Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

 Safety Evaluation of Intersection

Conflict Warning Systems

PUBLICATION NO. FHWA-HRT-16-035 JUNE 2016
FOREWORD

The research documented in this report was conducted as part of the Federal Highway 

Administration’s (FHWA) Evaluation of Low-Cost Safety Improvements Pooled Fund Study 

(ELCSI-PFS). FHWA established this pooled fund study in 2005 to conduct research on the 

effectiveness of the safety improvements identified by the National Cooperative Highway 

Research Program Report 500 Guides as part of the implementation of the American Association 

of State Highway and Transportation Officials Strategic Highway Safety Plan. The ELCSI-PFS 

studies provide a crash modification factor and benefit-cost economic analysis for each of the 

targeted safety strategies identified as priorities by the pooled fund member States.

Intersection conflict warning systems (ICWSs), evaluated for their safety effectiveness under this 

study, are intended to reduce the frequency of crashes by alerting drivers to conflicting vehicle 

paths on adjacent approaches at unsignalized intersections. For two-lane at two-lane 

intersections, results showed significant reductions for total, fatal and injury, right-angle, and 

rear-end crashes. For four-lane at two-lane intersections, results showed significant reductions 

for total, fatal and injury, right-angle, and nighttime crashes. The results suggest that the ICWSs 

can be cost-effective safety improvements. This report will benefit roadway designers and safety 

planners to provide greater intersection safety.

Monique R. Evans, P.E.

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. 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.

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 DOCUMENTATION PAGE

1. Report No.

FHWA-HRT-16-035

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

Safety Evaluation of Intersection Conflict Warning Systems

5. Report Date

June 2016

6. Performing Organization Code: 

7. Author(s)

Scott Himes, Frank Gross, Kimberly Eccles, and Bhagwant 

Persaud

8. Performing Organization Report No. 

9. Performing Organization Name and Address

VHB

8300 Boone Blvd., Ste. 700

Vienna, VA 22182-2626

10. Work Unit No.

11. Contract or Grant No.

DTFH61-13-D-00001

12. Sponsoring Agency Name and Address

U.S. Department of Transportation

Federal Highway Administration 

1200 New Jersey Avenue, SE

Washington, DC 20590

13. Type of Report and Period Covered

Safety Evaluation

14. Sponsoring Agency Code

FHWA

15. Supplementary Notes

The Federal Highway Administration (FHWA) Office of Safety Research and Development managed this study

under the Development of Crash Modification Factors program. The FHWA Office of Safety Research and 

Development Contracting Officer’s Representative was Roya Amjadi (202) 493-3383.

16. Abstract

FHWA organized a pooled fund study of 40 States to evaluate low-cost safety strategies as part of its strategic 

highway safety effort. One of the strategies selected for evaluation was intersection conflict warning systems 

(ICWSs). This strategy is intended to reduce the frequency of crashes by alerting drivers to conflicting vehicles 

on adjacent approaches at unsignalized intersections. Geometric, traffic, and crash data were obtained for fourlegged, rural, two-way stop-controlled intersections with ICWS installations in Minnesota, Missouri, and North 

Carolina. To account for potential selection bias and regression-to-the-mean, an empirical Bayes before–after 

analysis was conducted, using reference groups of similar four-legged, rural, two-way stop-controlled

intersections without ICWS installation. The analysis also controlled for changes in traffic volumes over time and 

time trends in crash counts unrelated to the strategy. The combined results for all States indicated statistically 

significant crash reductions for most crash types for two-lane at two-lane intersections and for four-lane at twolane intersections. For two-lane at two-lane intersections, the statistically significant crash modification factors 

(CMFs) for total, fatal and injury, right-angle, and rear-end crashes were 0.73, 0.70, 0.80, and 0.43, respectively. 

For four-lane at two-lane intersections, the statistically significant CMFs for total, fatal and injury, right-angle, 

and nighttime crashes were 0.83, 0.80, 0.85, and 0.61, respectively. The benefit-cost ratio estimated with 

conservative cost and service life assumptions was 27:1 for all two-lane at two-lane intersections and 10:1 for 

four-lane at two-lane intersections. The results suggest that the ICWS strategy, even with conservative 

assumptions on cost, service life, and the value of a statistical life, can be cost effective. Because this is an 

evolving strategy, the results of this study reflect installation practices to date.

17. Key Words

Intersection conflict warnings; Low-cost, Safety 

improvements, Safety evaluations, Empirical 

Bayesian

18. Distribution Statement

No restrictions. This document is available through the 

National Technical Information Service, Springfield, VA 

22161. http://www.ntis.gov/about/contact.aspx

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

76

22. Price

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

TABLE OF CONTENTS

EXECUTIVE SUMMARY............................................................................................................1

CHAPTER 1. INTRODUCTION .................................................................................................3

BACKGROUND ON THE ICWS STRATEGY .....................................................................3

BACKGROUND ON STUDY...................................................................................................4

LITERATURE REVIEW..........................................................................................................4

LIMITATIONS OF PREVIOUS RESEARCH.......................................................................7

CHAPTER 2. OBJECTIVE ..........................................................................................................9

CHAPTER 3. STUDY DESIGN .................................................................................................11

SAMPLE SIZE ESTIMATION OVERVIEW ......................................................................11

CHAPTER 4. METHODOLOGY ..............................................................................................15

CHAPTER 5. DATA COLLECTION........................................................................................17

MINNESOTA ...........................................................................................................................17

Installation Data .....................................................................................................................17

Reference Sites.......................................................................................................................17

Roadway Data ........................................................................................................................17

Traffic Data ............................................................................................................................18

Crash Data ..............................................................................................................................18

ICWS Cost Data .....................................................................................................................18

MISSOURI................................................................................................................................19

Installation Data .....................................................................................................................19

Reference Sites.......................................................................................................................19

Roadway Data ........................................................................................................................19

Traffic Data ............................................................................................................................20

Crash Data ..............................................................................................................................20

ICWS Cost Data .....................................................................................................................20

NORTH CAROLINA ..............................................................................................................20

Installation Data .....................................................................................................................20

Reference Sites.......................................................................................................................21

Roadway Data ........................................................................................................................21

Traffic Data ............................................................................................................................21

Crash Data ..............................................................................................................................22

ICWS Cost Data .....................................................................................................................22

DATA CHARACTERISTICS AND SUMMARY.................................................................23

CHAPTER 6. DEVELOPMENT OF SAFETY PERFORMANCE FUNCTIONS................27

MINNESOTA SAFETY PERFORMANCE FUNCTIONS .................................................28

MISSOURI SAFETY PERFORMANCE FUNCTIONS......................................................30

NORTH CAROLINA SAFETY PERFORMANCE FUNCTIONS ....................................32

CHAPTER 7. BEFORE–AFTER EVALUATION RESULTS................................................35

AGGREGATE ANALYSIS.....................................................................................................35

DISAGGREGATE ANALYSIS..............................................................................................39
iv

CHAPTER 8. ECONOMIC ANALYSIS ...................................................................................45

CHAPTER 9. SUMMARY AND CONCLUSIONS..................................................................47

APPENDIX A: EXAMPLE INSTALLATIONS BY STATE ..................................................49

MINNESOTA ...........................................................................................................................49

MISSOURI................................................................................................................................51

NORTH CAROLINA ..............................................................................................................52

APPENDIX B: ADDITIONAL INSTALLATION DETAILS.................................................55

RESPONSES FROM MINNESOTA......................................................................................56

RESPONSES FROM MISSOURI..........................................................................................58

RESPONSES FROM NORTH CAROLINA.........................................................................58

ACKNOWLEDGEMENTS.........................................................................................................65

REFERENCES.............................................................................................................................67
v

LIST OF FIGURES

Figure 1. Photo. ICWS visual display from Google Street ViewTM ................................................3

Figure 2. Equation. Estimated change in safety.............................................................................15

Figure 3. Equation. Empirical Bayes estimate of expected crashes...............................................15

Figure 4. Equation. Empirical Bayes weight..................................................................................16

Figure 5. Equation. Index of effectiveness.....................................................................................16

Figure 6. Equation. Standard deviation of index of effectiveness .................................................16

Figure 7. Equation. SPF model form for all States ........................................................................27

Figure 8. Photo. Major route blank-out sign with flashing beacon from Google Street 

ViewTM .....................................................................................................................................49

Figure 9. Photo. Minor route sign with LED arrow-shaped flashers from Google Street 

ViewTM .....................................................................................................................................50

Figure 10. Photo. Minor route visual display from Google Street ViewTM ...................................50

Figure 11. Photo. Major route static sign with flashing beacons from Google Street ViewTM......51

Figure 12. Photo. Minor route static sign with flashing beacons from Google Street ViewTM .....51

Figure 13. Photo. Dual major route static sign with flashing beacons from Google Street 

ViewTM .....................................................................................................................................52

Figure 14. Photo. Major and minor route overhead static signs with flashing beacons from 

Google Street ViewTM ..............................................................................................................52

Figure 15. Photo. Minor route overhead static sign with flashing beacons from Google 

Street ViewTM...........................................................................................................................53

Figure 16. Photo. Major route static sign with flashing beacons from Google Street ViewTM......53

Figure 17. Graphic. North Carolina pre-2012 crash reduction factor............................................60

Figure 18. Graphic. North Carolina post-2012 crash reduction factor...........................................60

Figure 19. Diagram. Example 1—overhead sign on major route ..................................................61

Figure 20. Diagram. Example 2—overhead sign on minor route ..................................................62

Figure 21. Diagram. Example 3—post-mounted sign on major approach.....................................63
vi

LIST OF TABLES

Table 1. CMFs for VEWF signs.......................................................................................................6

Table 2. Before period crash rate assumptions for four-legged, stop-controlled intersections......11

Table 3. Minimum required before period site-years for ICWS installation sites.........................12

Table 4. Sample analysis for crash effects (two-lane intersections) ..............................................13

Table 5. Sample analysis for crash effects (multilane intersections).............................................13

Table 6. Minnesota installation cost data .......................................................................................18

Table 7. Missouri installation cost and service life data ................................................................20

Table 8. North Carolina installation cost and service life data ......................................................22

Table 9. Definitions of crash types.................................................................................................23

Table 10. Data summary for installation sites................................................................................24

Table 11. Data summary for reference sites...................................................................................25

Table 12. Minnesota two-lane at two-lane intersection SPFs........................................................29

Table 13. Minnesota four-lane at two-lane intersection SPFs .......................................................29

Table 14. Missouri two-lane at two-lane intersection SPFs...........................................................31

Table 15. Missouri four-lane at two-lane intersection SPFs..........................................................31

Table 16. North Carolina two-lane at two-lane intersection SPFs.................................................33

Table 17. North Carolina four-lane at two-lane intersection SPFs................................................33

Table 18. Aggregate analysis results for Minnesota ......................................................................35

Table 19. Aggregate analysis results for Missouri.........................................................................36

Table 20. Aggregate analysis results for North Carolina ...............................................................37

Table 21. Aggregate analysis results for combined States.............................................................38

Table 22. CMFs by installation category .......................................................................................41

Table 23. CMFs by message (“WHEN FLASHING” versus not present) ....................................42

Table 24. CMFs by lighting presence ............................................................................................42

Table 25. CMFs by before period expected crash frequency.........................................................43

Table 26. Recommended CMFs (based on combined States)........................................................47
vii

LIST OF ABBREVIATIONS

AADT average annual daily traffic 

B/C benefit-cost

CAS collision avoidance system

CICAS Cooperative Intersection Collision Avoidance System

CMF crash modification factor

EB empirical Bayes 

FHWA Federal Highway Administration 

ICAS intersection collision avoidance system

ICWS intersection conflict warning system

KABCO Scale used to represent injury severity in crash reporting (K is fatal injury, A is 

incapacitating injury, B is non-incapacitating injury, C is possible injury, O is 

property damage only)

LED light-emitting diode

MnDOT Minnesota Department of Transportation

MoDOT Missouri Department of Transportation

MUTCD Manual on Uniform Traffic Control Devices

NCDOT North Carolina Department of Transportation

NHTSA National Highway Traffic Safety Administration

PDO property damage only

PRT perception-response time

RICWS rural intersection conflict warning system

RTM regression-to-the-mean

SPF safety performance function 

USDOT U.S. Department of Transportation

VEWF vehicle entering when flashing

1

EXECUTIVE SUMMARY

The Federal Highway Administration (FHWA) organized a pooled fund study of 40 States to 

evaluate low-cost safety strategies as part of its strategic highway safety effort. The purpose of 

the FHWA Evaluation of Low-Cost Safety Improvements Pooled Fund Study is to evaluate the 

safety effectiveness of high-priority, low-cost safety strategies selected by member States through 

scientifically rigorous crash-based studies. One of the strategies selected by member States for 

evaluation was the application of intersection conflict warning systems (ICWSs). This strategy is 

intended to reduce the frequency of crashes by alerting drivers to conflicting vehicles on adjacent 

approaches at unsignalized intersections, particularly those with one-way or two-way stop 

control. Few studies have explored the safety effectiveness of an ICWS; no studies have 

evaluated their effectiveness at four-legged intersections using a statistically rigorous 

methodology, such as the empirical Bayes (EB) before–after method. 

Geometric, traffic, and crash data were obtained for four-legged, rural, two-way stop-controlled 

intersections with ICWS installations in Minnesota, Missouri, and North Carolina. To account for 

potential selection bias and regression-to-the-mean (RTM), an EB before–after analysis was 

conducted using reference groups of similar four-legged, rural, two-way stop-controlled 

intersections without ICWS installation. Separate analyses were conducted for intersections with 

two lanes or four lanes on the major approaches. The analysis also controlled for changes in 

traffic volumes over time and time trends in crash counts unrelated to the strategy.

The combined results for all States indicated reductions for all crash types analyzed (i.e., total, 

fatal and injury, right-angle, rear-end, and nighttime) for both two-lane at two-lane intersections

and four-lane at two-lane intersections. The reductions were statistically significant at the 

95-percent confidence level for all crash types except nighttime crashes for two-lane at two-lane 

intersections. The reductions were statistically significant at the 95-percent confidence level for 

all crash types except rear-end crashes for four-lane at two-lane intersections.

For two-lane at two-lane intersections, the statistically significant crash modification factors 

(CMFs) for total, fatal and injury, right-angle, and rear-end crashes were 0.733, 0.701, 0.803, and 

0.425, respectively. Nighttime crashes had an estimated CMF of 0.898, which was not 

statistically significant at the 95-percent confidence level. It is important to consider the sample 

size used to develop the CMF when interpreting the results because some of the CMFs were 

based on relatively small samples. 

For four-lane at two-lane intersections, the statistically significant CMFs for total, fatal and 

injury, right-angle, and nighttime crashes were 0.827, 0.802, 0.850, and 0.612, respectively. 

Rear-end crashes had an estimated CMF of 0.973, which was not statistically significant at the 

95-percent confidence level. 

The disaggregate analysis sought to identify those conditions under which the ICWS strategy was

most effective. Because total, fatal and injury, and right-angle crashes were the focus of this 

strategy, these crash types were also the focus of the disaggregate analysis. Because installation

category was the main factor for the disaggregate analysis, the categories developed by the North 

Carolina Department of Transportation (NCDOT) were expanded for use in this study.(1)
2

Categories for further analysis included the following:

• Category 1—Overhead signs and flashers at the intersection on major; loop on minor.

• Category 2—Overhead signs and flashers at the intersection on minor; loop on major.

• Category 3a—Post-mounted signs and flashers in advance of the intersection on major; 

loop on minor.

• Category 3b—Post-mounted signs and flashers at the intersection on minor; loop on 

major.

• Category 4—Locations with a combination of category 1 through category 3.

The disaggregate analysis for two-lane at two-lane intersections indicated larger percentage crash 

reductions for sites with an ICWS installed on the major route, particularly for a post-mounted 

ICWS in advance of the intersection. Additional benefit may have been provided by including the 

“WHEN FLASHING” message as part of the system. The CMFs from the disaggregate analysis 

can be used in prioritizing installation sites, but interpretations should be made with caution. One

should pay particular attention to the sample size used to develop the CMFs.

The disaggregate analysis for four-lane at two-lane intersections indicated larger percentage crash 

reductions for sites with intersection lighting and for sites with a higher expected average crash 

frequency in the before period. There was no substantive difference for sites with warning on the 

major route versus warning on the minor route. The CMFs from the disaggregate analysis can be 

used in prioritizing installation sites, but interpretations should again be made with caution. 

The benefit-cost (B/C) ratio estimated with conservative cost and service life assumptions and,

only considering the benefits for total crashes, was 27:1 for all two-lane at two-lane intersections 

and 10:1 for four-lane at two-lane intersections. The benefits were calculated from the significant 

reduction found for combined States for all two-lane at two-lane intersections and based on the 

statistically significant reduction found for four-lane at two-lane. With the U.S. Department of 

Transportation (USDOT)-recommended sensitivity analysis, these values could range from 16:1 

to 39:1 for two-lane at two-lane intersections and 6:1 to 14:1 for four-lane at two-lane 

intersections. These results suggest that the ICWS strategy—even with conservative assumptions 

on cost, service life, and the value of a statistical life—can be cost effective.

Because ICWS is an evolving strategy, this study reflected installation practices to date. Future 

studies may show different results as installation practices change. In particular, the use of 

overhead ICWSs on the major route was limited to installations at the intersection (i.e., no 

advance warning), while post-mounted ICWSs on the major route were installed in advance of 

the intersection. Future research should compare these installation practices, considering 

placement of warning signs. Specifically, section 2C.05 of the Manual on Uniform Traffic 

Control Devices (MUTCD) provides guidance for the placement of warning signs so that they 

provide adequate perception-response time (PRT).(2)
3

CHAPTER 1. INTRODUCTION

BACKGROUND ON THE ICWS STRATEGY

The ICWS strategy involves installing ICWSs on the approaches of rural, four-legged,

unsignalized intersections. ICWSs may be installed on the major and/or minor approaches. These 

systems employ vehicle detectors to alert motorists of conflicting vehicles on an adjacent 

approach. Installation practices current at the time of this study used warning signs on the major 

approaches alerting motorists with the message “VEHICLE ENTERING WHEN FLASHING”

(VEWF), “CROSSING TRAFFIC WHEN FLASHING,” or “WATCH FOR ENTERING 

TRAFFIC.” Signs on the minor approaches alerted entering motorists with “TRAFFIC 

APPROACHING WHEN FLASHING,” “LOOK FOR TRAFFIC” (with yellow light-emitting 

diode (LED) arrow-shaped flashers), or visual graphic displays. Figure 1 presents a Google Street 

ViewTM image of an ICWS application in Missouri. Refer to appendix A for further examples of 

ICWS applications observed in this study.

©Google® 2016

Figure 1. Photo. ICWS visual display from Google Street ViewTM.

(3)

Use of ICWSs is one strategy employed at intersections with limited sight distance and/or 

intersections with a history of crashes involving gap acceptance problems. As Crowson and 

Jackels noted, there has been no specific guidance for the design, placement, and message of 

these systems, resulting in a broad range of approaches for States that are implementing these 

systems.(4) For this reason, the ENTERPRISE transportation pooled fund sponsored the research 

by Crowson and Jackels to develop a consistent approach for uniform deployment, provide 

further evaluation, and to recommend preliminary standards for the MUTCD.(2,4) Their research 

presented typical system components and developed recommended layouts for four scenarios

based on which road the alert was directed and the number of lanes of the intersection. This 
4

research served as an evaluation of the safety effectiveness of ICWS applications to date through 

a crash-based analysis.(4)

BACKGROUND ON STUDY

In 1997, the American Association of State Highway and Transportation Officials Standing 

Committee on Highway Traffic Safety, with the assistance of FHWA, the National Highway 

Traffic Safety Administration (NHTSA), and the Transportation Research Board Committee on 

Transportation Safety Management, met with safety experts in the field of driver, vehicle, and 

highway issues from various organizations to develop a strategic plan for highway safety. These

participants developed 22 key emphasis areas that affect highway safety.

The National Cooperative Highway Research Program published a series of guides to advance 

the implementation of countermeasures targeted to reduce crashes and injuries. Each guide 

addresses one of the emphasis areas and includes an introduction to the problem, a list of 

objectives for improving safety, and strategies for each objective. Each strategy is designated as 

proven, tried, or experimental. Many of the strategies discussed in these guides have not been 

rigorously evaluated; about 80 percent of the strategies are considered tried or experimental.

In 2005, to support the implementation of the guides, the FHWA organized a pooled fund study 

to evaluate low-cost safety strategies as part of this strategic highway safety effort. Over the 

years, the pooled fund has grown in size and now includes 40 States. The purpose of the pooled 

fund study is to evaluate the safety effectiveness of several tried and experimental, low-cost 

safety strategies through scientifically rigorous crash-based studies. The use of an ICWS was 

selected as a strategy to be evaluated as part of this effort. 

LITERATURE REVIEW

Literature on ICWSs was limited. This section summarizes the salient research related to specific 

strategies. Very few studies were identified that investigated the effects of ICWSs.

Lyle evaluated a series of progressively more informative (and emphatic) signs used to warn 

drivers of a hazardous intersection at two locations in Maine.(5) The most informative (and 

emphatic) device was a warning sign stating “Vehicles Entering When Flashing” with 

corresponding flashing beacons. The measures of effectiveness for this study were observed 

speed reductions and driver sign recall. The active sign was found to produce the greatest 

decrease in speed, but the decrease was not significantly different from that produced by the next 

most progressive sign (“Vehicles Entering” with continuous flashing beacons). Surveys showed 

that motorists who saw the active warning sign had better recall, not only of the sign but also of 

the presence of a vehicle in the intersection.

Bretherton and Miao developed guidelines for traffic-actuated warning signs at intersections with 

limited sight distance based on data from 18 intersections in Gwinnett County, GA.

(6) The 

85th percentile speed, existing sight distance, required minimum sight distance, and crash history 

were presented. The authors selected sites with at least three preventable crashes in 1 year, or at 

least one preventable crash for 3 consecutive years. A post-mounted “Vehicle Approaching” sign 

was used on the minor street approach for several intersections, and a post-mounted “Vehicle 

Entering Highway” sign was used on the major street approach for several intersections. The 
5

authors noted that “...the results show that the signs did effectively reduce the number of 

[preventable] accidents.”

(6)

(p. 12) Preventable crashes were defined as those related to limited

sight distance. It should be noted that, because of the method of site selection, the results suffered

from RTM bias. 

Hanscom conducted a test of a collision countermeasure system in Prince William County, VA,

using data from 1993 to 2000.(7) The primary measures of effectiveness were sign response 

speed, intersection arrival speed, first speed reduction, second speed reduction, and projected 

time to collision. Novelty speed effects were observed, but increased projected time to collision 

was sustained in the after period. A simple before–after crash analysis was conducted for sideimpact crashes, and 2.6 crashes were observed per year for the 5-year before period, and no 

crashes were observed in the 2-year after installation period. 

Peabody et al. also examined the effectiveness of a vehicle-activated warning system for stopcontrolled intersections in Norridgewock, ME.

(8) A conflict analysis showed a 35- to 40-percent 

reduction in intersection conflicts. A survey of drivers found that 67 percent said that the signs 

would prevent crashes, and 64 percent recommended the system for other intersections. Limited 

crash data were collected, and no crash effectiveness of the strategy was estimated.

The Pennsylvania Department of Transportation conducted a before–after analysis of a postmounted collision avoidance system (CAS) at two locations from 1999 to 2005.

(9) A speed study 

showed that operating speeds initially declined but increased after 3 years. A gap acceptance 

study found that typical gaps did not change from the before to the after period. Users were 

surveyed, and 97 percent said that the CAS was beneficial, and 93 percent said that the system 

should be installed at other locations. Summary statistics were presented for crash data. At 

one site, two crashes were observed in the 2-year before period, and no crashes occurred in the 

2-year after period. At the second site, two crashes were observed in the before period, and 

three crashes were observed in the after period. (One occurred while the system was 

malfunctioning.) The authors of the Pennsylvania study noted that the sample size was too small 

to conduct a safety analysis.(9)

The Missouri Department of Transportation (MoDOT) studied the safety effectiveness of postmounted warning systems at 9 stop-controlled intersection major street approaches and 10 stopcontrolled intersection minor street approaches.(10) A simple before–after study found 

28-, 72-, 37-, and 75-percent reductions in total, severe, angle, and severe-angle crashes, 

respectively, at the locations with the installation on the major street approach. They also found 

32-, 33-, 44-, and 38-percent reductions in total, severe, angle, and severe-angle crashes at the 

locations, respectively, with the installations on the minor street approaches. MoDOT noted that 

one-third of the individual locations showed little or no improvement.

Simpson and Troy evaluated VEWF signs at 56 two-lane at two-lane intersections in North 

Carolina.(1) Installation dates ranged from 1996 to 2010. A before–after analysis assessed the 

crash reduction factor for multiple crash types. The following definitions were provided for the 

four categories of signs used in North Carolina:(1)

• Category 1—Overhead signs and flashers on major; loop on minor.

• Category 2—Overhead signs and flashers on minor; loop on major.
6

• Category 3—Post-mounted signs and flashers on major; loop on minor.

• Category 4—Locations with a combination of category 1 through category 3.

Table 1 presents the results of the analyses for two-lane at two-lane intersections. The 

authors found that deployments with alerts on the major road in advance of the intersection 

and locations with a combination of both major and minor road alerts were the most effective for 

two-lane at two-lane stop-controlled intersections, with CMFs for total crashes of 0.68 and 

0.75, respectively.(1)

In addition, intersections with four lanes on the major route were considered; however, no 

apparent reductions in crashes were found for these sites. The authors suggested that VEWF 

systems may not be an appropriate strategy for most intersections with four lanes on the major 

route experiencing a strong frontal impact crash pattern.

Table 1. CMFs for VEWF signs.

(1)

VEWF Category CMF Standard Error

Total Crashes

All sites 0.897 0.047

1 1.059 0.098

2 0.953 0.084

3 0.675 0.076

4 0.749 0.115

Target Crashes

All sites 0.929 0.055

1 1.074 0.112

2 1.001 0.096

3 0.679 0.088

4 0.797 0.144

Injury Crashes

All sites 0.878 0.059

1 0.917 0.108

2 0.934 0.106

3 0.732 0.102

4 0.870 0.187

Severe Injury Crashes

All sites 0.697 0.159

1 0.613 0.236

2 0.761 0.268

3 0.699 0.301

4 0.242 0.212

Statistically significant results at the 95-percent confidence level are indicated 

in boldface.

Pierowicz et al. developed a prototype intersection collision avoidance system (ICAS) for use 

within vehicles.(11) The system was derived through the review of national databases such as the 

National Automotive Sampling System, General Estimates System, and Fatality Analysis 

Reporting System.(12) Four intersection crash scenarios were identified, as were three potential 

countermeasures. Two of the countermeasures, the Driver Advisory System and the Defensive 
7

System, were developed in a full-scale study for performance. Several recommendations were 

made from this research including the following:

• Integrate left turn across path sensor algorithms developed on the ICAS into the NHTSA 

Intelligent Vehicle Initiative.

• Continue development of map-based unsignalized intersection system.

• Fund development of forward-viewing, wide-field sensor.

• Investigate use of signal-to-vehicle communication to improve ICAS effectiveness.

• Continue investigation of driver-vehicle interface effectiveness and driver acceptance.

LIMITATIONS OF PREVIOUS RESEARCH

Most previous research studies focused on surrogate measures for intersection safety performance 

because typically only one or two applications were implemented. Some research studies were 

able to consider a simple before–after approach in an attempt to quantify a reduction in targeted 

crash types. However, these studies did not quantify a margin of error for the associated 

reductions, and they did not account for RTM bias. Only one of the studies reported in this 

chapter attempted to account for RTM, but details of how this was done were not provided. In 

addition, a linear assumption was used to account for changes in traffic volume experienced at 

the installation sites rather than safety performance functions (SPFs) typically used for EB

evaluations. The study did find statistically significant crash reductions at the 95-percent 

confidence level for certain crash types for two-lane at two-lane intersections when all 

installation categories were combined. Not enough intersections and reference sites were 

available to study four-lane at two-lane intersections, and CMFs for several crash types for 

individual installation categories were statistically insignificant. Further, there was no attempt to 

quantify the impact of system placement on the major road (i.e., in advance of or at the 

intersection).

9

CHAPTER 2. OBJECTIVE

This research examined the safety impacts of the application of ICWSs in Minnesota, Missouri,

and North Carolina. The objective was to estimate the safety effectiveness of this strategy as 

measured by crash frequency. Target crash types included the following: 

• Total crashes (all types and severities combined).

• Injury crashes (K (fatal injury), A (incapacitating injury), B (non-incapacitating injury), 

and C (possible injury) on KABCO scale).

• Right-angle crashes (all severities combined).

• Rear-end crashes (all severities combined).

• Nighttime crashes (all severities combined).

While the ICWS strategy specifically targets right-angle crashes, other crash types were 

considered to determine whether there were supplemental benefits or drawbacks. Rear-end

crashes were the only additional crash type determined to occur commonly enough to be 

reasonably considered independently in the analysis. The research team surmised that there was 

potential for other drivers to be alerted in addition to the conflicting vehicles for which the 

systems were designed. This could lead to reductions in rear-end crashes on both the major and 

minor routes, owing to increased awareness. This would differentiate the outcome from the 

effects of traffic signals, which typically produce an increase in rear-end crashes while reducing 

right-angle crashes.

A further objective was to address questions of interest, such as whether effects varied depending 

on the following characteristics:

• Type of installation (i.e., specific type or combination of ICWS).

• Location of installation (i.e., post mounted, overhead, or in advance or at the intersection).

• Intensity (i.e., number of approaches).

• Level of traffic volume.

• Posted speed limit on the major route or minor routes.

• Presence of turn lanes.

• Presence of intersection lighting.

• Before period expected crash frequency.

The evaluation of overall effectiveness also included the consideration of the installation costs 

and crash savings in terms of the B/C ratio. 
10

Meeting these objectives placed some special requirements on the data collection and analysis 

tasks, including the need to do the following:

• Select a large enough sample size to detect, with statistical significance, what may be 

small changes in safety for some crash types.

• Identify appropriate reference sites without ICWS installation.

• Properly account for changes in safety due to changes in traffic volume and other factors 

unrelated to ICWS installation.

• Pool data from multiple jurisdictions to improve reliability of the results and facilitate 

broader applicability of the products of the research.
11

CHAPTER 3. STUDY DESIGN

The study design involved a sample size analysis and prescription of needed data elements. The 

sample size analysis assessed the size of sample required to statistically detect an expected 

change in safety and also determined what changes in safety could be detected with available 

sample sizes.

SAMPLE SIZE ESTIMATION OVERVIEW

When planning a before–after safety evaluation study, it is vital to ensure that enough data are 

included such that the expected change in safety can be statistically detected. Even though in the 

planning stage, the expected change in safety is unknown, it is still possible to make a rough 

estimate of how many sites would be required based on the best available information about the 

expected change in safety. Alternatively, one could estimate, for the number of available sites, 

the change in safety that could be statistically detected. For a detailed explanation of sample size 

considerations, as well as estimation methods, see chapter 9 of Hauer.

(13) The sample size 

analysis presented here is limited to two cases: (1) how large a sample would be required to 

statistically detect an expected change in safety, and (2) what changes in safety could be detected 

with available sample sizes.

For case 1, it was assumed that a conventional before–after study with comparison group design 

would be used because available sample size estimation methods were based on this assumption. 

The sample size estimates from this method would be conservative in that the EB methodology 

would likely require fewer sites. To facilitate the analysis, it was also assumed that the number of 

comparison sites was equal to the number of installation sites and the duration of the before and 

after periods were equal, which, again, was a conservative assumption.

Table 2 provides the crash rate assumptions. The locations of interest for the ICWS strategy were

four-legged, stop-controlled intersections. Intersection crash rates differ substantially depending 

on a number of factors (e.g., traffic control, traffic volume, geometric configuration, and area 

type). Therefore, the intersection crash rates assumed for these computations represented the 

before data for installation sites in North Carolina, Missouri, and Minnesota. Rates A and B were 

calculated as the weighted average crash rate for two-lane and multilane major routes, 

respectively. 

Table 2. Before period crash rate assumptions for four-legged, stop-controlled intersections.

Crash Type

Crash Rate (crashes/intersection/yr)

North Carolina Missouri Minnesota Rate A Rate B

TwoLane MultilaneTwoLane MultilaneTwoLane MultilaneTwoLane Multilane

Total 3.817 4.317 1.932 3.712 1.535 5.929 3.300 4.246

Fatal and

injury 2.228 2.867 0.886 1.962 0.744 3.786 1.877 2.596

Right-angle 2.430 3.150 1.091 2.077 0.698 3.571 2.049 2.754

Rear-end 0.304 0.217 0.159 0.519 0.209 0.500 0.274 0.373

Nighttime 0.494 0.583 0.295 0.885 0.209 1.071 0.434 0.762
12

Table 3 provides estimates of the required number of before and after period intersection-years 

for statistical significance at both a 90- and 95-percent confidence level for crash rates A and B. 

The minimum sample indicates the level for which a study seemed worthwhile; that is, it was

feasible to detect with the level of confidence the largest effect that could reasonably be expected 

based on what was currently known about the ICWS strategy. These sample size calculations 

were based on specific assumptions regarding the number of crashes per intersection and years of 

available data. Rate A (from table 2) was used for two-lane at two-lane intersections, and rate B 

(from table 2) was used for four-lane at two-lane intersections. Site-years are the number of sites

where the strategy was implemented multiplied by the number of years of data before or after 

implementation. For example, if a strategy was implemented at nine sites and data were available 

for 3 years since implementation, then there would be a total of 27 site-years of after period data 

available for the study.

Table 3. Minimum required before period site-years for ICWS installation sites.

Expected Percent 

Reduction in Crashes

Minimum Before Period Site-Years1

95-Percent

Confidence

90-Percent

Confidence

Rate A Rate B Rate A Rate B

Total

10 564 439 351 273

20 85 66 59 46

30 29 23 21 16

40 13 10 9 7

Fatal and injury

10 991 717 616 446

20 149 108 103 75

30 51 37 36 26

40 22 16 16 12

Right-angle

10 908 676 564 420

20 136 102 94 70

30 47 35 33 25

40 20 15 14 11

Rear-end

10 6,788 4,987 4,218 3,098

20 1,017 747 703 516

30 346 254 242 178

40 149 110 105 77

Nighttime

10 4,286 2,441 2,663 1,517

20 642 366 444 253

30 219 125 153 87

40 94 54 66 38 1

Assumes equal number of site-years for ICWS installation and comparison sites 

and equal length of before and after periods.

Boldface indicates the sample size values recommended in this study.

The sample size values recommended in this study are highlighted in bold in table 3. These were

recommended based on the likeliness of obtaining the estimated sample size as well as the 

anticipated effects of the ICWS strategy. As noted, the sample size estimates provided were

conservative in that the state-of-the-art EB methodology proposed for the evaluations would 

require fewer sites than the less robust conventional before–after study with a comparison group 

that had to be assumed for the calculations. Estimates could be predicted with greater confidence 
13

or a smaller reduction in crashes would be detectable if there were more site-years of data 

available in the after period. The same holds true if the actual data used for the analysis had a 

higher crash rate for the before period than was assumed.

Case 2 considers the data collected for both the before and after periods. The total site-years of 

data available for two-lane major roadways was 360 for the before period and 255 for the after 

period. The total site-years of data available for multilane major roadways was 126 for the before 

period and 100 for the after period. The statistical accuracy attainable for a given sample size is 

described by the standard deviations of the estimated percent change in safety. From this, 

P-values were estimated for various sample sizes and expected changes in safety for a given 

crash history. A set of such calculations is shown in table 4 and table 5. The calculations are 

based on the methodology in Hauer.

(13)

For the available data, the minimum percentage change in crash frequency that could be 

statistically detected at 90- and 95-percent significance levels were estimated using the same 

crash rates in table 2. The results indicate that the data should be able to detect the anticipated 

crash reduction effects highlighted in table 3 (i.e., 20-percent reductions for all crash types except 

for rear-end and nighttime crashes for both two-lane and multilane roadways), if such an effect 

were present. Using these results, a decision was made to proceed with the evaluation using the 

data available at the time.

Table 4. Sample analysis for crash effects (two-lane intersections).

Crash Type

Intersection-Years

in Before Period

Intersection-Years in 

After Period

Minimum Percent

Reduction Detectable for 

Crash Rate Assumption1

P = 0.10 P = 0.05

Total

360 255

10 15

Fatal and injury 15 15

Right-angle 15 15

Rear-end 30 30

Nighttime 25 25 1

Results are to nearest 5-percent interval, and the crash rate assumption is based on actual crash rate for the 

before period.

Table 5. Sample analysis for crash effects (multilane intersections).

Crash Type

Intersection-Years

in Before Period

Intersection-Years

in After Period

Minimum Percent

Reduction Detectable for 

Crash Rate Assumption1

P = 0.10 P = 0.05

Total

126 100

15 15

Fatal and injury 20 20

Right-angle 15 20

Rear-end 35 40

Nighttime 30 30 1

Results are to nearest 5-percent interval, and the crash rate assumption is based on actual crash rate for the 

before period.

15

CHAPTER 4. METHODOLOGY

The EB methodology for observational before–after studies was used for the evaluation. This 

methodology was considered rigorous in that it accounted for RTM using a reference group of 

similar sites without ICWS installation. In the process, SPFs were used for the following reasons:

• They overcome the difficulties of using crash rates in normalizing for volume differences 

between the before and after periods.

• They account for time trends.

• They reduce the level of uncertainty in the estimates of safety effect.

• They properly account for differences in crash experience and reporting practice in 

amalgamating data and results from diverse jurisdictions.

• The methodology also provides a foundation for developing guidelines for estimating the 

likely safety consequences of a contemplated strategy.

In the EB approach, the change in safety ( ) for a given crash type at a site is given by figure 2.

Figure 2. Equation. Estimated change in safety.

Where:

= Expected number of crashes that would have occurred in the after period without the 

strategy.

= Number of reported crashes in the after period. 

In estimating , the effects of RTM and changes in traffic volume were explicitly accounted for 

using SPFs, relating crashes of different types to traffic flow and other relevant factors for each 

jurisdiction based on reference sites. Annual SPF multipliers were calibrated to account for 

temporal effects on safety (e.g., variation in weather, demography, and crash reporting).

In the EB procedure, the SPF was used to first estimate the number of crashes that would be 

expected in each year of the before period at locations with traffic volumes and other 

characteristics similar to the one being analyzed (i.e., reference sites). The sum of these annual 

SPF estimates (P) was then combined with the count of crashes (x) in the before period at an 

installation site to obtain an estimate of the expected number of crashes (m) before installation, 

as shown in figure 3. 

Figure 3. Equation. Empirical Bayes estimate of expected crashes.

Where w is estimated from the mean and variance of the SPF estimate, as shown in figure 4.

∆SSSSSSSSSSSS = λλ − ππ

λ

π
16

Figure 4. Equation. Empirical Bayes weight.

Where:

k = Constant for a given model, which is estimated from the SPF calibration process with the use 

of a maximum likelihood procedure. In that process, a negative binomial distributed error 

structure is assumed with k being the overdispersion parameter of this distribution.

A factor was then applied to m to account for the length of the after period and differences in 

traffic volumes between the before and after periods. This factor was the sum of the annual SPF 

predictions for the after period divided by P, the sum of these predictions for the before period. 

The result, after applying this factor, was an estimate of . The procedure also produced an 

estimate of the variance of .

The estimate of was then summed over all installation sites in a group of interest (to obtain 

sum) and compared with the count of crashes observed during the after period in that group 

( sum). The variance of was also summed over all sites in the strategy group. 

The index of effectiveness ( ) is estimated in figure 5.





 



 +

=

2

( ) 1 sum

sum

sum

sum

Var

λ

λ

λ

π

θ

Figure 5. Equation. Index of effectiveness.

The standard deviation of is given in figure 6.

2

2

2 2

2

( ) 1

( ) ( )

( )

















+

















+

=

sum

sum

sum

sum

sum

sum

Var

Var Var

StDev

λ

λ

λ

λ

π

π θ

θ

Figure 6. Equation. Standard deviation of index of effectiveness.

The percent change in crashes was calculated as 100(1 − ); thus, a value of = 0.7 with a 

standard deviation of 0.12 indicates a 30-percent reduction in crashes with a standard deviation 

of 12 percent.

λ

λ

λ

λ

π λ

θ

θ

θ θ
17

CHAPTER 5. DATA COLLECTION 

Minnesota, Missouri, and North Carolina provided data containing locations and dates of ICWS

installations. Each State also identified approximately 30 reference sites for four-legged

intersections with two lanes on the major route and 30 reference sites for four-legged

intersections with four lanes on the major route. These States also provided roadway geometry, 

traffic volumes, and crash data for both installation and reference sites. Additional details about 

the design, installation, and maintenance of ICWSs, as well as lessons learned, can be found in 

appendix B.

MINNESOTA

Installation Data

The Minnesota Department of Transportation (MnDOT) provided a list of intersections where 

ICWSs had been installed, along with information about whether the installations were on the 

major and/or minor routes. In addition, the list provided by MnDOT included information about 

the specific messages shown on each of the signs or whether the ICWS consisted of a visual 

display. The final list of installation sites comprised 10 two-lane at two-lane intersections and 

3 four-lane at two-lane intersections (13 total installation sites). All Minnesota installation sites 

were post mounted, and all sites had a warning sign on the minor roadway approach. Six of the 

two-lane at two-lane intersections also had an installation on the major approaches of the 

intersections. The four-lane at two-lane intersections had visual displays for minor route 

approaches. All two-lane at two-lane installations specified “WHEN FLASHING” on the 

messages provided on the warning signs. Twenty more installation sites were identified by 

MnDOT, but these were still in the process of being installed and thus could not be used in this 

study. 

Reference Sites

Reference sites were provided by MnDOT separately for two-lane at two-lane intersections and 

for four-lane at two-lane intersections. Data were provided for 28 two-lane at two-lane 

intersections and 35 four-lane at two-lane intersections. Intersections were identified that were in 

close proximity to the installation sites, preferably along the same major route as installation

sites. Sites were selected if they had similar traffic and geometric characteristics to installation

sites. Selecting sites in close proximity reduced the effects of differences in driver population 

and spatial factors, such as weather or terrain. 

Roadway Data

MnDOT provided roadway data for the installation and reference sites. Various roadway 

characteristics were coded by the project team from the records provided, and from Google 

EarthTM, including the following: 

• Number of lanes on the major route approaches.

• Presence of right- and left-turn lanes on the major and minor approaches.

• Intersection angle.
18

• Median presence.

• Median width.

• Presence of channelization on the minor approaches.

• Presence of intersection lighting.

• Presence of overhead flashers.

• Posted speed limit.

• Presence of multiple STOP signs.

• Presence of advance intersection warning sign.

• Presence of STOP AHEAD warning sign.

Traffic Data

MnDOT also provided traffic volume data for the installation and reference sites. Traffic data 

were typically available for State highways every 2 to 3 years. County highway data were 

provided for every 4 to 5 years. The years of average annual daily traffic (AADT) counts were 

provided for each value of AADT. The counts covered both the before and after periods for 

installation and reference sites. For years with missing data, linear interpolation of AADT counts 

were used, or an extrapolation was used if the after period counts did not cover the latest year. If 

no apparent trend was observed in the AADT data, the extrapolated value was defined as the 

same as the previous year’s AADT value.

Crash Data

MnDOT provided crash data for installation and reference intersections from 2006 to 2012. 

Because crash data were provided separately for each intersection, no linking was necessary, but 

the data had to be manually coded for each intersection. 

ICWS Cost Data

MnDOT provided cost estimates of the installations for use in conducting a B/C analysis of the 

ICWS strategy. Table 6 provides itemized cost data for post-mounted signs for two-lane at 

two lane intersections and four-lane at two-lane intersections. The project team noted that 

intersection warning systems included static signs on the major road, blank-out signs on the 

minor road, micro-loops on the major road, loops or micro-loops on the minor road, controller 

cabinets, and onsite contractor warranty, which included a 72-h response to address any system 

malfunction. Maintenance and operations costs were not provided, nor was an estimate of 

lifespan.

Table 6. Minnesota installation cost data.

Countermeasure Mobilization Engineering Construction

Design Build 

Oversight

Post-mounted on all approaches 

for two-lane major approach ~ $5,000 $11,807 $75,650 $17,000

Post-mounted on all approaches 

for four-lane major approach ~ $5,000 $13,130 $103,833 $17,000
19

MISSOURI

Installation Data

MoDOT provided a list of projects where ICWSs had been installed, along with information 

about whether the installations were on the major and/or minor routes. In addition, MoDOT 

provided details on how the ICWS signs were activated, the mounting type, the specific message 

on each sign, any additional signs/countermeasures, and any additional improvements made at the 

site during the analysis years. The final list of sites consisted of 6 two-lane at two-lane 

intersections and 8 four-lane at two-lane intersections (14 total installation sites). All Missouri 

installation sites were post mounted. Five of the six two-lane at two-lane intersections had ICWSs

on the minor approaches. Two of the six had ICWSs on the major approaches. Five of eight fourlane at two-lane intersections had an ICWS on the minor approaches, and four had an ICWS on 

the major approaches. Two-lane at two-lane sites with an ICWS on the minor approaches had 

“WHEN FLASHING” plaques, while only one four-lane at two-lane site had the plaque.

Reference Sites

Reference sites were provided by MoDOT separately for two-lane at two-lane intersections and 

for four-lane at two-lane intersections. Data were provided for 35 two-lane at two-lane

intersections and 28 four-lane at two-lane intersections. Intersections were identified that were in 

close proximity to the installation sites, preferably along the same major route. Sites were 

selected if they had similar traffic and geometric characteristics to installation sites. Selecting 

sites in close proximity reduced the effects of differences in driver population and spatial factors, 

such as weather or terrain.

Roadway Data

MoDOT provided roadway data for the installation and reference sites. Various roadway 

characteristics were coded by the project team from the records provided, and from Google 

EarthTM, including the following: 

• Number of lanes on the major route approaches.

• Presence of right- and left-turn lanes on the major and minor approaches.

• Intersection angle.

• Median presence.

• Median width.

• Presence of channelization on the minor approaches.

• Presence of intersection lighting.

• Presence of overhead flashers.

• Posted speed limit.

• Presence of multiple STOP signs.

• Presence of advance intersection warning sign.

• Presence of STOP AHEAD warning sign.
20

Traffic Data

MoDOT also provided traffic volume data for the installation and reference sites. Traffic data 

were typically available for State highways every 2 to 3 years. County highway data were 

provided for every 4 to 5 years. The years of AADT counts were provided for each value of 

AADT. The counts covered both the before and after periods for installation and reference sites. 

For years with missing data, linear interpolation of AADT counts were used, or an extrapolation 

was used if the after period counts did not cover the latest year. If no apparent trend was 

observed in the AADT data, the extrapolated value was defined as the same as the previous 

year’s AADT value.

Crash Data

MoDOT provided crash data for the installation and reference intersections from 2000 to 2012. 

The crash data were linked to each intersection using the intersection identifier.

ICWS Cost Data

MoDOT provided estimates of the costs and services lives of the installations for use in 

conducting a B/C analysis of the ICWS strategy. Table 7 provides the approximate cost and 

lifespan for a post-mounted ICWS on the major approaches as reported by MoDOT. In addition, 

maintenance costs were noted to vary substantially. Annual maintenance costs for mainline 

warning systems with loops on the minor routes were estimated to be $800 per year. For 

intersections with mainline detection using probes or microwave and wireless communication, 

the estimated annual maintenance was $3,000 per intersection. Ignoring the cost of intersection 

lighting, utility costs were estimated to average $275 for mainline flashers and $400 for sidestreet flashers.

Table 7. Missouri installation cost and service life data.

Intersection Type Installation Type Cost Lifespan

Two-lane at two-lane 

intersection

Post-mounted ICWS on major 

approach $25,000 to $33,500 10 years minimum

Four-lane at two-lane 

intersection

Post-mounted ICWS on minor 

approaches ~ $75,000 10 years minimum

NORTH CAROLINA

Installation Data

NCDOT provided a list of intersections where an ICWS had been installed, along with 

information about whether the installations were on the major and/or minor routes. In addition, 

the list provided by NCDOT included information about the specific messages shown on each of 

the signs, the project improvement description, statement of existing physical conditions, 

statement of problem, additional countermeasures, sign size details, detector types, detector 

locations, and detector timings. The final list of installation sites consisted of 53 two-lane at twolane intersections and 13 four-lane at two-lane intersections (66 total installation sites). All 

four-lane at two-lane installations were on major approaches, and nine had post-mounted ICWS 
21

signs. Four of the sites had overhead ICWS signs, and nine sites specifically stated “WHEN 

FLASHING.” Thirty-eight two-lane at two-lane sites had ICWS signs on the major approaches,

and 23 had ICWS signs on the minor approaches. Post-mounted ICWS signs were present at 

16 two-lane at two-lane sites, and 40 had overhead ICWS signs. 

Reference Sites

Reference sites were provided by NCDOT separately for two-lane at two-lane intersections and 

for four-lane at two-lane intersections. Data were provided for 35 two-lane at two-lane 

intersections and 35 four-lane at two-lane intersections. These intersections were provided based 

on reference sites NCDOT had obtained for other projects, and all were used in this study. 

Roadway Data

NCDOT provided roadway data for the installation and reference sites. Various roadway 

characteristics were coded by the project team from the records provided, and from Google 

EarthTM, including the following: 

• Number of lanes on the major route approaches.

• Presence of right- and left-turn lanes on the major and minor approaches.

• Intersection angle.

• Median presence.

• Median width.

• Presence of channelization on the minor approaches.

• Presence of intersection lighting.

• Presence of overhead flashers.

• Posted speed limit.

• Presence of multiple STOP signs.

• Presence of advance intersection warning sign.

• Presence of STOP AHEAD warning sign.

Traffic Data

NCDOT also provided traffic volume data for the installation and reference sites. Traffic data 

were available for State highways every 2 years. Because NCDOT is responsible for State and 

county roads, all roads are considered to be State maintained. The years of AADT counts were 

provided for each value of AADT. The counts covered both the before and after periods for 

installation and reference sites. For years with missing data, linear interpolation of AADT counts 

was used, or an extrapolation was used if the after period counts did not cover the latest year. If 

no apparent trend was observed in the AADT data, the extrapolated value was defined as the 

same as the previous year’s AADT value.
22

Crash Data

NCDOT provided crash data for the installation and reference intersections from 1992 to 2012. 

The crash data were linked to each intersection using the intersection ID. All data were used for 

SPF development; however, a maximum of 5 years before and after were used for the analysis of 

installation sites.

ICWS Cost Data

NCDOT provided estimates of the costs and services lives of the installation for use in 

conducting a B/C analysis of the ICWS strategy. Total cost estimates were provided for each of 

the installations from 1996 to 2011. Owing to the difference in time for cost estimates, the cost 

estimates were normalized by consumer price index to develop an average cost based on 2014. 

Table 8 provides installation cost data for sites based on the type of ICWS and based on which 

approaches were installed. In addition, Table 8 contains information for annual maintenance 

cost, annual operations cost, and estimated lifespan for installations used by NCDOT for 

economic analysis.

North Carolina assumed an annual maintenance cost of $500 per year, an operations cost of 

$125 per year, and a lifespan of 10 years for installations. These values did not differ by 

installation type. The average installation cost of an overhead sign on a single approach was 

approximately $30,000, with a maximum value of approximately $50,000. The average 

installation cost of a post-mounted installation on the major approach only was approximately 

$20,000 for two-lane at two-lane intersections, with a maximum value of approximately $50,000. 

For four-lane at two-lane intersections, the average cost was $117,000, and the maximum cost 

was $142,500. For two-lane at two-lane intersections with overhead signs on all approaches, the 

average cost was approximately $50,000, and the maximum cost was $78,000. 

Table 8. North Carolina installation cost and service life data.

Condition

Installation Cost Annual Costs

Lifespan 

Minimum Mean Maximum (years)

Maintenance

Cost

Operations

Cost

Overhead on minor 

only $20,000 $29,500 $46,000 $500 $125 10

Overhead on major and 

minor $20,000 $49,000 $78,000 $500 $125 10

Overhead on major 

only $13,500 $28,000 $49,000 $500 $125 10

Post-mounted only two 

lane $9,000 $21,600 $49,000 $500 $125 10

Post-mounted only four 

lane $49,000 $117,000 $142,500 $500 $125 10
23

DATA CHARACTERISTICS AND SUMMARY

Table 9 defines the crash types used by each State. The project team attempted to make the crash 

type definitions consistent.

Table 9. Definitions of crash types.

Crash Type

State

Minnesota Missouri North Carolina

Total Identified as all crashes, 

without exclusion

Identified as all crashes, 

without exclusion

Identified as all crashes, 

without exclusion

Fatal and

injury

Resulted in a fatality, or 

A, B, or C injury

Resulted in fatal, 

disabling injury, or 

minor injury

Resulted in K, A, B, or C 

severity

Right-angle Diagram is coded as 5—

Right-angle

Accident class name is 

coded as right-angle

First harmful event is 

angle

Rear-end Diagram is coded as 1—

Rear-end

Accident class name is 

coded as rear-end

First harmful event is 

rear-end

Nighttime Lighting condition is 

coded as sunrise, sunset, 

or any value of dark

Lighting condition is 

coded as any value of 

dark

Lighting condition is 

coded as dusk, dawn, or 

any value of dark

Table 10 summarizes information for the data collected for the installation sites. The information 

in table 10 should not be used to make simple before–after comparisons of crashes per site-year

because it does not account for factors, other than the ICWS strategy, that may cause a change in 

safety between the before and after periods. Such comparisons are properly done with the EB

analysis as presented later. Table 11 summarizes information for the reference site data.
24

Table 10. Data summary for installation sites.

Variable

Two-Lane Sites Four-Lane Sites

Minnesota Missouri

North 

Carolina Minnesota Missouri

North 

Carolina

Number of sites 10 6 53 3 8 13

Site-years before 43 44 263 14 52 60

Site-years after 16 28 211 4 41 55

Total crashes 

before1 1.54 1.93 3.82 5.93 3.71 4.32

Total crashes after1 1.25 1.32 2.91 4.00 2.90 4.55

Fatal and injury 

crashes before1 0.74 0.89 2.23 3.79 1.96 2.87

Fatal and injury

crashes after1 0.38 0.64 1.60 2.25 1.15 2.84

Right-angle crashes 

before1 0.70 1.09 2.43 3.57 2.08 3.15

Right-angle crashes 

after1 0.81 0.71 1.83 2.25 1.49 3.31

Rear-end crashes 

before1 0.21 0.16 0.30 0.50 0.52 0.22

Rear-end crashes 

after1 0.00 0.14 0.18 0.25 0.39 0.29

Nighttime crashes

before1 0.21 0.30 0.49 1.07 0.89 0.58

Nighttime crashes

after1 0.19 0.11 0.52 0.50 0.68 0.42

Major AADT before

Avg 2,374

Min 810

Max 6,300

Avg 2,547

Min 1,420

Max 4,846

Avg 4,076

Min 299

Max 11,450

Avg 11,293

Min 6,400

Max 17,800

Avg 14,773

Min 9,104

Max 37,504

Avg 9,193

Min 1,323

Max 27,635

Major AADT after

Avg 2,345

Min 900

Max 6,500

Avg 2,334

Min 973

Max 5,123

Avg 4,041

Min 830

Max 10,000

Avg 13,225

Min 7,300

Max 18,600

Avg 16,530

Min 9,285

Max 33,685

Avg 10,868

Min 1,934

Max 30,500

Minor AADT 

before

Avg 1,257

Min 600

Max 3,250

Avg 618

Min 196

Max 1,846

Avg 1,776

Min 420

Max 4,100

Avg 1,934

Min 1,200

Max 3,350

Avg 957

Min 269

Max 3,000

Avg 2,044

Min 568

Max 5,500

Minor AADT after

Avg 1,512

Min 550

Max 3,700

Avg 723

Min 243

Max 1,431

Avg 1,906

Min 370

Max 4,300

Avg 1,700

Min 1,250

Max 2,950

Avg 965

Min 404

Max 2,742

Avg 2,268

Min 890

Max 5,700 1

Crash rates are presented as crashes/site/year.

Avg = Average.

Min = Minimum.

Max = Maximum.
25

Table 11. Data summary for reference sites.

Variable

Two-Lane Sites Multilane Sites

Minnesota Missouri

North 

Carolina Minnesota Missouri

North 

Carolina

Number of sites 28 35 35 35 28 35

Site-years 196 455 672 245 364 630

Total crashes1 1.34 0.90 1.36 1.49 2.35 1.91

Fatal and injury 

crashes1 0.67 0.33 0.71 0.70 0.96 1.02

Right-angle crashes1 0.62 0.35 0.53 0.76 1.02 0.88

Rear-end crashes1 0.26 0.20 0.31 0.18 0.31 0.32

Nighttime crashes1 0.41 0.17 0.26 0.35 0.54 0.42

Major AADT

Avg 6,286

Min 2,033

Max 12,200

Avg 2,432

Min 79

Max 6,895

Avg 5,462

Min 720

Max 17,000

Avg 10,119

Min 3,250

Max 21,000

Avg 6,687

Min 3,169

Max 12,770

Avg 12,111

Min 3,541

Max 28,000

Minor AADT

Avg 1,462

Min 390

Max 4,400

Avg 330

Min 18

Max 1,176

Avg 1,095

Min 235

Max 5,300

Avg 1,337

Min 310

Max 4,400

Avg 493

Min 106

Max 1,455

Avg 1,049

Min 100

Max 5,600 1

Crash rates are presented as crashes/site/year.

Avg = Average.

Min = Minimum.

Max = Maximum.

27

CHAPTER 6. DEVELOPMENT OF SAFETY PERFORMANCE FUNCTIONS

This chapter presents the SPFs developed for each State, which are subsequently used in the EB 

methodology.(13) Generalized linear modeling was used to estimate model coefficients assuming 

a negative binomial error distribution, which was consistent with the state of research in 

developing these models. In specifying a negative binomial error structure, the overdispersion 

parameter, k, was estimated iteratively from the model and the data. For a given dataset, smaller 

values of k indicated relatively better models.

SPFs were calibrated separately for intersections with two-lane and multilane major routes for 

Minnesota, Missouri, and North Carolina using the corresponding reference sites from each 

State. The SPFs developed are presented by State in the following sections. 

The form of the SPFs for all States is given in figure 7. 

Figure 7. Equation. SPF model form for all States.

Where: 

TotalEntering = Total entering volume (major route AADT + minor route AADT). 

characteristic = Intersection characteristics included in SPF, defined by associated estimated 

parameters c through s. 

The following definitions were used for all States for parameters with intersection 

characteristics:

c = Proportion of total entering volume from minor approach. 

d = Presence of flashers on the minor route approaches.

e = Presence of flashers on the major route approaches. 

f = Number of major route left-turn lanes. 

g = Number of minor route left-turn lanes. 

h = Number of major route right-turn lanes. 

i = Number of minor route right-turn lanes. 

j = Advance intersection warning sign on major route approaches.

l = Posted speed limit on major route. 

m = Presence of lighting at intersection.

n = Advance STOP AHEAD warning sign on minor route approaches. 

o = Intersection angle.

p = Presence of channelization on minor route approaches. 

q = Presence of dual stop signs on the minor route approaches. 

r = Median width on major route. 

s = Posted speed limit on minor route. 

t = Presence of a median on major route. 

cccc cch = eeee × TTTT TTTTTT ii iibb × eeee(cch × cc + ... + cch × )
28

In addition, the following parameter was provided for each SPF: 

k = Overdispersion parameter of the model. 

MINNESOTA SAFETY PERFORMANCE FUNCTIONS

The SPFs for two-lane at two-lane intersections are provided in table 12, and the SPFs for 

four-lane at two-lane intersections are provided in table 13.
29 

Table 12. Minnesota two-lane at two-lane intersection SPFs. 

Crash 

Type

Parameter Estimates (Standard Error)

a b c d e f g h i j l 

Total -7.913

(1.314)

0.866

(0.143)

2.976

(0.569)

-0.527

(0.195) — — — — — — — 

Fatal and

injury

-5.791

(1.786)

0.557

(0.195)

2.611

(0.779)

-0.841

(0.337) — — 0.473(0.266) — — — — 

Rightangle-14.557

(4.266)

0.825

(0.272)

2.010

(1.129)

-0.856

(0.412) — -0.364(0.174)

0.620

(0.313) — — 0.742(0.420)

0.115

(0.053)

Rear-end -20.508

(4.817)

2.259

(0.583)

3.350

(1.709)

-0.932

(0.495)

1.024

(0.511) — — -0.833(0.391)

0.379

(0.215) — — 

Nighttime -12.177

(3.036)

1.251

(0.335)

2.588

(1.361) — — — — — — — — 

— Indicates not applicable. 

Table 13. Minnesota four-lane at two-lane intersection SPFs. 

Crash 

Type

Parameter Estimates (Standard Error)

a b c e j n o p q 

Total -8.481

(1.581)

1.198

(0.151)

4.121

(0.839)

0.966

(0.237)

-0.337 

(0.190)

-0.394

(0.144)

-0.032

(0.005)

-0.417

(0.191)

0.712

(0.192

Fatal and

injury

-5.123

(1.808)

0.772

(0.180)

2.391

(0.985)

1.144

(0.260)

-1.038

(0.300) — -0.034(0.007) — 0.787(0.228

Right-angle -9.030

(2.105)

1.227

(0.203)

2.914

(1.160)

1.051

(0.276)

-0.763

(0.258) — -0.039(0.007)

-0.505

(0.261)

0.605

(0.255

Rear-end -7.843

(3.385)

0.730

(0.363) — — — -0.717(0.386) — — — 

Nighttime -10.665

(2.672)

1.202

(0.262)

5.010

(1.458)

1.357

(0.408) — -0.816(0.261)

-0.021

(0.009) — — 

— Indicates not applicable. 
30

MISSOURI SAFETY PERFORMANCE FUNCTIONS

The SPFs for two-lane at two-lane intersections are provided in table 14, and the SPFs for 

four-lane at two-lane intersections are provided in table 15.
31 

Table 14. Missouri two-lane at two-lane intersection SPFs. 

Crash 

Type

Parameter Estimates (Standard Error)

a b c e f h i j m n o p 

Total -8.314

(1.252)

0.700

(0.130)

2.912

(0.593) — — 0.788(0.298)

0.187

(0.091)

0.298

(0.141)

0.601

(0.157) — 0.023(0.009) — 

Fatal and

injury

-14.938

(2.243)

1.166

(0.193)

5.192

(0.726) — — — — — 0.701(0.207) — 0.044(0.013) — 

Rightangle-19.026

(2.821)

1.454

(0.241)

6.575

(0.921)

-1.531

(0.556)

0.518

(0.205) -- 0.986(0.315)

0.372

(0.218)

0.812

(0.235) — 0.045(0.016)

-2.147

(0.655)

Rear-end -12.296

(2.431)

1.258

(0.300) — — — 1.928(0.356) — 0.824(0.324) — -0.931(0.313) — 0.916(0.257) 

Nighttime -11.085

(2.625)

0.639

(0.246)

2.786

(1.022) — -0.321(0.192)

1.064

(0.504) — — 0.841(0.280) — 0.043(0.021) — 

— Indicates not applicable. 

Table 15. Missouri four-lane at two-lane intersection SPFs. 

Crash 

Type

Parameter Estimates (Standard Error)

a b c d e f h j l n o p 

Total -2.179

(1.355)

0.871

(0.118)

10.185

(1.824) — -0.462(0.199)

0.455

(0.117)

0.246

(0.077)

0.925

(0.163)

-0.057

(0.011)

0.286

(0.138)

-0.034

(0.007)

0.319

(0.098)

Fatal and 

injury

-1.058

(1.675)

0.729

(0.124)

5.733

(2.031) — — 0.361(0.151) — 0.890(0.156)

-0.037

(0.010) — -0.056(0.008)

0.288

(0.121)

Rightangle-3.586

(1.427)

0.502

(0.146)

13.626

(2.097)

0.804

(0.207) — — — 1.319(0.211) — 0.429(0.164)

-0.037

(0.008)

0.581

(0.136) 

Rear-end -9.471

(2.289)

1.738

(0.294)

8.643

(3.508) — -1.607(0.428) — 0.419(0.166)

1.181

(0.321)

-0.155

(0.023)

1.064

(0.297) — — 

Nighttime -12.962

(1.738)

1.381

(0.188)

5.658

(2.807) — -0.532(0.254) — — 0.727(0.212) — — — — 

— Indicates not applicable. 
32

NORTH CAROLINA SAFETY PERFORMANCE FUNCTIONS

The SPFs for two-lane at two-lane intersections are provided in table 16, and the SPFs for 

four-lane at two-lane intersections are provided in table 17.
33 

Table 16. North Carolina two-lane at two-lane intersection SPFs. 

Crash 

Type

Parameter Estimates (Standard Error)

a b c g h i l o q 

Total -5.148

(0.578)

0.683

(0.054)

1.977

(0.223)

0.131 

(0.074) — — -0.012 (0.005)

-0.008 

(0.002) — 

Fatal and

injury

-6.098

(0.676)

0.656

(0.067)

1.725

(0.284) — — 0.139 (0.080) — — 0.211 (0.061)

-0

(0

Rightangle-5.567

(0.675)

0.577

(0.071)

2.233

(0.302) — — 0.201(0.084) — -0.009 (0.003)

0.290 

(0.066) 

Rear-end -10.130

(1.527)

1.331

(0.151) — — — -0.861 (0.269)

-0.039

(0.012)

-0.010 

(0.005) — 

Nighttime -8.312

(1.219)

0.669

(0.116)

2.379

(0.437) — -0.296 (0.112) — — -0.007(0.004) — 0(0

— Indicates not applicable. 

Table 17. North Carolina four-lane at two-lane intersection SPFs. 

Crash Type

Parameter Estimates (Standard Error)

a b c f h i l o p q 

Total -4.414

(1.083)

0.642

(0.093)

3.742

(0.451) — — — -0.037 (0.012) — 0.306 (0.142)

0.301 

(0.124) 

Fatal and 

injury

-5.128

(1.072)

0.472

(0.109)

3.838

(0.537)

-0.254 

(0.132)

-0.158 

(0.071)

0.114

(0.056) — — 0.345 (0.189)

0.760 

(0.187) 

Right-angle -2.584

(1.232)

0.217

(0.123)

3.791

(0.625) — — 0.319 (0.058) — — — 0.642 (0.168) 

Rear-end -12.819

(2.966)

0.562

(0.228)

2.553

(0.946) — — 0.417 (0.098)

0.074 

(0.040)

0.032 

(0.008) — — -0(0.

Nighttime -8.531

(1.727)

0.803

(0.173)

2.315

(0.750) — — — — — — 0.314 (0.209)

-0

(0.

— Indicates not applicable. 

35

CHAPTER 7. BEFORE–AFTER EVALUATION RESULTS

AGGREGATE ANALYSIS

Table 18 through table 21 provide the estimates of expected crashes in the after period without 

installation, the observed crashes in the after period, and the estimated CMF and its standard 

error for all crash types considered. Results are provided separately for each State as well as for 

all States combined. Results are provided separately for individual States because the application 

practices varied for all three. Minnesota used a variety of post-mounted signs, including static 

signs with flashers, blank-out signs with flashers, visual displays, and signs with LED arrows 

indicating the direction of conflicting vehicles. Missouri used post-mounted static signs with 

flashers exclusively. North Carolina used static signs that were a mix of post mounted and 

overhead where the overhead signs were installed at the intersection. The results were combined 

to further draw inferences on the overall effect of ICWSs. 

The results for Minnesota in table 18 were inconsistent across crash types and by number of 

through lanes on the major route. Statistically significant reductions at the 95-percent confidence

level were found for fatal and injury crashes for two-lane at two-lane intersections only. 

However, the sample sizes were quite small, so readers should use caution when attempting to 

draw meaningful conclusions from the results.

Table 18. Aggregate analysis results for Minnesota.

Statistic Total

Fatal and

Injury

RightAngleRearEnd Nighttime

Two-Lane at Two-Lane

EB estimate of crashes 

expected in the after period 

without strategy

23.00 11.13 6.49 7.62 2.89

Count of crashes observed in 

the after period 20 6 13 0 3

Estimate of CMF 0.856 0.525 1.945 0.000 1.003

Standard error of estimate of 

CMF 0.216 0.225 0.618 N/A 0.588

Four-Lane at Two-Lane

EB estimate of crashes 

expected in the after period 

without strategy

21.50 8.27 13.80 1.09 1.99

Count of crashes observed in 

the after period 16 9 9 1 2

Estimate of CMF 0.737 1.052 0.642 0.811 1.003

Standard error of estimate of 

CMF 0.196 0.388 0.225 0.764 0.710

Statistically significant results at the 95-percent confidence level are indicated in boldface.

The results for Missouri in table 19 indicate reductions for nearly all crash types for both 

two-lane at two-lane intersections and four-lane at two-lane intersections. No statistically 
36

significant results were found for two-lane at two-lane intersections because of small sample 

sizes; however, significant reductions were found for four-lane at two-lane intersections. 

Reductions were found for total crashes, fatal and injury crashes, and nighttime crashes that were 

significant at the 95-percent confidence level.

Table 19. Aggregate analysis results for Missouri.

Statistic Total

Fatal and

Injury

RightAngleRearEnd Nighttime

Two-Lane at Two-Lane

EB estimate of crashes 

expected in the after period 

without strategy

47.08 16.18 25.42 5.74 3.71

Count of crashes observed in 

the after period 37 18 20 4 3

Estimate of CMF 0.777 1.088 0.771 0.642 0.810

Standard error of estimate of 

CMF 0.151 0.298 0.200 0.343 0.467

Four-Lane at Two-Lane

EB estimate of crashes 

expected in the after period 

without strategy

164.2

6 84.06 75.42 19.51 46.00

Count of crashes observed in 

the after period 119 47 61 16 28

Estimate of CMF 0.719 0.554 0.799 0.778 0.594

Standard error of estimate of 

CMF 0.089 0.096 0.134 0.252 0.143

Statistically significant results at the 95 percent confidence level are indicated in boldface.
37

The results for North Carolina in table 20 indicate statistically significant reductions at the 

95-percent confidence level for all crash types except nighttime crashes for two-lane at two-lane 

intersections. A statistically significant reduction at the 95-percent confidence level was found 

for nighttime crashes for four-lane at two-lane intersections; however, insignificant decreases 

were found for total crashes, fatal and injury crashes, and right-angle crashes. These findings are 

explored further in the Disaggregate Analysis section of this chapter.

Table 20. Aggregate analysis results for North Carolina.

Statistic Total

Fatal and

Injury

RightAngleRearEnd Nighttime

Two-Lane at Two-Lane

EB estimate of crashes 

expected in the after period 

without strategy

842.71 488.25 490.26 87.10 122.25

Count of crashes observed in 

the after period 613 338 387 39 110

Estimate of CMF 0.727 0.691 0.788 0.444 0.897

Standard error of estimate of 

CMF 0.037 0.046 0.050 0.081 0.099

Four-Lane at Two-Lane

EB estimate of crashes 

expected in the after period 

without strategy

278.74 163.86 206.25 12.47 37.52

Count of crashes observed in 

the after period 250 156 182 16 23

Estimate of CMF 0.893 0.947 0.877 1.224 0.595

Standard error of estimate of 

CMF 0.081 0.104 0.094 0.388 0.157

Statistically significant results at the 95-percent confidence level are indicated in boldface.
38

The combined results in table 21 indicate reductions for all crash types analyzed for both 

two-lane at two-lane and four-lane at two-lane intersections. The reductions were statistically 

significant at the 95-percent confidence level for all crash types except nighttime crashes for 

two-lane at two-lane intersections. The reductions were statistically significant at the 95-percent 

confidence level for all crash types except for rear-end crashes for four-lane at two-lane 

intersections.

Table 21. Aggregate analysis results for combined States.

Statistic Total

Fatal and

Injury

RightAngleRearEnd Nighttime

Two-Lane at Two-Lane

EB estimate of crashes 

expected in the after period 

without strategy

912.79 515.56 522.17 100.46 128.84

Count of crashes observed in 

the after period 670 362 420 43 116

Estimate of CMF 0.733 0.701 0.803 0.425 0.898

Standard error of estimate of 

CMF 0.035 0.045 0.049 0.073 0.096

Four-Lane at Two-Lane

EB estimate of crashes 

expected in the after period 

without strategy

464.50 263.56 295.47 33.07 85.52

Count of crashes observed in 

the after period 385 212 252 33 53

Estimate of CMF 0.827 0.802 0.850 0.973 0.612

Standard error of estimate of 

CMF 0.059 0.072 0.075 0.224 0.108

Statistically significant results at the 95-percent confidence level are indicated in boldface.

For two-lane at two-lane intersections, the crash type with the smallest CMF (which translates to 

the greatest reduction) was rear-end with a CMF of 0.425, which was statistically significant at 

the 95-percent confidence level. Total, fatal and injury, and right-angle crashes had estimated 

CMFs of 0.733, 0.701, and 0.803, respectively, which were also statistically significant at the 

95-percent confidence level. Nighttime crashes had an estimated CMF of 0.898, which was not 

statistically significant at the 95-percent confidence level. It is important to consider the sample 

size used to develop each CMF when interpreting the results. For example, the sample sizes used 

to develop CMFs for rear-end and nighttime crashes were relatively low, resulting in larger 

standard errors and confidence intervals compared with the CMFs for total, fatal and injury, and 

right-angle crashes.

For four-lane at two-lane intersections, the crash type with the smallest CMF (which was 

statistically significant at the 95-percent confidence level) was nighttime crashes, with a CMF of 

0.612. Total, fatal and injury, and right-angle crashes had estimated CMFs of 0.827, 0.802, and 

0.850, respectively, which were also statistically significant at the 95-percent confidence level. 
39

Rear-end crashes had an estimated CMF of 0.973, which was not statistically significant at the 

95-percent confidence level. 

As discussed in the literature review, the most comprehensive study to date of ICWS 

applications was conducted by Simpson and Troy using data from North Carolina.(1) This 

report includes recommended CMFs for two-lane at two-lane intersection but does not provide 

recommended CMFs for four-lane at two-lane intersections because the small sample size 

precluded a rigorous analysis. Simpson and Troy recommended a CMF of 0.897 for total crashes 

and 0.878 for injury crashes at two-lane at two-lane intersections.

(1) Greater crash benefits were 

indicated in the present study, which were attributed to the following characteristics of the 

present study:

• Included only four-legged intersections.

• Limited the number of study years to no more than 5 years before and 5 years after 

installation.

• Used SPFs to account for changes in traffic volumes.

• Used annual multipliers to account for trends at reference sites. 

• Used a multistate database.

DISAGGREGATE ANALYSIS

The disaggregate analysis sought to identify those conditions under which the ICWS strategy

was most effective. Because total, fatal and injury, and right-angle crashes were the focus of this 

strategy, these crash types were the focus of the disaggregate analysis. Several variables were 

identified as being of interest and available for all three States, including installation category, 

message, presence of turn lanes, presence of lighting, presence of additional countermeasures, 

major and minor route AADT, major and minor route posted speed limit, and expected crash 

frequency in the before period.

For installation category, the categories developed by NCDOT were expanded for use in this 

study. Categories for further analysis were as follows:

• Category 1—Overhead signs and flashers at the intersection on major; loop on minor.

• Category 2—Overhead signs and flashers at the intersection on minor; loop on major.

• Category 3a—Post-mounted signs and flashers in advance of the intersection on major; 

loop on minor.

• Category 3b—Post-mounted signs and flashers at the intersection on minor; loop on 

major.

• Category 4—Locations with a combination of category 1 through category 3.
40

For two-lane at two-lane intersections, all categories were considered in the disaggregate 

analysis. For four-lane at two-lane intersections, categories 3a and 3b were included in the 

disaggregate analysis. Category 1 and category 2 systems were found only in North Carolina,

and these systems were installed at the intersection on both the major and minor road.

Category 3a signs were found only in Missouri and North Carolina and were installed in advance 

of the intersection. Category 3b systems were found only in Minnesota and Missouri and were 

installed at the intersection. 

Table 22 provides the disaggregate results by category for two-lane at two-lane intersections and 

four-lane at two-lane intersections. The number of intersections is indicated for each installation

category. For each crash type, the estimated CMF, standard error (in parentheses), and sample 

size in terms of observed crashes in the after period is provided. It is important to consider the 

sample size used to develop the CMFs when applying the CMFs. 

For two-lane at two-lane intersections, results indicate statistically significant reductions at the 

95-percent confidence level for all crash types for category 1, 3a, and 4 systems. Considering the 

standard errors of the CMFs, it was difficult to draw a conclusion about the relative effectiveness 

of categories 1, 3a, and 4; with the exception of the CMFs for right-angle crashes, the results 

were not statistically different at the 95-percent confidence level. The majority of the category 4 

sites consisted of a combination of categories 1 and 2 or a combination of categories 3a and 3b.

For four-lane at two-lane intersections, the results indicate statistically significant reductions at 

the 95-percent confidence level for all crash types for category 3a and for total crashes only for 

category 3b systems. The CMFs for categories 3a and 3b were not significantly different for any 

crash type.
41

Table 22. CMFs by installation category.

Crash Type

Installation Category

1 2 3a 3b 4

Two-Lane at Two-Lane

No. of sites (N) 16 15 14 8 16

Total 0.740 (0.070)

173

0.892 (0.075)

241

0.519 (0.056)

120

0.886 (0.162)

42

0.704 (0.087)

94

Fatal and injury 0.600 (0.075)

91

0.944 (0.101)

144

0.450 (0.069) 

58

1.064 (0.287)

18

0.742 (0.122)

51

Right-angle 0.807 (0.096)

111

1.084 (0.110)

169

0.454 (0.067) 

61

1.247 (0.299)

25

0.697 (0.113)

54

Four-Lane at Two-Lane

No. of sites (N) N/A N/A 12 7 N/A

Total N/A N/A 0.745 (0.068) 

243

0.690 (0.127)

35 N/A

Fatal and injury N/A N/A 0.734 (0.083) 

138

0.896 (0.210)

22 N/A

Right-angle N/A N/A 0.769 (0.082) 

174

0.763 (0.173)

23 N/A

Statistically significant results at the 95-percent confidence level are indicated in boldface.

In each cell containing results is the estimated CMF, standard error (in parentheses), and the sample size in terms of 

observed crashes in the after period.

N/A = Not applicable.

It was not appropriate to compare the effectiveness of overhead versus post-mounted 

applications on the major route from the study results because the placement of treatment 

differed for the two groups. Post-mounted ICWSs were installed in advance of the intersection,

whereas all overhead signs were installed at the intersection. Ideally, to address the difference 

between post-mounted and overhead signs, the placement should be taken into consideration. 

The MUTCD states that warning signs should be placed to provide an adequate PRT. This 

suggests that the findings in table 22 may have been influenced by system placement, which 

could not be addressed in this research.(2)

Table 23 presents the disaggregate results for intersections by sign message. For two-lane at 

two-lane intersections, the ICWS strategy appeared to be slightly more effective when the 

message specifically stated “WHEN FLASHING,” compared with signs that did not have the 

message. Considering the standard errors of the CMFs, there was no statistical difference 

between the two conditions. All systems in Minnesota located at two-lane at two-lane 

intersections had a “WHEN FLASHING” message and were therefore considered in this 

category. There was no apparent difference by message for four-lane at two-lane intersections.
42

Table 23. CMFs by message (“WHEN FLASHING” versus not present).

Lanes Crash Type Message Expected Observed CMF

Standard 

Error

2

Total crashes Present 656.20 458 0.697 0.040

Not present 256.59 212 0.824 0.070

Fatal and

injury crashes

Present 373.70 242 0.646 0.050

Not present 141.85 120 0.842 0.095

Right-angle 

crashes

Present 364.80 275 0.752 0.056

Not present 157.38 145 0.918 0.095

Statistically significant results at the 95-percent confidence level are indicated in boldface.

Table 24 presents the disaggregate results by the presence of intersection lighting. There was no 

apparent difference by lighting presence for two-lane at two-lane intersections. For four-lane at 

two-lane intersections, the strategy appeared to be more effective at sites with intersection 

lighting. The difference was statistically significant at the 95-percent confidence level for fatal 

and injury crashes. 

Table 24. CMFs by lighting presence.

Lanes Crash Type Lighting Expected Observed CMF

Standard 

Error

4

Total crashes Present 169.49 119 0.697 0.085

None 295.01 266 0.898 0.079

Fatal and

injury crashes

Present 87.27 48 0.545 0.093

None 176.29 164 0.925 0.099

Right-angle 

crashes

Present 78.89 62 0.777 0.127

None 216.57 190 0.872 0.090

Statistically significant results at the 95-percent confidence level are indicated in boldface.

Table 25 presents the disaggregate results by expected crash frequency in the before period. 

There was no apparent difference by expected crash frequency for two-lane at two-lane 

intersections. For four-lane at two-lane intersections, the ICWS strategy was more effective 

when the expected crash frequency was higher in the before period. This is logical because the 

strategy was often used at intersections with unusually high crashes or issues related to limited 

sight distance. For total crashes, there did not appear to be a benefit if the expected crash 

frequency was less than or equal to three crashes per year before installation; however, there was 

a significant reduction for sites with more than three expected crashes per year in the before 

period. The results for right-angle crashes were significantly different from each other for sites 

with less than or equal to 2.5 expected crashes per year versus sites with more than 2.5 expected 

crashes per year before installation. There did not appear to be a benefit if the expected fatal and 

injury crash frequency was less than or equal to two crashes per year before installation; 

however, there was a significant reduction for sites with more than two expected fatal and injury 

crashes per year in the before period.
43

Table 25. CMFs by before period expected crash frequency.

Lanes Crash Type

Crashes 

Per Year Expected Observed CMF

Standard 

Error

4

Total crashes ≤ 3 114.23 121 1.047 0.147

> 3 350.27 264 0.751 0.062

Fatal and

injury crashes

≤ 2 66.28 74 1.101 0.179

> 2 197.28 138 0.696 0.075

Right-angle 

crashes

≤ 2.5 93.32 116 1.228 0.176

> 2.5 202.15 136 0.669 0.075

Statistically significant results at the 95-percent confidence level are indicated in boldface.

45

CHAPTER 8. ECONOMIC ANALYSIS

An economic analysis was conducted to estimate the B/C ratio for using the ICWS strategy for 

two-lane at two-lane intersections and four-lane at two-lane intersections. The statistically 

significant reduction in total crashes for combined States was used as the benefit for two-lane at 

two-lane intersections and for four-lane at two-lane intersections.

Based on details provided by NCDOT, the analysis used the average cost estimate for each 

installation type (e.g., overhead signs on both approaches) by major route number of approach 

lanes. Approximate costs provided by MoDOT were used for installations in Missouri. For sites 

in Minnesota, the cost estimates provided by MnDOT were used. The average installation cost 

for all two-lane at two-lane intersections was $41,590. The average installation cost was 

$106,150 for four-lane at two-lane intersections. In addition, an annual maintenance and 

operations cost of $1,075 was assumed for two-lane at two-lane intersections based on 

information provided by MoDOT. A value of $1,200 for maintenance and utility costs was 

assumed for four-lane at two-lane sites based on information provided by MoDOT for sites with 

loop detectors. A value of $3,400 was used for four-lane at two-lane sites with wireless 

communication. These values were more conservative than the estimated value of $625 used by 

NCDOT. In total, 69 two-lane at two-lane intersections and 24 four-lane at two-lane intersections 

were installed.

The analysis assumed that the useful service life for safety benefits was 10 years. This was based 

on information provided from Minnesota, Missouri, and North Carolina. This was likely 

conservative in that this was the minimum service life reported from the three States. MoDOT 

noted that loop detectors might need to be replaced every 5 years, and this cost was considered in 

the annual maintenance cost. 

The FHWA Office of Safety Research and Development suggested using the Office of 

Management and Budget Circular A-4: Regulatory Analysis to determine the conservative real 

discount rate of 7 percent that was applied to calculate the annual cost of the treatment for the 

10-year service life.(14) With this information, the capital recovery factor was computed to be 

7.024 for all intersection types.

For the benefit calculations, the most recent FHWA mean comprehensive crash costs 

disaggregated by crash severity and location type were used as a base.

(15) These costs were 

developed based on 2001 crash costs, and the unit cost (in 2001 dollars) for fatal and injury 

crashes was $158,177 and $7,428 for property damage only (PDO) crashes. This was updated to 

2014 dollars by applying the ratio of the USDOT 2014 value of a statistical life of $9.2 million to 

the 2001 value of $3.8 million.

(16,17) Applying this ratio of 2.42 to the unit costs for PDO and

fatal and injury crashes and then weighting by the frequencies of these two crash types in the 

after period resulted in an aggregate 2014 unit cost for total crashes of $202,060 for two-lane at 

two-lane intersections and $219,876 for four-lane at two-lane intersections. 

The total crash reduction was calculated by subtracting the actual crashes in the after period from 

the expected crashes in the after period if the ICWS strategy had not been implemented. The 

total crash reduction was then divided by the average number of after period years per site to 
46

compute the total crashes saved per year. The number of total crashes avoided per year was 

65.69 for all two-lane at two-lane intersections and 19.08 for four-lane at two-lane intersections.

Considering the number of intersections installed, this resulted in an average “savings” 

(avoidance) of 0.95 crashes per intersection per year for two-lane at two-lane intersections and 

0.79 crashes per intersection per year for four-lane at two-lane intersections.

The annual benefits (i.e., dollar value of crash avoidance) were obtained by multiplying the crash 

reduction per site per year by the cost of a crash, with all severities combined. The B/C ratio was 

calculated as the ratio of the annual benefit to the annual cost. The B/C ratio was estimated to be 

27:1 for two-lane at two-lane intersections and 10:1 for four-lane at two-lane intersections. 

USDOT recommended that sensitivity analysis be conducted by assuming values of a statistical 

life of 0.57 and 1.41 times the recommended 2014 value.

(16) These factors can be applied directly 

to the estimated B/C ratios to obtain a range of 16:1 to 39:1 for two-lane at two-lane intersections

and 6:1 to 14:1 for four-lane at two-lane intersections. These results suggest that the ICWS

strategy, even with conservative assumptions on cost, service life, and the value of a statistical 

life, can be cost effective in reducing total crashes at four-legged intersections with stop-control 

on the minor approaches.
47

CHAPTER 9. SUMMARY AND CONCLUSIONS

The objective of this study was to undertake a rigorous before–after evaluation of the safety 

effectiveness of ICWS as measured by crash frequency. The study used data from three States, 

Minnesota, Missouri, and North Carolina, to examine the effects for the following specific crash 

types: total, fatal and injury, right-angle, rear-end, and nighttime crashes. Based on the combined 

results, the CMFs shown in table 26 are recommended for the various crash types. 

Table 26. Recommended CMFs (based on combined States).

Statistic Total

Fatal and

Injury

RightAngleRearEnd Nighttime

Two-Lane at Two-Lane

Estimate of CMF 0.733 0.701 0.803 0.425 0.898

Standard error of estimate of CMF 0.035 0.045 0.049 0.073 0.096

Four-Lane at Two-Lane

Estimate of CMF 0.827 0.802 0.850 0.973 0.612

Standard error of estimate of CMF 0.059 0.072 0.075 0.224 0.108

Statistically significant results at the 95-percent confidence level are indicated in boldface.

The aggregate results indicated statistically significant crash reductions at the 95-percent 

confidence level for all crash types except nighttime crashes for two-lane at two-lane 

intersections. The results also indicated statistically significant crash reductions in all crash types 

except rear-end crashes for four-lane at two-lane intersections.

The disaggregate analysis sought to identify those conditions under which the ICWS strategy

was most effective. Because total, fatal and injury, and right-angle crashes were the focus of this 

strategy, these crash types were the focus of the disaggregate analysis. The disaggregate analysis 

of the results for two-lane at two-lane intersections indicated larger percentage crash reductions 

for sites with ICWSs installed on the major route, particularly for post-mounted ICWSs in 

advance of the intersection. An additional benefit may be provided by including the “WHEN 

FLASHING” message as part of the system. The disaggregate CMFs can be used in prioritizing 

installation sites, but interpretations should be made with caution. One should pay particular 

attention to the sample size used to develop the CMFs.

The disaggregate analysis for four-lane at two-lane intersections indicated larger percentage 

crash reductions for sites with intersection lighting and for sites with a higher expected average 

crash frequency in the before period. There was no substantive difference for sites with warning 

on the major route versus warning on the minor route. The disaggregate CMFs can be used in 

prioritizing installation sites, but again, interpretations should be made with caution. 

The B/C ratio estimated with conservative cost and service life assumptions and only considering 

the benefits for total crashes was 27:1 for all two-lane at two-lane intersections and 10:1 for 

four-lane at two-lane intersections. The benefits were calculated from the significant reduction 

found for combined States for all two-lane at two-lane intersections and based on the statistically 

significant reduction found for four-lane at two-lane intersections. With the USDOTrecommended sensitivity analysis, these values could range from 16:1 to 39:1 for two-lane at 
48

two-lane intersections and 6:1 to 14:1 for four-lane at two-lane intersections. These results 

suggest that the ICWS strategy—even with conservative assumptions on cost, service life, and 

the value of a statistical life—can be cost effective.

Because this is an evolving strategy, this study reflects installation practices to date. Future 

studies may show different results as installation practices change. In particular, the use of an 

overhead ICWS on the major route was limited to the installations at the intersection (i.e., no 

advance warning), while post-mounted ICWSs on the major route were installed in advance of 

the intersection. Future research should compare these installation practices, considering 

placement of warning signs. Specifically, section 2C.05 of the MUTCD provides guidance for 

the placement of warning signs so that they provide adequate PRT.(2)
49

APPENDIX A: EXAMPLE INSTALLATIONS BY STATE

The following appendix presents Google Street ViewTM images of ICWS installations used in 

each State.

MINNESOTA

All sites in Minnesota were post mounted. Figure 8 through figure 10 present examples of signs 

used in Minnesota. 

©Google® 2016

Figure 8. Photo. Major route blank-out sign with flashing beacon from Google Street 

ViewTM.

(18)
50

©Google® 2016

Figure 9. Photo. Minor route sign with LED arrow-shaped flashers from Google Street 

ViewTM.

(19)

©Google® 2016

Figure 10. Photo. Minor route visual display from Google Street ViewTM.

(20)
51

MISSOURI

All sites in Missouri were post mounted. Figure 11 through figure 13 present examples of signs 

used in Missouri. 

©Google® 2016

Figure 11. Photo. Major route static sign with flashing beacons from Google Street 

ViewTM.

(21)

©Google® 2016

Figure 12. Photo. Minor route static sign with flashing beacons from Google Street 

ViewTM.

(22)
52

©Google® 2016

Figure 13. Photo. Dual major route static sign with flashing beacons from Google Street 

ViewTM.

(23)

NORTH CAROLINA

Sites in North Carolina were post mounted and/or mounted overhead. Figure 14 through 

figure 16 present examples of signs used in North Carolina. 

©Google® 2016

Figure 14. Photo. Major and minor route overhead static signs with flashing beacons from 

Google Street ViewTM.

(24)
53

©Google® 2016

Figure 15. Photo. Minor route overhead static sign with flashing beacons from Google 

Street ViewTM.

(25)

©Google® 2016

Figure 16. Photo. Major route static sign with flashing beacons from Google Street 

ViewTM.

(26)

55

APPENDIX B: ADDITIONAL INSTALLATION DETAILS 

The following appendix presents additional details provided by Minnesota, Missouri, and North 

Carolina. States were asked to provide responses to the following questions:

1. How were treatment signs, messages, and approaches selected for treatment? For 

example, how were sites selected to have treatment on the major or minor approaches 

only, or how were sites selected to be treated on BOTH the major and minor routes? 

2. How were signs and messages selected (e.g., visual display versus message, “When 

Flashing” versus no message, or overhead versus post-mounted)?

3. Do you know if other geometric changes or countermeasures (e.g., addition of turn lanes) 

were implemented concurrently with the ICWS?

4. We would like to provide a summary of the ICWS characteristics below. Do you have 

any standard drawings that applied to the treatment sites considered by the study?

a. Location of sign on major approach and/or minor approaches.

b. Location/type of detection on major and/or minor approaches.

c. Messages on the signs.

d. Sign size.

e. Detector timing parameters.

5. Was crash history the major criteria for site selection? Were any specific crash types 

targeted? Please specify any criteria.

6. Were there any requirements for ICWS implementation (e.g., minimum major/minor 

route volumes, minimum/maximum speed limits)?

7. Please describe any notable challenges related to ICWS installation and how you 

overcame them.

8. Please describe any notable challenges related to ICWS maintenance and how you 

overcame them.

9. What lessons learned or recommendations would you share with another State interested 

in the application of ICWS?

10. Can you provide any estimates on cost of ICWS operation and maintenance for 

intersections with installations only on minor approaches, only on major approaches, or 

on both the major and minor approaches? Are there any noted differences for overhead 

versus post-mounted installations?
56

RESPONSES FROM MINNESOTA

Minnesota responded to all 10 questions. Their responses are listed in numeric order. The 

responses are listed separately for cooperative intersection collision avoidance system (CICAS)

sites and for ICWS sites.

The following responses were received regarding CICAS installations:

1. The locations for the CICAS system were selected based on their crash history. These 

were locations with a documented crash history of angle crashes at expressway

intersections. Only the minor approach received treatment via the CMS that was installed 

at the intersection. A report describing the location selection and the preliminary 

candidate message types can be found at http://www.dot.state.mn.us/guidestar/

2001_2005/ids/2007_33.pdf.

2. Details on the various signs and messages where researched over a several year 

period. Details on these studies can be found at http://www.dot.state.mn.us/guidestar/

2006_2010/cicas.html. Ultimately, the design that was implemented was informed by this 

research as well as feasibility aspects brought forward by the University.

3. No additional geometric changes were incorporated into the deployment of the CICAS 

system.

4. Additional details are discoverable but would require some effort (detector parameters, 

etc.) or may be included in reports published at the hyperlink in A2. See page 13/27 of 

the report found at http://www.dot.state.mn.us/guidestar/2001_2005/ids/2007_33.pdf.

5. The TH 52/CSAH 9 intersection had a known crash history of high speed right-angle 

crashes and that brought the project to its location. While there was no specific site 

criteria that was used to identify site selection factors used for other locations the 

following factors were used to identify the other deployment sites: number and severity 

of right-angle crashes (looking for high frequency and high severity in terms of fatal and 

serious injury crashes) and expressway intersections with stop control. A report 

describing the location selection can be found at http://www.dot.state.mn.us/guidestar/

2001_2005/ids/2007_33.pdf.

6. No. There was concern about higher volume roads where the system would be frequently 

showing a no movement recommendation due to smaller gaps based on the traffic 

composition. This pushed the system out of “super” high volume expressways in urban 

areas to more rural environments where the demand was not as consistent throughout the 

day.

7. The installation was fairly straight forward but there were some challenges with post 

design and guardrail requirements for the system. Engineering judgment was used to 

move forward with an acceptable layout.

8. Since this system was a prototype keeping the system operational 24/7 was a challenge.

This was confounded by having remote locations not near the university staff office 
57

facilities. Diagnostics could be done remotely to check system status but diagnosing 

repairs required a site visit and potentially ordering replacement parts. It should also be 

noted that the vendor for the signs at one of the locations went out of business, making it 

difficult to repair the signs and get parts. Also, electricity for the large DMS signs was 

very expensive.

9. A plethora of information was used to develop and design this new elaborate 

system. Substantial information was gathered at the locations that provided details on 

drivers and their gap acceptance. In the end, drivers still made bad choices at 

intersections and crashes occurred even after the system was installed. Every crash that 

occurred after the system was installed had the correct message displayed, indicating that 

no movement was advised. A system with lower installation and maintenance cost would 

be more likely to be built.

10. Due to the prototype nature of this project we are not able to provide any meaningful 

costs on the installation and operation of this system. The project costs were high due to 

the research and development that went into the system, the goal was that someone would 

commercialize the product with standard detection and logic in the future based on the 

results of this research.

The following responses were received regarding the ICWS installations:

1. The locations for the [rural intersection conflict warning system] RICWS system were 

selected based on their crash history and local input into intersections to test this new 

technology—these locations in general were not considered black spots. The default 

system design was to employ warning signs for all approaches to the intersection

2. The goal was to provide a system that leveraged off the shelf technology so the project 

looked to leverage existing signs (MUTCD compliant) and compliment them with 

technology to flash when conditions warranted.

3. No additional geometric changes were incorporated into the deployment of the RICWS 

system.

4. Attached are files that contain some of the information requested in the above bullets.

5. The locations selected were based on a variety of factors including the perception of a 

crash problem (i.e., lots of near misses, less than standard site distance, limited crash 

history). Angle crashes were the focus of this intersection treatment.

6. No.

7. The installation was fairly straight forward but there were some challenges keeping the 

system operational 100% of the time.

8. Since this system was a prototype keeping the system operational 24/7 was a 

challenge. This was confounded by not having an easy way to diagnose whether the 

system was operational or not.
58

9. While this system is relatively simple to design and install substantial effort was focused 

on keeping the system operational throughout the deployment. A system with lower 

installation and maintenance cost would be more desirable in the future.

10. Due to the prototype nature of this project we are not able to provide any meaningful 

costs on the installation and operation of this system. The project costs were high due to 

the research and development that went into the system, the goal was that someone would 

commercialize the product with standard detection and logic in the future based on the 

results of this research. A new system has been designed and deployed based on the 

lessons learned from this system, however the costs are well in excess of $50k per 

intersection.

RESPONSES FROM MISSOURI

Missouri provided responses to four of the questions.

Response to question 2: For the message, we have a couple that have been used, but it seems like 

the “traffic approaching when flashing” is being taken out of service due to litigation concerns 

(may not always flash).

Response to question 5. These locations were driven by crash issues.

Response to question 9. If we see a continued trend in angle collisions after installation, we may 

decide to modify the access and potentially install a j-turn design (RCUT).

Response to question 10. I am confident we are hoping to get a 10 year plus lifespan out of the 

locations we have installed this countermeasure.

RESPONSES FROM NORTH CAROLINA

North Carolina responded to all 10 questions. Its responses are listed in numeric order. Figure 17

and Figure 18 provide the pre- and post-2012 crash reduction factors used by NCDOT. Figure 19

through figure 21 present example diagrams of ICWS applications in North Carolina.

1. Treatment sites were selected by the local traffic engineering staff, for the most part 

based on an observed crash experience. The decision of where to place signs (overhead in 

the intersection or in advance of the intersection, an on which approaches) likely 

depended upon:

a. Whether there was an existing standard overall flasher in the intersection.

i. At some of the locations, a standard overhead flasher was already in place 

(likely as a safety treatment that hadn’t worked well). If there was an existing 

overhead flasher, the ICWS likely replaced the standard flasher in the same 

spot.
59

b. Whether there were site distance issues at the intersection.

i. In some cases, if the intersection was located in a curve, the flashers may be 

placed in advance of the intersection on the major road so that drivers could 

better see the warning.

c. The personal preference of the local Division/Regional traffic engineers in the area 

where it was placed (this may be the biggest reasoning behind the design at each site).

2. The decision on which messages to use also depended upon the preference of the local 

traffic engineering staff. Some feel more comfortable adding the “When Flashing” to the 

message than others due to potential/perceived liability issues.

3. The ICWS should be the only change made to the sites during the study periods.

4. See the attached drawings of the countermeasures for examples (Examples 1–3).[See 

figure 19 through figure 21.]

5. Yes, most of the sites were selected based on an existing crash pattern. The number of 

total crashes in the before period at each site varies from 0 crashes to 9.5 crashes per 

year, with an average of 3.7 crashes per year at 74 sites. Note, there was only 1 site with 

0 crashes in the before period. Target crashes were frontal impact, specifically angle 

crash types where a vehicle pulled out from the stop-controlled leg. The number of target 

crashes in the before period at each site varied from 0 crashes per year to 8.5 crashes per 

year, with an average of 3.0 crashes per year at 74 sites. Many of the sites were funded 

through the Spot Safety program, where they competed with other safety projects based 

on the B/C ratio, among other items. A site with a strong pattern of crashes, including 

some high severity crashes, may be more likely to be funded depending upon the total 

cost of the project.

6. There were not volume or speed thresholds. Intersection AADTs ranged from 

approximately 3,000 to 30,000, with an average of 7,300 at 74 sites. Major road speed 

limits ranged from 35 to 55 mph, although a majority of sites were located on high-speed 

facilities.

7. We have no notable challenges related to ICWS installation to report. We have been 

installing these countermeasures since 1997, so there may have been some installation 

issues to overcome initially, but we do not have a record of those items.

8. We also do not have any notable challenges related to ICWS maintenance. We have been 

installing these countermeasures since 1997, so there may have been some maintenance 

issues to overcome initially, but we do not have a record of those items.

9. In our experience, ICWS may work best when signs are posted on the major road in 

advance of the intersection. A combination of signs (i.e., minor road signs with major 

road signs in advance of the intersection) may be most effective. Also, ICWS appears to 

be more effective when the major road is a two-lane cross-section as opposed to a fourlane divided cross-section. Probably the biggest thing we learned was the crash reduction 

factor estimates we used for this type of project pre-2012 to post-2012 in our B/C process 
60

within our Spot Safety program. Since there was no good crash reduction factor research 

available prior to 2012, we decided to use a 25 percent reduction in total crashes for these 

types of countermeasures. After our evaluation of the sites we shared, we adjusted our 

crash reduction factor estimates to match the data results our analysis provided. Based on 

the new information, I believe there were a lot of sites we would have never installed this 

countermeasure; those with a low opportunity for same improvements (see values for 

Post-2012 below).

© NCDOT.

Figure 17. Graphic. North Carolina pre-2012 crash reduction factor.

© NCDOT.

Figure 18. Graphic. North Carolina post-2012 crash reduction factor.

10. We are currently using $500 for the annual maintenance costs and $125 for the annual 

utility costs in our B/C process within our Spot Safety program. We do not differentiate 

the costs between the following four categories:

• Overhead signs and flashers on major, loop in minor.

• Overhead signs and flashers on minor, loop on major.

• Post mounted signs and flashers on major, loop on minor.

• Combination of signs and flashers on major/minor, loops on major/minor.
61 

© NCDOT.

Figure 19. Diagram. Example 1—overhead sign on major route. 
62 

© NCDOT.

Figure 20. Diagram. Example 2—overhead sign on minor route. 
63 

© NCDOT.

Figure 21. Diagram. Example 3—post-mounted sign on major approach.

65

ACKNOWLEDGEMENTS

This report was prepared for the Federal Highway Administration, Office Safety Research and 

Development, under Contract DTFH61-13-D-00001. The FHWA Contracting Officer’s Technical 

Representative for this project was Roya Amjadi. 

The project team gratefully acknowledges the participation of the following organizations for 

their assistance in this study:

• Minnesota Department of Transportation.

• Missouri Department of Transportation.

• North Carolina Department of Transportation.

67

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21. Google®, Inc. (2016) Major route static sign with flashing beacons from Google Street 

ViewTM. Available at https://goo.gl/maps/2Pmdkjx1wkv. Accessed February 25, 2016.

22. Google®, Inc. (2016) Minor route static sign with flashing beacons from Google Street 

ViewTM. Available at https://goo.gl/maps/Dq3iTnPAkV42. Accessed February 25, 2016.

23. Google®, Inc. (2016) Major route static sign with flashing beacons from Google Street 

ViewTM. Available at https://goo.gl/maps/P3ajE6JKrjA2. Accessed February 25, 2016.

24. Google®, Inc. (2016) Major and minor route overhead static signs with flashing beacons 

from Google Street ViewTM. Available at https://goo.gl/maps/wR5yNuQdizp. Accessed 

February 25, 2016.

25. Google®, Inc. (2016) Minor route overhead static sign with flashing beacons from Google 

Street ViewTM. Available at https://goo.gl/maps/NTCEffZnUp82. Accessed February 25, 

2016.

26. Google®, Inc. (2016) Major route static sign with flashing beacons from Google Street 

ViewTM. Available at https://goo.gl/maps/HEi8PrSMYSp. Accessed February 25, 2016.

HRDS-20/06-16(500)E Recycled

Recyclable