly affect comprehension.

17. Key Words

Intersection conflict warning system, ICWS, sign comprehension, sign placement, driver preferences

18. Distribution Statement

No restrictions. This document is available through the National Technical Information Service, Springfield, VA 22161.
http://www.ntis.gov

19. Security Classification
(of this report)

Unclassified

20. Security Classification
(of this page)

Unclassified

21. No. of Pages

53

22. Price

N/A

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

SI* (Modern Metric) Conversion Factors

TABLE OF CONTENTS

CHAPTER 1. OBJECTIVE

CHAPTER 2. BACKGROUND

CHAPTER 3. METHOD

CHAPTER 4. RESULTS

CHAPTER 5. DISCUSSION

CHAPTER 6. RECOMMENDATIONS

REFERENCES

LIST OF FIGURES

Figure 1. Graph. The number of crashes at each of 1,306 intersections shown as a function of AADT
Figure 2. Illustration. ICWS warning installation at stop sign at intersection with four-lane divided highway
Figure 3. Screenshot. Blank-out sign with “WHEN FLASHING” placard along two-lane approach to a stop-controlled cross street
Figure 4. Screenshot. ICWS signs on four-lane divided highway approach
Figure 5. Screenshot. ICWS sign on four-lane undivided highway approach
Figure 6. Screenshot. ICWS warning at intersection with two-lane highway
Figure 7. Screenshot. ICWS warning at stop-controlled intersection with four-lane undivided highway
Figure 8. Screenshot. ICWS at stop-controlled approach to four-lane divided highway
Figure 9. Screenshot. Explanation of ICWS sign on minor road approach
Figure 10. Screenshot. Example of screen used for obtaining agreement ratings
Figure 11. Screenshot. Example of rating screen for nine alternative minor road warnings
Figure 12. Screenshot. Example of screen shown while participants were asked to explain their preferences
Figure 13. Screenshot. Screen examining comprehension of ICWS signs with placard when the beacons are flashing
Figure 14. Screenshot. Screen examining comprehension of ICWS signs with placard when the beacons are not flashing
Figure 15. Screenshot. Screen examining whether driver would check for traffic when beacons are not flashing
Figure 16. Screenshot. Screen used to explain why blank-out signs are used
Figure 17. Screenshot. Screen used to assess comprehension of blank-out message when beacons are active
Figure 18. Screenshot. Screen used to assess blank-out sign understanding when the beacons are not active
Figure 19. Screenshot. Screen used to asses blank-out sign understanding when the sign is blank
Figure 20. Screenshot. Screen used to assess interpretation of a blank-out sign when the sign is on but the beacons are not active
Figure 21. Screenshot. Second screen used to assess interpretation of a blank-out sign when the sign is on but the beacons are not active
Figure 22. Screenshot. Exemplar from video showing the ICWS sign placement across from the stop and yield lines
Figure 23. Screenshot. Exemplar from video showing the ICWS sign placements up stream on the cross road
Figure 24. Screenshot. Screen from which participants indicated which sign placement they thought would be most effective
Figure 25. Scatter plot. Comprehension and safety factors
Figure 26. Scatter plot. Safety and affinity factors
Figure 27. Photo. Sign accompanied by the statement “When the beacons aren’t flashing, it is not necessary to look for approaching traffic”
Figure 28. Illustration. Recommended ICWS signing for minor road approaches to two major roads
Figure 29. Illustration. Recommended ICWS signing for minor road approaches to four-lane divided highways

LIST OF TABLES

Table 1. Reported frequency of driver injuries
Table 2. Reported frequency of crashes by crash type
Table 3. Crash frequency of 1,306 stop-controlled intersections on California State highways in 2010
Table 4. ICWS treatment crash reduction factors from Empirical Bayes analysis of 67 North Carolina intersections
Table 5. ELCSI PFS Empirical Bayes results as a function of crash type for ICWS treatments at intersections of two-lane stop-controlled roads with two-lane through highways
Table 6. ELCSI PFS Empirical Bayes results as a function of crash type for ICWS treatments at intersections of two-lane stop-controlled roads with four-lane through highways
Table 7. Video animation content
Table 8. Percent of correct responses to “What does the warning sign mean?” as a function of presence or absence of a “WHEN FLASHING” placard
Table 9. Percent of correct responses to “What would you do in response to the warning sign?” as a function of the presence or absence of a “WHEN FLASHING” placard
Table 10. Percent of correct responses to “What does the warning sign mean?” as a function of sign type
Table 11. Percent of correct responses to “What would you do in response to the warning sign?” as a function of sign type
Table 12. Percent of correct responses to “What does the warning sign mean?” (comprehension) and “What would you do in response to the warning sign?” (reaction) as a function of sign location
Table 13. Perception ratings for individual statements
Table 14. Factor loadings for three-factor solution
Table 15. Correlations between factors
Table 16. Condorcet results for minor road messages accompanied by the “WHEN FLASHING” placard
Table 17. Top three messages using three methods of analyzing the rank data
Table 18. Five most frequent explanations for major road ranking choices
Table 19. Five most frequent explanations for message minor road ranking choices

LIST OF ABBREVIATIONS

AADTaverage annual daily traffic 
CMFcrash modification factor 
ELCSIEvaluation of Low Cost Safety Improvement 
FHWAFederal Highway Administration 
ICWSintersection conflict warning system 
LCDliquid crystal display 
MUTCDManual on Uniform Traffic Control Devices 
PFSPooled Fund Study 

 

 

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