Safety Evaluation of Multiple

Strategies at Signalized Intersections

PUBLICATION NO. FHWA-HRT-17-062 MAY 2018

Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296
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 PFS 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 (AASHTO) 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.

This study evaluated multiple low-cost safety improvements at signalized intersections for basic 

signing, pavement marking, and signal enhancements. This strategy was intended to reduce the 

frequency and severity of crashes at signalized intersections by alerting drivers to the presence, 

type, and configuration of the approaching intersection. The results indicate reductions for all 

crash types analyzed (i.e., total, fatal and injury, rear-end, right-angle, and nighttime crashes). 

The economic analysis results suggest that implementation of multiple low-cost treatments at 

signalized intersections, even with conservative assumptions for cost, service life, and the value 

of a statistical life, can be cost effective. This report is intended for safety engineers, highway 

designers, planners, and practitioners at State and local agencies involved with AASHTO

Strategic Highway Safety Plan implementation.

Monique R. Evans, P.E., CPM

Director, Office of Safety

Research and Development

Notice

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

(USDOT) in the interest of information exchange. The U.S. Government assumes no liability for 

the use of the information contained in this document.

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document.

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

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

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

ensure continuous quality improvement.
TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

FHWA-HRT-17-062

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

4. Title and Subtitle

Safety Evaluation of Multiple Strategies at Signalized Intersections

5. Report Date

May 2018

6. Performing Organization Code

7. Author(s)

Thanh Le, Frank Gross, Bhagwant Persaud, Kimberly Eccles, and 

Jonathan Soika

8. Performing Organization Report No.

9. Performing Organization Name and Address

VHB Persaud and Lyon, Inc

8300 Boone Boulevard, Suite 700 87 Elmcrest Road

Vienna, VA 22182-2626 Toronto, ON M9C 3R7

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 20590-3660

13. Type of Report and Period Covered

Safety Evaluation; October 2010–

March 2017

14. Sponsoring Agency Code

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 Program and Task Manager was Roya Amjadi (HRDS-20).

16. Abstract

The Development of Crash Modification Factors program conducted safety evaluations of multiple strategies at 

signalized intersections for the Evaluation of Low Cost Safety Improvements Pooled Fund Study. This study 

evaluated multiple low-cost treatments at signalized intersections. Improvements included basic signing, pavement 

marking, and signal enhancements. This strategy was intended to reduce the frequency and severity of crashes at 

signalized intersections by alerting drivers to the presence, type, and configuration of the approaching intersection. 

Geometric, traffic, and crash data were obtained at three- and four-legged, two- and four-lane major road, urban, and 

rural signalized intersections in South Carolina. To account for potential selection bias and regression-to-the-mean, 

an empirical Bayes before–after analysis was conducted using reference groups of untreated intersections with 

characteristics similar to the treated sites to develop safety performance functions for the intersection configurations 

included in this study. The analysis also controlled for changes in traffic volumes over time and time trends in crash 

counts unrelated to the treatments. The aggregate results indicate reductions for all crash types analyzed (i.e., total, 

fatal and injury, rear-end, right-angle, and nighttime). The crash modification factors (CMFs) for fatal and injury and 

right-angle crashes were 0.893 and 0.883, respectively, which were statistically significant at the 95-percent 

confidence level. The CMFs for total, rear-end, and nighttime crashes were 0.955, 0.974, and 0.969, respectively, 

which were not statistically significant at the 95-percent confidence level. Note that the CMF for total crashes was

statistically significant at the 90-percent confidence level. The benefit–cost ratio estimated with conservative cost 

and service life assumptions was 4.1:1 for total crashes at signalized intersections. The results suggest that the 

implementation of multiple low-cost treatments, even with conservative assumptions on cost, service life, and the 

value of a statistical life, can be cost effective.

17. Key Words

Signalized, intersection, low-cost, safety improvements, 

safety evaluations, empirical Bayesian, before–after, 

multistrategy, multiple treatments

18. Distribution Statement.

No restrictions. This document is available through the 

National Technical Information Service, 

Springfield, VA 22161.

http://www.ntis.gov

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

103

22. Price

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

TABLE OF CONTENTS

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

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

BACKGROUND ON MULTIPLE STRATEGIES AT SIGNALIZED 

INTERSECTIONS.............................................................................................................. 3

BACKGROUND ON STUDY.................................................................................................. 5

CHAPTER 2. LITERATURE REVIEW .................................................................................... 7

WARNING SIGNS.................................................................................................................... 7

RETROREFLECTIVE SIGN POSTS .................................................................................... 7

REFRESHING EXISTING PAVEMENT MARKINGS ...................................................... 8

INSTALLING ONE SIGNAL FACE PER LANE................................................................. 8

RETROREFLECTIVE BORDERS ON BACKPLATES ..................................................... 8

TWELVE-INCH LIGHT-EMITTING DIODE (LED) LENSES......................................... 9

LIMITATIONS OF PREVIOUS RESEARCH.................................................................... 10

CHAPTER 3. OBJECTIVE ....................................................................................................... 11

CHAPTER 4. STUDY DESIGN ................................................................................................ 13

SAMPLE SIZE ESTIMATION OVERVIEW ..................................................................... 13

REFERENCE SITE OVERVIEW ........................................................................................ 19

CHAPTER 5. METHODOLOGY ............................................................................................. 21

CHAPTER 6. DATA COLLECTION....................................................................................... 23

INSTALLATION DATA........................................................................................................ 23

REFERENCE SITES.............................................................................................................. 25

TRAFFIC DATA..................................................................................................................... 26

CRASH DATA ........................................................................................................................ 27

TREATMENT COST DATA................................................................................................. 28

DATA CHARACTERISTICS AND SUMMARY................................................................ 29

CHAPTER 7. DEVELOPMENT OF SPFs............................................................................... 31

SPFs FOR 3 X 22 INTERSECTIONS................................................................................... 32

SPFs FOR 4 X 22 INTERSECTIONS................................................................................... 34

SPFs FOR 3 X 42 INTERSECTIONS................................................................................... 37

SPFs FOR 4 X 42 INTERSECTIONS................................................................................... 39

BEFORE–AFTER ADJUSTMENT FACTORS .................................................................. 42

CHAPTER 8. BEFORE–AFTER EVALUATION RESULTS............................................... 45

AGGREGATE ANALYSIS.................................................................................................... 45

DISAGGREGATE ANALYSIS............................................................................................. 45

CHAPTER 9. ECONOMIC ANALYSIS .................................................................................. 51

CHAPTER 10. SUMMARY AND CONCLUSIONS............................................................... 53

APPENDIX. ADDITIONAL INSTALLATION DETAILS.................................................... 55

EXAMPLE OF DOCUMENTS USED DURING THE LOW-COST

INTERSECTION IMPROVEMENT PROJECT.......................................................... 55
iv

GENERAL SIGNING AND PAVEMENT MARKING NOTES FOR ALL 

INTERSECTIONS............................................................................................................ 56

STANDARD MARKINGS FOR INTERSECTIONS.......................................................... 57

Application of Markings at Intersections..........................................................................57

Arrows and Word Messages...............................................................................................59

Additional Guidance through Intersections .....................................................................59

Crosswalks ...........................................................................................................................59

TYPICAL MARKINGS FOR TURN LANE INSTALLATIONS...................................... 60

Notes .....................................................................................................................................60

STANDARD PAVEMENT MARKINGS ............................................................................. 64

ADDITIONAL SIGN INVENTORY FOR REPLACEMENT........................................... 70

RETROREFLECTIVE SIGN POST PANELS.................................................................... 74

STANDARD REVIEW GUIDELINES................................................................................. 82

General Notes.......................................................................................................................82

Non‐Signalized Locations ...................................................................................................83

Signalized Locations............................................................................................................84

Field Notes............................................................................................................................85

Final Plans............................................................................................................................85

Submissions..........................................................................................................................86

Checklist for Submitting Packets.......................................................................................87

ACKNOWLEDGMENTS .......................................................................................................... 89

REFERENCES............................................................................................................................ 91
v

LIST OF FIGURES

Figure 1. Illustration. Example of signalized intersection improvements........................................4

Figure 2. Equation. Estimated change in safety. ............................................................................21

Figure 3. Equation. EB estimate of expected crashes. ...................................................................22

Figure 4. Equation. EB weight. ......................................................................................................22

Figure 5. Equation. Index of effectiveness.....................................................................................22

Figure 6. Equation. Standard deviation of index of effectiveness. ................................................22

Figure 7. Illustration. Collecting the number of legs and verifying the number of lanes from 

the work plan............................................................................................................................25

Figure 8. Equation. Total crash SPF for 3 x 22 intersections.........................................................33

Figure 9. Equation. Rear-end crash SPF for 3 x 22 intersections. .................................................33

Figure 10. Equation. Right-angle crash SPF for 3 x 22 intersections. ...........................................33

Figure 11. Equation. Nighttime crash SPF for 3 x 22 intersections...............................................34

Figure 12. Equation. Total crash SPF for 4 x 22 intersections.......................................................34

Figure 13. Equation. Fatal and injury crash SPF for 4 x 22 intersections......................................35

Figure 14. Equation. Rear-end crash SPF for 4 x 22 intersections. ...............................................35

Figure 15. Equation. Right-angle crash SPF for 4 x 22 intersections. ...........................................36

Figure 16. Equation. Nighttime crash SPF for 4 x 22 intersections...............................................36

Figure 17. Equation. Total crash SPF for 3 x 42 intersections.......................................................37

Figure 18. Equation. Fatal & injury crash SPF for 3 x 42 intersections. .......................................37

Figure 19. Equation. Rear-end crash SPF for 3 x 42 intersections. ...............................................38

Figure 20. Equation. Right-angle crash SPF for 3 x 42 intersections. ...........................................38

Figure 21. Equation. Nighttime crash SPF for 3 x 42 intersections...............................................39

Figure 22. Equation. Total crash SPF for 4 x 42 intersections.......................................................39

Figure 23. Equation. Fatal and injury crash SPF for 4 x 42 intersections......................................40

Figure 24. Equation. Rear-end crash SPF for 4 x 42 intersections. ...............................................40

Figure 25. Equation. Right-angle crash SPF for 4 x 42 intersections. ...........................................41

Figure 26. Equation. Nighttime crash SPF for 4 x 42 intersections...............................................41

Figure 27. Equation. Before–after adjustment factor calculation. .................................................42

Figure 28. Chart. The relationship between CMF (total crashes) and total intersection 

AADT (2014). ..........................................................................................................................49

Figure 29. Chart. The relationship between CMF (fatal and injury crashes) and total 

intersection AADT (2014). ......................................................................................................49

Figure 30. Chart. The relationship between CMF (total crashes) and expected total crashes 

during the before period. ..........................................................................................................50

Figure 31. Chart. The relationship between CMF (fatal and injury crashes) and expected 

fatal and injury crashes during the before period.....................................................................50

Figure 32. Illustration. SCDOT Standard Drawing 625-305-00 excerpts for application of 

markings at intersections..........................................................................................................58

Figure 33. Illustration. SCDOT Standard Drawing 625-305-00 excerpt for guidance 

through intersections. ...............................................................................................................59

Figure 34. Illustration. SCDOT Standard Drawing 625-305-00 excerpts for crosswalk 

markings. ..................................................................................................................................60

Figure 35. Illustration. SCDOT Standard Drawing 625-310-00 excerpts for turn lane 

installations (part 1)..................................................................................................................61
vi

Figure 36. Illustration. SCDOT Standard Drawing 625-310-00 excerpts for turn lane 

installations (part 2)..................................................................................................................62

Figure 37. Illustration. SCDOT Standard Drawing 625-310-00 excerpts for turn lane 

installations (part 3)..................................................................................................................63

Figure 38. Illustration. SCDOT Standard Drawing 625-310-00 excerpts for chevron 

marking details. ........................................................................................................................64

Figure 39. Illustration. SCDOT Standard Drawing 625-410-00 excerpt for straight arrow 

standard pavement marking. ....................................................................................................65

Figure 40. Illustration. SCDOT Standard Drawing 625-410-00 excerpt for right or left turn 

arrow and combination straight and left or right turn arrow standard pavement marking.......66

Figure 41. Illustration. SCDOT Standard Drawing 625-410-00 excerpt for right lane drop 

arrow and left lane drop arrow standard pavement marking....................................................67

Figure 42. Illustration. SCDOT Standard Drawing 625-410-00 excerpt for “ONLY” 

standard pavement marking. ....................................................................................................68

Figure 43. Illustration. SCDOT Standard Drawing 625-410-00 excerpt for right or left turn 

markings application. ...............................................................................................................69

Figure 44. Illustration. Additional sign inventory for replacement (part 1)...................................70

Figure 45. Illustration. Additional sign inventory for replacement (part 2)...................................71

Figure 46. Illustration. Additional sign inventory for replacement (part 3)...................................72

Figure 47. Illustration. Additional sign inventory for replacement (part 4)...................................73

Figure 48. Illustration. SCDOT nonsignalized intersection design for pavement marking 

and sign installations. ...............................................................................................................76

Figure 49. Illustration. SCDOT traffic engineering rumble strip typical.......................................77

Figure 50. Illustration. SCDOT typical for a signalized intersection.............................................78

Figure 51. Illustration. SCDOT typical for a four-way stop controlled intersection. ....................79

Figure 52. Illustration. SCDOT typical for a cross-type stop controlled intersection. ..................80

Figure 53. Illustration. SCDOT typical for a t-type stop controlled intersection...........................81

Figure 54. Illustration. SCDOT street name sign typical...............................................................82
vii

LIST OF TABLES

Table 1. Phase V safety strategies and participating States. ............................................................6

Table 2. Before-period crash rate assumptions for four-legged signalized intersections in 

South Carolina..........................................................................................................................14

Table 3. Minimum required before-period intersection-years for treated sites at 95-percent 

confidence. ...............................................................................................................................15

Table 4. Minimum required before period intersection-years for treated sites at 90-percent 

confidence. ...............................................................................................................................16

Table 5. Sample analysis for crash effects at rural two-lane intersections. ...................................17

Table 6. Sample analysis for crash effects at urban two-lane intersections...................................17

Table 7. Sample analysis for crash effects at rural four-lane intersections....................................18

Table 8. Sample analysis for crash effects at urban four-lane intersections. .................................18

Table 9. p-value for 10 percent change in crashes. ........................................................................19

Table 10. Reference groups and desirable sample sizes. ...............................................................19

Table 11. Definitions of crash types...............................................................................................28

Table 12. Treatment cost summary. ...............................................................................................28

Table 13. Summary of treatment sites............................................................................................29

Table 14. Summary of reference sites............................................................................................29

Table 15. SPF parameters for total crashes at 3 x 22 intersections................................................33

Table 16. SPF parameters for rear-end crashes at 3 x 22 intersections..........................................33

Table 17. SPF parameters for right-angle crashes at 3 x 22 intersections. ....................................34

Table 18. SPF parameters for nighttime crashes at 3 x 22 intersections........................................34

Table 19. SPF parameters for total crashes at 4 x 22 intersections................................................35

Table 20. SPF parameters for fatal and injury crashes at 4 x 22 intersections...............................35

Table 21. SPF parameters for rear-end crashes at 4 x 22 intersections..........................................36

Table 22. SPF parameters for right-angle crashes at 4 x 22 intersections. ....................................36

Table 23. SPF parameters for nighttime crashes at 4 x 22 intersections........................................37

Table 24. SPF parameters for total crashes at 3 x 42 intersections................................................37

Table 25. SPF parameters for fatal and injury crashes at 3 x 42 intersections...............................38

Table 26. SPF parameters for rear-end crashes at 3 x 42 intersections..........................................38

Table 27. SPF parameters for right-angle crashes at 3 x 42 intersections. ....................................39

Table 28. SPF parameters for nighttime crashes at 3 x 42 intersections........................................39

Table 29. SPF parameters for total crashes at 4 x 42 intersections................................................40

Table 30. SPF parameters for fatal and injury crashes at 4 x 42 intersections...............................40

Table 31. SPF parameters for rear-end crashes at 4 x 42 intersections..........................................41

Table 32. SPF parameters for right-angle crashes at 4 x 42 intersections. ....................................41

Table 33. SPF parameters for nighttime crashes at 4 x 42 intersections........................................42

Table 34. Before–after adjustment factor for total crashes. ...........................................................43

Table 35. Before–after adjustment factor for fatal and injury crashes...........................................43

Table 36. Before–after adjustment factor for rear-end crashes......................................................43

Table 37. Before–after adjustment factor for right-angle crashes..................................................43

Table 38. Before–after adjustment factor for nighttime crashes....................................................43

Table 39. Aggregate results for EB before-after study. .................................................................45

Table 40. Disaggregate results by area type...................................................................................46

Table 41. Disaggregate results by number of legs. ........................................................................47

Table 42. Disaggregate results by number of lanes. ......................................................................47
viii

Table 43. Disaggregate results by number of legs and number of lanes........................................48

Table 44. B/C ratios. ......................................................................................................................52

Table 45. Recommended CMFs.....................................................................................................53

Table 46. Advance placement distance for signal ahead, stop, or intersection warning signs. .....74
ix

LIST OF ABBREVIATIONS 

AADT annual average daily traffic

B/C benefit–cost

CMF crash modification factor

DCMF Development of Crash Modification Factors (program)

EB empirical Bayes 

ELCSI‐PFS Evaluation of Low‐Cost Safety Improvements Pooled Fund Study

FHWA Federal Highway Administration

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, and O is 

property damage only)

LED light-emitting diode

MUTCD Manual on Uniform Traffic Control Devices

NCHRP National Cooperative Highway Research Program

PDO property damage only

PE preliminary engineering

RTM regression to the mean

SCDOT South Carolina Department of Transportation

SPF safety performance function

TWLTL two-way left-turn lane

USD U.S. dollars

USDOT U.S. Department of Transportation

1

EXECUTIVE SUMMARY

The Federal Highway Administration established the Development of Crash Modification Factors 

(DCMF) program in 2012 to address highway safety research needs for evaluating new and 

innovative safety strategies (improvements) by developing reliable quantitative estimates of their 

effectiveness in reducing crashes. The ultimate goal of the DCMF program is to save lives by 

identifying new safety strategies that effectively reduce crashes and to promote those strategies 

for nationwide implementation by providing measures of their safety effectiveness and benefit–

cost (B/C) ratios through research. State transportation departments and other transportation 

agencies need to have objective measures for safety effectiveness and B/C ratios before investing 

in broad applications of new strategies for safety improvements. Forty State transportation 

departments have provided technical feedback on safety improvements to the DCMF program 

and have implemented new safety improvements to facilitate evaluations. These States are 

members of the Evaluation of Low-Cost Safety Improvements Pooled Fund Study, which 

functions under the DCMF program. This study evaluated multiple low-cost treatments at 

signalized intersections. Improvements included basic signing, pavement marking, and signal 

enhancements. This strategy was intended to reduce the frequency and severity of crashes at 

signalized intersections by alerting drivers to the presence, type, and configuration of the 

approaching intersection. Both urban and rural signalized intersections on divided and undivided 

State-maintained roads (non-freeways) were selected as locations for treatments. Study results 

have shown that by making improvements such as those described here, the South Carolina 

Department of Transportation was able to achieve a small but statistically significant crash

reduction. Although the expected crash savings per location were not as large as for some higher 

cost treatments (e.g., converting conventional intersections to roundabouts), the low cost of these 

treatments has allowed many more locations to be treated.

Geometric, traffic, and crash data were obtained at three- and four-legged, two- and four-lane 

major road, urban and rural signalized intersections in South Carolina. To account for potential 

selection bias and regression-to-the-mean, an empirical Bayes before–after analysis was 

conducted. Safety performance functions were developed using reference groups of untreated 

intersections with characteristics similar to the treated sites. The analysis also controlled for 

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

The aggregate results indicate reductions for all crash types analyzed (i.e., total, fatal and injury, 

rear-end, right-angle, and nighttime). The crash modification factors (CMFs) for fatal and injury 

and right-angle crashes were 0.893 and 0.883, respectively, which were statistically significant at 

the 95-percent confidence level. The CMFs for total, rear-end, and nighttime crashes were 0.955, 

0.974, and 0.969, respectively, which were not statistically significant at the 95-percent 

confidence level. Note that the CMF for total crashes was statistically significant at the 90-

percent confidence level.

The disaggregate analysis identified specific CMFs by crash type and different conditions. The 

analysis revealed those conditions under which the combination of treatments may be more

effective. Variables of interest for this analysis included area type (urban or rural), number of legs 

(three or four), lane configuration of the mainline and the cross street, traffic volumes, and 

expected crashes without treatment. For area type, the results showed larger crash reductions at 
2

urban intersections for total, fatal and injury, and rear-end crashes. The results disaggregated by 

number of legs indicated the treatments were more effective for total, fatal and injury, and rearend crashes at three-legged intersections. The disaggregate analysis by lane configuration showed 

larger reductions for all crash types at intersections with two-lane major roads. For total entering 

volume, the disaggregate analysis indicated the combination of treatments was slightly more 

effective on average for intersections with lower traffic volumes. While the disaggregate analysis 

indicated conditions under which the treatments may be more effective, the variables are likely 

correlated, and caution should be exercised in interpreting and applying the disaggregate analysis 

results. The disaggregate analysis also showed the multitreatment strategy could yield similar 

crash reductions across the range of expected crashes without the treatments. This confirms the 

need for caution when interpreting the disaggregate results because the net effect of multiple 

correlations is a negligible effect on the expected number of crashes, which collectively captures 

the effects of other variables.

Assuming a 7-year service life, conservative costs, and the benefits for total crashes, the B/C ratio 

was 4.1:1. With the U.S. Department of Transportation recommended sensitivity analysis, these 

values could range from 2.3:1 up to 5.8:1. The B/C ratio when excluding the cost of pedestrian 

improvements was 11.7:1. These results suggest that implementation of multiple low-cost 

treatments, even with conservative assumptions on cost, service life, and the value of a statistical 

life, can be cost effective in reducing crashes at signalized intersections.
3

CHAPTER 1. INTRODUCTION

This chapter presents background information on the low-cost strategies implemented by South 

Carolina at signalized intersections. It also provides a brief overview of the Evaluation of Low‐Cost 

Safety Improvements Pooled Fund Study (ELCSI-PFS), of which the study reported here is a part.

BACKGROUND ON MULTIPLE STRATEGIES AT SIGNALIZED INTERSECTIONS

In recent years, there has been an increased interest in systemic installations of low-cost safety 

treatments throughout an entire jurisdiction. South Carolina Department of Transportation 

(SCDOT) embraced this approach in its intersection safety improvement plan and identified a 

number of low-cost strategies for implementation at stop-controlled and signalized intersections 

statewide. Typical low‐cost treatments at signalized intersections in South Carolina included 

improvements to basic signing, pavement markings, and traffic signals. Figure 1 shows an 

example of the systemic, low-cost improvements at a four-legged signalized intersection with 

four through lanes on both the major and minor roads.
4

©SCDOT

Figure 1. Illustration. Example of signalized intersection improvements.

The following list presents the types of traffic signal improvements used in South Carolina:

• Replace all signal heads.

• Replace pedestrian signal heads, pushbuttons, and signs.

• Install backplates with retroreflective borders on all signal heads.

• Re-stripe stop lines.

• Re-stripe crosswalks.

• Install warning signs.

• Install overhead signs (e.g., R10‐12, R3‐5L, R3‐5R).

• Install curb ramps.

The appendix of this report provides further details and considerations. Each treatment was 

installed when appropriate but not at all intersections. Each intersection received a unique 

package of improvements suited for implementation at that site.
5

BACKGROUND ON STUDY

The goal of the ELCSI‐PFS is to develop reliable estimates of the effectiveness of the safety 

improvements that are identified as strategies in the National Cooperative Highway Research 

Program (NCHRP) Report 500 Guides.

(1) These estimates are determined by conducting 

scientifically rigorous evaluations at sites in the United States where these strategies are being 

implemented. The study has spanned multiple phases. In March 2005, the first Technical 

Advisory Committee Meeting of the ELCSI‐PFS was held at the Turner‐Fairbank Highway 

Research Center. The purpose of the meeting was to discuss the study and applicable strategies 

from the NCHRP Report 500 Guides and to develop a prioritization of those strategies for 

potential evaluation in the study.(1) Since this initial meeting, several phases have been 

undertaken to evaluate strategies.

Phase V of the ELCSI‐PFS has been a “build-to-evaluate” effort in which States volunteered to 

install a variety of promising low‐cost safety countermeasures and contribute the appropriate data 

to allow a rigorous crash‐based evaluation of their safety effectiveness. This phase has had two

parts—implementation and evaluation. The implementation portion (part 1) defined the before 

period, including installation data (location and date), roadway data, traffic data, and crash data. 

The evaluative portion (part 2) began within 3 years of the conclusion of the installation phase. 

Four safety strategies were identified for implementation and evaluation in phase V. Five States 

volunteered and provided data for the phase V evaluations. Table 1 shows these safety 

improvement strategies and the volunteering States for each.
6

Table 1. Phase V safety strategies and participating States.

Safety Strategy/

Participating State

Combination 

of Cable 

Median 

Barrier and 

Rumble 

Strips

Combination 

of CenterLine Rumble 

Strips and 

Edge-Line 

Rumble 

Strips

Multistrategy 

Improvements 

at Signalized 

Intersections

Multistrategy 

Improvements 

at StopControlled 

Intersections

Illinois X — — —

Kentucky X X — —

Missouri X X — —

Pennsylvania — X — —

South Carolina — — X X

—Not used.

As the implementing agency in a volunteering State, SCDOT initiated a project to improve safety 

at more than 2,200 intersections statewide through low‐cost engineering techniques focused 

primarily on signing and markings in 2009. These intersections—600 of which were classified as 

rural—comprise only 2 percent of all State‐maintained intersections but account for nearly half of 

all intersection crashes and fatalities. It was envisioned that the project would span 3 years and 

implement improvements at approximately 700–800 intersections each year.

This report documents the safety effectiveness evaluations of multiple strategies at signalized 

intersections implemented in South Carolina. The evaluation of multiple strategies at stopcontrolled intersections can be found in the companion report entitled Safety Evaluation of 

Multiple Strategies at Stop-Controlled Intersections.

(2)
7

CHAPTER 2. LITERATURE REVIEW

The research team identified literature on the signalized intersection strategies of interest. 

Although previous research had been conducted for several of the individual treatments, literature 

on some of the related strategies was limited. There were also very few studies that investigated 

the effects of multiple strategies. The following is a summary of the salient research related to 

specific strategies. 

WARNING SIGNS

Polanis conducted an evaluation of installing advance warning signs (i.e., providing positive 

guidance) for signalized intersections.(3) The results of the evaluation indicated that right-angle 

crashes decreased by 35 percent with a standard error of 1 percent. The reduction was highly 

statistically significant, but the study relied on a simple before–after study design and a limited 

dataset with a small sample size (11 sites) from one city.

A number of studies have examined the use of fluorescent yellow sheeting on warning signs. 

Using fluorescent yellow sheeting is an inexpensive method of increasing the conspicuity of 

signs without violating the provisions contained in the Manual on Uniform Traffic Control 

Devices (MUTCD).

(4) Multiple studies confirm the superiority of fluorescent signs in terms of 

conspicuity. Jenssen et al. conducted a comparative evaluation of fluorescent and nonfluorescent 

signs on a closed track in Norway.(5) Subjects seated in moving railcars were asked to indicate 

when they could detect and recognize the size, shape, and content of fluorescent and 

nonfluorescent signs. The performance of the fluorescent signs proved to be superior; the subjects 

were able to detect and recognize the fluorescent signs well before they could detect and 

recognize the nonfluorescent signs. Burns and Johnson studied fluorescent and nonfluorescent 

materials and found that the photometric properties of fluorescent materials explained their 

superior visibility and conspicuity.(6)

RETROREFLECTIVE SIGN POSTS

A recent study by the Virginia Department of Transportation directly relates to the retroreflective 

sign post strategy. This study examined the effectiveness of retroreflective material on stop sign 

posts with respect to visibility and driver compliance.

(7) The authors measured performance with 

respect to visibility using a video survey in which participants were asked to pinpoint when a stop

sign with retroreflective material and another without retroreflective material could be detected. 

The results indicated that, during daytime conditions, the vast majority of participants could 

detect the stop sign without retroreflective material sooner than the stop sign with retroreflective 

material. In contrast, during nighttime conditions, the vast majority of participants could detect 

the stop sign with retroreflective material sooner than the stop sign without retroreflective 

material. In terms of compliance, the behavior of drivers approaching a stop sign with 

retroreflective material was not observed to be different from that of a driver approaching a stop 

sign without retroreflective material.(7)
8

REFRESHING EXISTING PAVEMENT MARKINGS

The research team did not identify any studies analyzing the safety effects of refreshing existing 

pavement markings at signalized intersections. 

INSTALLING ONE SIGNAL FACE PER LANE

Agent studied several low-cost strategies aimed at reducing crashes related to red-light running.(8)

Among the strategies examined in the study was the installation of one signal face per lane. In the 

study, high-crash intersection locations were identified in Kentucky using the critical rate method 

with an emphasis on red-light running crashes. The low-cost strategies were then employed and 

evaluated at the identified intersection locations. A simple before–after study design was used in 

the evaluation. The evaluation of installing one signal face per lane was encouraging. The results 

of the evaluation indicated that this strategy reduced total crash frequency by 24 percent, angle 

crash frequency by 63 percent, and rear-end crash frequency by 10 percent.(8)

Sayed et al. analyzed the safety effects of various combinations of signal visibility-related 

treatments at signalized intersections in British Columbia, Canada.(9) The addition of signal heads 

was included in some of these treatment combinations. The results from all the combinations of 

intersection treatments indicated declines of 7.3 percent for total crashes, 6.6 percent for 

nighttime crashes, 5.9 percent for daytime crashes, 2.6 percent for severe crashes, and 8.5 percent 

for property damage only (PDO) crashes. All of the reductions were statistically significant at the 

5-percent level except the reduction in severe crashes. Based on the aggregate results, it appears 

that the addition of signal heads, at least when coupled with other signal visibility-related 

countermeasures, produced meaningful declines in crash frequency.(9)

Felipe et al. conducted an evaluation of the effects of adding a primary head to signalized 

intersections.(10) The treatment locations were four-legged, urban intersections. The evaluation 

revealed that the treatment reduced total crashes by 28 percent, severe crashes by 17 percent, and 

PDO crashes by 31 percent. The evaluation also indicated that this countermeasure decreased 

right-angle crashes by 35 percent and rear-end crashes by 28 percent.(10)

RETROREFLECTIVE BORDERS ON BACKPLATES

Agent studied several low-cost strategies aimed at reducing crashes related to red-light running.(8)

Among the strategies examined in the study was the installation of retroreflective borders on

backplates. In the study, high-crash intersection locations were identified in Kentucky using the 

critical rate method with an emphasis on red-light running crashes. The low-cost strategies were 

then employed and evaluated at the identified intersection locations. A simple before–after study 

design was used in the evaluation. The evaluation of the retroreflective borders on backplates 

revealed that the total crash frequency decreased from 5.1 crashes per year in the before period to 

4.1 crashes per year in the after period. This change represents a 20-percent reduction in total 

crash frequency. The study also revealed that angle crashes decreased by 44 percent and rear-end 

crashes decreased by 10 percent.(8)

Another instance of this treatment was documented in a Federal Highway Administration

(FHWA) case study in South Carolina.(11) Three traffic signals that had backplates were 

retrofitted with retroreflective borders. These three signalized intersections had a history of red-
9

light running, and the addition of retroreflective borders was intended to increase the visibility of 

the traffic signals. A simple before–after evaluation was then conducted. The results indicated 

that total crash frequency decreased by 29 percent, injury crash frequency decreased by 37 

percent, and late-night/early-morning crash frequency decreased by 50 percent. In addition to its 

apparent effectiveness, application of the treatment specific to this effort had a low cost of $1,500 

per intersection.(11)

Sayed et al. performed an evaluation of different combinations of signal visibility-related 

countermeasures using an empirical Bayes (EB) technique.(9) One of these combinations of 

treatments was the installation of new backplates with reflective tape along with the upgrading of 

signal lenses. The treatment sites for this combination of countermeasures were all located in the 

North Vancouver district of British Columbia. The study estimated that this combination of 

countermeasures caused reductions of approximately 9 percent in total crashes, 14 percent in 

nighttime crashes, 8 percent in daytime crashes, 10 percent in PDO crashes, and 10 percent in 

severe crashes. Another combination of treatments evaluated in this study was the upgrading of 

signal lenses and the addition of reflective tape to existing backplates. The treatment sites for this 

combination of treatments were all in the New Westminster district of British Columbia. The 

results indicated reductions of about 12 percent in total crashes, 17 percent in nighttime crashes, 

10 percent in daytime crashes, 14 percent in PDO crashes, and 10 percent in severe crashes.(9)

Sayed et al. evaluated the effects of adding 3-inch yellow retroreflective sheeting to signal 

backplates.(12) Their evaluation used an EB before–after study design. The treatment sites were all 

located in urban areas. The results of the evaluation were that total crashes decreased by 

15 percent with a standard error of 0.5 percent. Therefore, the results of this study were 

statistically significant at the 95-percent confidence level.(12)

TWELVE-INCH LIGHT-EMITTING DIODE (LED) LENSES

Sayed et al. assessed the performance of signal heads with 300 mm (equivalent to approximately

12 inches) lenses relative to signals with 200 mm (equivalent to approximately 8 inches) lenses in

terms of crash frequency.(13) The assessment was based on 10 treatment sites in British Columbia, 

Canada, and used the EB method described by Pendleton, Higle, and Witkowski.(14,15) The results 

of the EB analysis indicated that total crash frequency decreased by 24 percent and injury crash 

frequency decreased by 16 percent. 

A study of 12-inch lenses was also conducted in Michigan by FHWA.(16) Two improvements 

were made to 33 signalized intersections with a history of angle crashes: replacement of 8-inch

lenses with 12-inch lenses and the implementation of an all-red clearance interval. The 

combination of treatments proved to be effective with a 33-percent reduction in total crashes, a 

46-percent reduction in injury crashes, and a 76-percent reduction in angle crashes.(16)

Sayed et al. performed an evaluation of combinations of intersection treatments that included

upgrading signal lenses using an EB technique.(9) The treatment group consisted of 

139 intersections in 6 municipalities in British Columbia, Canada. The results for total, nighttime, 

daytime, severe, and PDO crashes were reductions of 7.3, 6.6, 5.9, 2.6, and 8.5 percent, 

respectively. The reductions in all but the severe crashes were statistically significant.(9)
10

Sayed et al. evaluated the safety effects of installing larger 12-inch signal lenses.(13) The dataset 

for this study included 10 urban intersections in British Columbia, Canada, and an EB before–

after analysis was used. The effect on total crashes was estimated to be a 24-percent reduction, 

and the effect on severe crashes was estimated to be a 16-percent reduction.(13)

As part of the study reported in NCHRP Report 617, Accident Modification Factors for Traffic 

Engineering and ITS Improvements, an evaluation of installing larger signal lenses was 

conducted.(17) The treatment group consisted of 26 intersections where the existing 8-inch lenses 

were replaced by 12-inch lenses. The EB method was employed in this evaluation. The 

evaluation indicated that right-angle crashes were reduced by 42 percent (statistically significant 

at the 5-percent level), while total crashes remained unchanged. In order for the total crashes to 

have remained unchanged while right-angle crashes decreased, there must have been an increase 

in other crash types. The authors hypothesized that the increase occurred in rear-end crashes. An 

economic evaluation was also performed to assess the impact of this treatment. The results of the

economic evaluation indicated that the treatment produced an estimated $11,800 reduction in 

total harm per intersection per year.(17)

LIMITATIONS OF PREVIOUS RESEARCH

Literature on the signalized intersection strategies of interest was scarce in some cases and 

nonexistent in others. Further, many of the existing studies did not provide a rigorous evaluation 

of the safety effects (i.e., several potential confounding factors were not considered). The simple 

before–after methodology, which was used in many of the evaluations, does not properly account 

for factors such as traffic volume changes, changes in crash reporting practices, and regression to 

the mean (RTM). The EB before–after methodology is the preferred method to account for the 

identified potential confounding factors. While a few of the evaluations employed the EB 

method, they generally lacked a robust treatment group (i.e., they were based on a limited sample 

or were limited to one jurisdiction). Finally, there were few studies that considered the 

implementation of the combination of the treatments of interest. Thus, additional research was

warranted based on a more rigorous analysis of these treatments, especially research that would 

consider the effects of multiple low-cost strategies implemented in combination.
11

CHAPTER 3. OBJECTIVE

This research examined the safety impacts of multiple strategies implemented at signalized

intersections throughout South 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, including K (fatal injury), A (incapacitating injury), B (non-incapacitating 

injury), and C (possible injury) as rated using the KABCO scale.

• Rear-end crashes (all severities combined).

• Right-angle crashes (all severities combined).

• Nighttime crashes (all severities combined).

A further objective was to address the following questions of interest:

• Do effects vary by area type (i.e., urban versus rural)?

• Do effects vary by approach configuration of intersection (i.e., three-legged versus

four-legged)?

• Do effects vary by lane configuration of intersection (e.g., four mainline lanes and 

two cross-street lanes versus two mainline lanes and two cross-street lanes)?

• Do effects vary by traffic volume?

• Do effects vary by expected crashes?

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

crash savings in terms of the benefit–cost (B/C) ratio. 

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 might be 

small changes in safety for some crash types.

• Identify appropriate untreated reference sites.

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

unrelated to the strategy.

13

CHAPTER 4. 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.(18) The sample size analysis 

presented in this report 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 that the durations of the before 

and after periods were equal, which again, was were conservative assumptions.

Table 2 provides the crash rate assumptions. The locations of interest for this strategy were threeand four-legged, signalized intersections with two- and four-lane major roads. 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 a general estimate based on the reference sites identified for this study. 

Rates A and B represent rural and urban, four-legged, signalized intersections with two-lane 

major roads, respectively. Rates C and D represent rural and urban, signalized intersections with 

four-lane major roads, respectively.
14

Table 2. Before-period crash rate assumptions for four-legged signalized intersections in 

South Carolina.

Crash Type

Rate A

Rural

Signalized 

Intersections 

With TwoLane Major 

Road

Rate B

Urban

Signalized 

Intersections 

With Two-Lane 

Major Road

Rate C

Rural Signalized 

Intersections 

With Four-Lane 

Major Road

Rate D

Urban

Signalized 

Intersections 

With Four-Lane 

Major Road

Total 2.685 2.628 3.136 3.787

Injury 0.856 0.747 1.143 1.221

Rear-end 0.949 0.928 0.912 1.229

Right-angle 1.019 1.014 1.447 1.617

Crash Rate = crashes/intersection/year.

Table 3 and table 4 provide estimates of the minimum number of before and after period 

intersection-years for four-legged signalized intersections assuming that would be required for the 

four site types described above at both the 90- and 95-percent confidence levels. The minimum 

sample indicates the amount of data necessary to detect the safety effects with a desirable level of 

statistical significance. Larger safety effects require less data to achieve the same confidence 

level. These sample size calculations were based on specific assumptions regarding the number of 

crashes per intersection and years of available data. Intersection-years were the number of 

intersections where the strategy was implemented multiplied by the number of years of data 

before or after implementation. For example, if a strategy were implemented at nine intersections

and data were available for 3 years since implementation, then there would be a total of 27 

intersection-years of after period data available for the study. Note that the number of 

intersection-years was estimated by first estimating the required number of intersection-related 

crashes and then dividing by the appropriate intersection crash rate.
15

Table 3. Minimum required before-period intersection-years for treated sites at 

95-percent confidence.

Crash Type

Expected 

Percent 

Reduction in 

Crashes

Number of Intersection-Years

Type A Type B Type C Type D

Total 10 691 706 592 490

Total 20 104 106 89 74

Total 30 35 36 30 25

Total 40 15 16 13 11

Fatal and injury 10 2,167 2,483 1,623 1,519

Fatal and injury 20 326 373 244 229

Fatal and injury 30 111 127 83 78

Fatal and injury 40 48 55 36 34

Rear-end 10 1,955 1,999 2,034 1,509

Rear-end 20 294 301 306 227

Rear-end 30 100 102 104 77

Rear-end 40 43 44 45 33

Right-angle 10 1,820 1,829 1,282 1,147

Right-angle 20 274 275 193 173

Right-angle 30 93 94 66 59

Right-angle 40 40 40 28 25

Note: Assumes equal number of site-years for treatment and comparison sites and equal length of before and after 

periods. Types A, B, C, and D represent four-legged intersection types. A represents rural, signalized intersections 

with two-lane major roads. B represents urban, signalized intersections with two-lane major roads. C represents rural, 

signalized intersections with four-lane major roads. D represents urban, signalized intersections with four-lane major 

roads.
16

Table 4. Minimum required before period intersection-years for treated sites at 

90-percent confidence.

Crash Type

Number of Intersection-Years

Expected 

Percent 

Reduction in 

Crashes Type A Type B Type C Type D

Total 10 429 439 368 304

Total 20 72 73 62 51

Total 30 25 25 21 18

Total 40 11 11 9 8

Fatal and injury 10 1,347 1,544 1,009 944

Fatal and injury 20 225 258 169 158

Fatal and injury 30 78 90 59 55

Fatal and injury 40 34 39 25 24

Rear-end 10 1,215 1,242 1,264 938

Rear-end 20 203 208 212 157

Rear-end 30 71 72 73 55

Rear-end 40 31 31 32 24

Right-angle 10 1,132 1,137 797 713

Right-angle 20 189 190 133 119

Right-angle 30 66 66 46 41

Right-angle 40 28 29 20 18

Note: Assumes equal number of site-years for treatment and comparison sites and equal length of before and after 

periods. Types A, B, C, and D represent four-legged intersection types. A represents rural, signalized intersections 

with two-lane major roads. B represents urban, signalized intersections with two-lane major roads. C represents rural, 

signalized intersections with four-lane major roads. D represents urban, signalized intersections with four-lane major 

roads.

Case 2 considered the data collected for both the before and after periods. The statistical accuracy 

attainable for a given sample size was 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 5

through table 8. The calculations were based on the methodology in Hauer.(18) The tables indicate 

the total intersection-years of data available in the before and after period.
17

Table 5. Sample analysis for crash effects at rural two-lane intersections.

Crash Type

IntersectionYears in 

Before 

Period

Intersection-Years 

in After Period

(Assumes 2-Year 

After Period for 

Each Site)

Minimum 

Percent

Reduction 

Detectable for 

Crash Rate 

Assumption,

p = 0.10*

Minimum 

Percent

Reduction 

Detectable for 

Crash Rate 

Assumption, 

p = 0.05*

Total 705 282 10 15

Fatal and

injury 705 282 15 20

Rear-end 705 282 15 20

Right-angle 705 282 15 20

Note: Results are to nearest 5-percent interval.

*Crash rate assumption is based on actual crash rate for the before period from table 2.

Table 6. Sample analysis for crash effects at urban two-lane intersections.

Crash Type

IntersectionYears in 

Before 

Period

Intersection-Years 

in After Period

(Assumes 2-Year

After Period for 

Each Site)

Minimum 

Percent

Reduction 

Detectable for 

Crash Rate 

Assumption,

p = 0.10*

Minimum 

Percent

Reduction 

Detectable for 

Crash Rate 

Assumption, 

p = 0.05*

Total 705 282 10 15

Fatal and

injury 705 282 15 20

Rear-end 705 282 15 20

Right-angle 705 282 15 20

Note: Results are to nearest 5-percent interval.

*Crash rate assumption is based on actual crash rate for the before period from table 2.
18

Table 7. Sample analysis for crash effects at rural four-lane intersections.

Crash Type

IntersectionYears in 

Before 

Period

Intersection-Years 

in After Period

(Assumes 2-Year

After Period for 

Each Site)

Minimum 

Percent

Reduction 

Detectable for 

Crash Rate 

Assumption,

p = 0.10*

Minimum 

Percent

Reduction 

Detectable for 

Crash Rate 

Assumption, 

p = 0.05*

Total 705 282 10 10

Fatal and

injury 705 282 15 15

Rear-end 705 282 15 20

Right-angle 705 282 15 15

Note: Results are to nearest 5-percent interval.

*Crash rate assumption is based on actual crash rate for the before period from table 2.

Table 8. Sample analysis for crash effects at urban four-lane intersections.

Crash Type

IntersectionYears in 

Before 

Period

Intersection-Years 

in After Period

(Assumes 2-Year

After Period for 

Each Site)

Minimum 

Percent

Reduction 

Detectable for 

Crash Rate 

Assumption,

p = 0.10*

Minimum 

Percent

Reduction 

Detectable for 

Crash Rate 

Assumption, 

p = 0.05*

Total 705 282 10 10

Fatal and

injury 705 282 15 15

Rear-end 705 282 15 15

Right-angle 705 282 15 15

Note: Results are to nearest 5-percent interval

*Crash rate assumption is based on actual crash rate for the before period from table 2.

Another strategy would be to estimate the level of significance (i.e., the p-value) for which a 

minimum desired effect can be detected. For instance, assume the minimum desired level of 

effect is 10 percent for total and target crashes. Based on the current knowledge of available data, 

table 9 indicates the p-value associated with a 10-percent change in crashes based on the before 

period data. These calculations used the crash rates from table 2. Given the existing sample size, 

it is likely this study can detect moderate treatment effects (e.g., a 10-percent change in total 

crashes) at the 10-percent level of significance.
19

Table 9. p-value for 10 percent change in crashes.

Crash Type

p-Value Rural 

Two-Lane

p-Value Urban 

Two-Lane

p-Value Rural 

Four-Lane

p-Value Urban 

Four-Lane

Total 0.11 0.11 0.09 0.07

Fatal and injury 0.33 0.36 0.27 0.25

Rear-end 0.31 0.31 0.32 0.25

Right-angle 0.29 0.29 0.22 0.20

REFERENCE SITE OVERVIEW

A reference group was required for the various intersection groups, including rural and urban, threeand four-legged, signalized intersections with two- and four-lane major roads. Each reference group 

should consist of untreated sites adjacent to or in the vicinity of the treated sites. The untreated sites 

in each reference group should have geometric, traffic, and crash data for the same years as treated 

sites. Each reference group should be similar to its corresponding treatment group—particularly in 

terms of area type (e.g., urban or rural), geometric configuration (e.g., number of legs and number 

of through lanes), and annual average daily traffic (AADT)—except that these intersections were 

not treated during the study period. Based on previous experience in similar analyses, the research 

team determined that at least 30 intersections for each intersection type in the reference group 

would be desirable, as shown in table 10. Where it is impractical or infeasible to obtain the required 

sample size for one or more intersection groups, it was possible to combine groups and account for 

the differences in the process of statistical model development. 

Table 10. Reference groups and desirable sample sizes.

Number of 

Legs

Number of Through Lanes on 

Major Road Rural Urban Total

3 Two-lane 30 30 60

3 Four-lane 30 30 60

4 Two-lane 30 30 60

4 Four-lane 30 30 60

Note: Sample included 120 urban sites and 120 rural sites for a total sample size of 240 sites.

21

CHAPTER 5. METHODOLOGY

This study employed the EB methodology for observational before–after studies. The EB method 

is considered rigorous in that it accounts for RTM bias using a reference group of similar but 

untreated sites. As a result, safety performance functions (SPFs) were developed and used in this 

study for the following reasons:

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

between the before and after periods.

• They can account for time trends. (The final SPFs did not use yearly indicator variables to 

account for time trend; more detailed discussions are provided in the section Before–After 

Adjustment Factors in chapter 7.)

• They reduce the level of uncertainty in the estimates of countermeasure effectiveness.

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

amalgamating data and results from diverse jurisdictions.

• They provide 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, which relate 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 application of the EB procedure, a 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 

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

shown in figure 3. 

∆Safety = λ − π
22

Figure 3. Equation. EB estimate of expected crashes.

Where w, the EB weight, is estimated from the mean and variance of the SPF estimate, as shown 

in figure 4.

Figure 4. Equation. EB weight.

Where k is the 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 treatment sites in each 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 treatment group. 

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

Figure 5. Equation. Index of effectiveness.

The standard deviation of is given in figure 6.

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.

m = w(P)+(1-w)(x)

w =

1

1+ kP

















2

( )

1

sum

sum

sum

sum

Var











2

2

2 2

2

( )

1

( ) ( )

( )







































sum

sum

sum

sum

sum

sum

Var

Var Var

StDev
















23

CHAPTER 6. DATA COLLECTION 

SCDOT provided the majority of data for this study. The dataset included the following data 

elements:

• Installation data: SCDOT provided information related to treatment sites and start and 

completion dates for each improvement. SCDOT also provided the research team with 

work orders, drawings, and sketches for these locations. The research team used some of 

these additional data to verify the intersection configurations.

• Reference site data: SCDOT provided a list of intersections that had not been treated. 

The research team used these intersections as potential reference sites.

• Traffic data: SCDOT provided a statewide AADT data file for 2014. This file had 

information for almost all mainline routes and cross streets. The research team obtained 

additional AADT files for 2006 to 2014 from SCDOT’s website. These publicly available 

files did not have all the details necessary. Many AADT for minor routes were missing.

The research team used these files to calculate traffic growth factors and estimated 

AADTs for other years.

• Crash data: SCDOT provided the research team with crash data files for 2005 to 2014.

The research team collected additional data using Google® EarthTM and Google® MapsTM.

INSTALLATION DATA

SCDOT administered the low-cost intersection improvements through the following two 

centralized contracts:

• All ground-level improvements, including all signing and pavement marking installations.

• All traffic signal improvements.

SCDOT provided the data in two separate files. The first included all installation data for the 

ground-level contract (918 locations). The other provided information on the work from the 

signal contract (158 locations). The improvements at signalized intersections included both 

ground-level and signal work. For this reason, the installation period often spanned 3 calendar 

years (e.g., the ground-level work began and was completed in 2010, and the signal work began 

later and was completed in 2012).

SCDOT assigned each intersection a unique identification number, where the location is

determined by the following:

• County.

• Mainline route designation and number (e.g., US 25).

• Crossing route designation and number (e.g., SC 12).
24

The data files also included the following other related information:

• Work order number.

• Start and completion dates for each task (e.g., sign installation and signal installation)

showing proposed signs and general intersection layout.

In addition to these two data files for ground-level and signal contracts, SCDOT also provided the 

research team with work orders and work plans from both ground-level and signal contracts. Each 

work plan provided a sketch of the intersection (including number of legs, lane configuration, and 

general geometric layout), as well as notes on proposed signs, markings, and other improvements.

Because the ground-level contract covered both the improvements at signalized and stopcontrolled intersections, the first step was to separate signalized intersections from the file. Key 

pieces of information from this data file included county, route designations and numbers for both 

mainline and cross street (e.g., US 25, SC 12), start and completion dates of installation, number 

of lanes (e.g., two or four lanes on the mainlines, two lanes on the cross street), and area type 

(i.e., urban, rural).

The list with confirmed installation dates for signal improvements included 158 intersections. The 

research team merged the list from the ground-level contract with the signal contract to create a 

list of candidate treatment sites for signalized intersections.

The first step of processing the data was to convert route designation and number for both 

mainline and cross street into the following three identification codes:

• Route type code (i.e., US = 2, SC = 4, and S = 7).

• Route number.

• Route auxiliary (i.e., Mainline = 0, Alternate = 2, Business = 7).

These three identifiers would later be used to link the crash and traffic data files to each 

intersection. Once crash and traffic data were linked to each intersection, the research team 

summarized the number of crashes per year by type for each location.

The start and completion dates allowed the team to identify the before and after periods. Beforeand after-periods included complete calendar years during which there was no installation activity. 

For example, if the work at a given intersection started at the ground level in December 2009 and 

the signal work was not completed until January 2011, 3 full calendar years (2009, 2010, and 2011) 

were considered installation years and removed from the dataset. The before period in this research 

was from 2005 (the first year of available data) to 2008 (the last full year of no construction 

activity). Similarly, the after period was from 2012 to 2014 (the last year of available data).

The result of this process was a list of signalized intersections with location identifiers in a 

uniform format across different data files. This list included the before and after periods. These 

intersections were candidates for the treatment group used in the EB evaluation. The team 

checked all work orders and work plans to collect the number of legs and verify the number of 

lanes for each intersection. Figure 7 shows an example of this process with the four-legged 

intersection between S-2 and S-65 in Spartanburg County, collected from the work plan for Work 
25

Order #4. The research team performed a manual process of verifying candidate intersections in 

Google® EarthTM (i.e., visual verification) to select the final treatment group.

©SCDOT

Figure 7. Illustration. Collecting the number of legs and verifying the number of lanes from 

the work plan.

Following SCDOT’s advice, the research team decided to exclude all intersections in Beaufort 

County because there were some changes in route names and numbers in this county that could 

have led to inaccurate matching of traffic and crash data.

REFERENCE SITES

SCDOT provided the research team with a list of more than 3,000 intersections—both stopcontrolled and signalized—for reference sites. Similar to the installation data, this list of 

intersections included key location identifiers (e.g., county, route designations, and numbers) and 

intersection characteristics (e.g., number of lanes on the mainline and cross street, area type, and 

type of traffic control). Therefore, the research team followed similar steps to process the raw 

data. The route identifiers were converted to a common format to link crash and traffic data for 
26

each intersection from different files. However, this data file did not provide the number of legs, a 

key variable for these potential reference sites.

The research team decided to collect the number of legs using Google® EarthTM and Google®

MapsTM. It was infeasible to locate and collect the number of legs from Google® EarthTM for 

every intersection because of resource constraints. Instead, the team randomly sampled at least 

30 intersections for each group from the pool of candidate reference sites and took the 

following steps:

• Step 1: Separate the pool of signalized intersections into different categories using the 

available information (e.g., rural intersections with two lanes on the mainline and two 

lanes on the cross street, urban intersections with four lanes on the mainline).

• Step 2: Randomize the order in each intersection category using a random number 

generator.

• Step 3: Start from the top of the list for each category, locate the intersection in Google®

MapsTM and Google® EarthTM, determine the number of legs, and verify the number of 

lanes.

The research team repeated these steps until there were at least 30 sites for each group (e.g., threelegged, rural intersections with two mainline lanes and two cross-street lanes). In two cases, the 

total number of intersections in the candidate pools was approximately 30 sites. (The rural 2 x

2 category had 28 candidate locations, and the rural 4 x 2 category had 33 candidate locations.) In 

these two cases, the research team reviewed all sites and retained as many as possible. 

TRAFFIC DATA

SCDOT provided the research team with a statewide traffic volume data file for 2014. The 

research team merged data in this file with both candidate treatment and reference sites. This data 

file had more details with AADT information for both mainline and cross streets for most 

intersections. The research team made a request, but SCDOT was not able to provide similar data 

for other years. With SCDOT’s advice, the research team downloaded AADT files for 2006 to 

2014 publicly available on SCDOT’s website. However, these data files were much less detailed 

than the 2014 file the team received from SCDOT staff. AADT information from these files was 

not available for many mainline streets at the studied intersections and at a majority of 

intersection cross streets. The research team used these data files to create growth factors by 

county. The research team used the growth factors and the detailed 2014 data file to estimate 

AADT information for 2006 to 2013. AADT for 2005 was not available from SCDOT’s website,

so the team extrapolated 2005 AADT based on data for 2006 to 2008. If AADT for either the 

mainline or cross street was still missing after the data processing, the team dropped that 

intersection from the pools of treatment or reference sites.
27

CRASH DATA

SCDOT provided 10 years of crash data (2005 to 2014). A unique accident number (i.e., an 

“ano”) identifies each crash in the data files. A combination of the following variables was used 

to identify the location of each crash:

• County number (e.g., 1 = Abbeville, 2 = Aiken, 3 = Allendale).

• Route type code (e.g., 2 = US, 4 = SC, and 7 = S).

• Route number.

• Route auxiliary (e.g., 0 = Mainline, 2 = Alternate, 7 = Business).

• Crossing route type code (2 = US, 4 = SC, and 7 = S).

• Crossing route number.

• Crossing route auxiliary (e.g., 0 = Mainline, 2 = Alternate, 7 = Business).

• Base distance offset from the intersection (e.g., 1 = 0.01 mi, 5 = 0.05 mi, 10 = 0.1 mi).

Note that the team used crossing route, in this context, as a reference point, and the offset 

determined the distance from that reference point to the crash location. Route and crossing route,

in crash data files, did not necessarily mean the mainline and minor routes in the same context of 

an intersection. The route indicated the roadway on which the crash occurred, and the crossing 

route indicated the crossing street at the nearest intersection (reference point). Both could be the 

mainline or the minor roads of the intersection used as the reference point.

The research team screened crash location information to identify and count crashes at each 

intersection. The crash data files did not provide a specific code to determine intersection-related

crashes. Therefore, the process of locating and counting crashes at each intersection relied solely 

on crash location. As recommended by SCDOT staff, the research team considered a crash 

intersection-related and counted it toward the number of crashes at an intersection if the location 

information indicated the crash along a particular mainline or cross street occurred within 0.05 mi 

(264 ft) of the intersection.

The research team used the number of fatalities and injuries coded for each crash to determine 

crash severity. The manner of collision determined rear-end and right-angle crashes. Light 

condition information was also available and identified nighttime crashes.

Table 11 presents the crash type definitions for South Carolina crash data.
28

Table 11. Definitions of crash types.

Total

Fatal and

Injury Rear-End Right-Angle Nighttime

Crashes of 

all types 

and 

severity 

levels

One of the 

following 

conditions:

• At least one 

fatality 

(fat ≥ 1)

• At least one 

injury (inj ≥ 1)

Manner of 

collision 

coded as 

“rear-end” 

(rims_mac =

10)

Manner of collision coded 

as “Angle 1” (rims_mac =

41), “Angle 2” (rims_mac =

42), or “Angle 3” 

(rims_mac = 43)

Light condition 

coded as 

anything other 

than 

“Daylight” (alc

= 1)

TREATMENT COST DATA

SCDOT provided actual construction cost data for improvements at signalized intersections. 

Intersection construction costs were separated into subtotal pavement marking, signing, signal, 

and pedestrian-related treatment costs. Each intersection received a package of those treatments 

appropriate for implementation at the site from the list of potential treatments. The treatment 

costs varied at each intersection based on the unique package of treatments it received. Table 12

summarizes the marking and signing costs.

Table 12. Treatment cost summary.

Statistic Pavement Marking Signing Total

Minimum $384.34 $382.58 $552.45

Average $4,696.33 $2,753.60 $6,958.70

Maximum $19,042.12 $19,498.44 $29,740.51

Note that some intersections only received pavement marking or signing improvements, but all 

intersections had at least some treatments installed. 

The signal improvements included upgrades such as installing retroreflective backplates, adding

signal heads, and replacing signal heads. Some intersections also had pedestrian signals installed. 

These improvements were in addition to the marking and signing countermeasures. Costs for 

signal upgrades were provided as a typical cost quote. A typical signal head replacement, which 

included replacing eight signal heads, adding retroreflective borders on backplates, installing new 

electric service hardware, and installing overhead signs on the span wires, was quoted at $13,300. 

Pedestrian signal heads and poles with push-button assemblies were estimated at $18,300 per 

intersection with a $1,680 present value maintenance cost to change lamps every 3.5 years. 

Americans with Disabilities Act–compliant curb ramps were estimated at $2,500 each.

Maintenance costs were dependent on the countermeasures installed at a given intersection. 

Without a record of the countermeasures installed at each intersection, it was difficult to estimate 

maintenance costs and service life. In addition, preliminary engineering (PE) costs were not 

supplied by SCDOT. PE costs often represent 10 to 30 percent of the total project costs for 

systemic projects.
29

DATA CHARACTERISTICS AND SUMMARY

Table 13 and table 14 provide summary information for the data collected for the treatment and 

reference sites, respectively. The information in table 13 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 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 13. Summary of treatment sites.

Data Element

Before 

Period

After 

Period

Number of sites 84 84

Three-legged, two lanes on the mainline and two lanes on the cross street 5 5

Four-legged, two lanes on the mainline and two lanes on the cross street 15 15

Three-legged, four lanes on the mainline and two lanes on the cross street 9 9

Four-legged, four lanes on the mainline and two lanes on the cross street 55 55

Number of site-years 411 209

Total crashes 5,132 2,675

Fatal and injury crashes 1,278 551

Right-angle crashes 2,061 921

Rear-end crashes 2,136 1,349

Nighttime crashes 1,263 581

Maximum mainline AADT 39,037 41,100

Average mainline AADT 20,668 20,447

Minimum mainline AADT 4,272 4,270

Maximum minor road AADT 18,970 20,000

Average minor road AADT 5,972 5,864

Minimum minor road AADT 115 111

Table 14. Summary of reference sites.

Data Element Value

Number of sites 368

Number of site-years 3,680

Total crashes 11,365

Fatal and injury crashes 3,457

Right-angle crashes 4,538

Rear-end crashes 4,102

Nighttime crashes 2,473

Maximum mainline AADT 48,308

Average mainline AADT 12,146

Minimum mainline AADT 1,563

Maximum minor road AADT 18,893

Average minor road AADT 3,393

Minimum minor road AADT 113

31

CHAPTER 7. DEVELOPMENT OF SPFs

This section presents the SPFs developed for each crash type. The SPFs support the use of the EB 

methodology to estimate the safety effectiveness of a strategy.

(18) The research team estimated

negative binomial regression models to predict the number of crashes. In specifying a negative 

binomial error structure, the dispersion parameter, k, was estimated iteratively from the model 

and the data. For a given dataset, smaller values of k indicate relatively better models. The 

research team developed one SPF for each of the following intersection configurations:

• 3 x 22: Three-legged intersections with two lanes on the mainline and two lanes on the 

cross street.

• 4 x 22: Four-legged intersections with two lanes on the mainline and two lanes on the 

cross street.

• 3 x 42: Three-legged intersections with four lanes on the mainline and two lanes on the 

cross street.

• 4 x 42: Four-legged intersections with four lanes on the mainline and two lanes on the 

cross street.

The research team developed correlation matrices for variables and used them as a guide for the 

SPF development process, which helped avoid highly correlated variables in the models. The 

model development followed a process of forward selection for selecting variables with the best 

fit. The team started with mainline and cross-street traffic volumes and their variants (e.g., natural 

logarithm and ratio of cross-street AADT to mainline AADT). Other candidate explanatory 

variables were then added, one by one, to the model. The model was reestimated, and the 

goodness of fit was reevaluated with each variable addition.

The research team initially included annual adjustment variables (i.e., indicators for years 2005 to 

2014) in the SPFs during the first iteration of model development. However, most of these 

variables did not result in statistically significant parameters or help improve the fit of the SPFs. 

The inclusion of annual adjustment variables also led to heavily underpredicted crashes for some 

years (i.e., small coefficients on the negative side and far from being well fit), especially for the 

later years that covered the after period. The team eventually decided to drop these annual 

adjustment variables from the models and considered another approach to account for the annual 

trend (discussed later in this chapter).

In some cases, the research team could not develop an adequate model for a specific crash type. 

In these cases, the team used the SPF for total crashes and adjusted by the proportion of the 

number of crashes for the given crash type in total crashes.

The definition of variables included in the final SPFs are as follows:

• Totalaxbc: The predicted number of total crashes (all types and severity levels) for an 

intersection with a legs, b lanes on the mainline, and c lanes on the cross street (e.g., 3 x
32

42 for three-legged intersections with four lanes on the mainline and two lanes on the 

cross street).

• FIaxbc: The predicted number of fatal and injury crashes for an intersection with a legs, 

b lanes on the mainline, and c lanes on the cross street.

• Rear-Endaxbc: The predicted number of rear-end crashes for an intersection with a legs, 

b lanes on the mainline, and c lanes on the cross street.

• Right-Angleaxbc: The predicted number of right-angle crashes for an intersection with 

a legs, b lanes on the mainline, and c lanes on the cross street.

• Nightaxbc: The predicted number of nighttime crashes for an intersection with a legs, 

b lanes on the mainline, and c lanes on the cross street.

• aadt: ml_aadt + xst_aadt, total traffic of intersection (vehicles/day).

• ml_aadt: AADT on the mainline (vehicles/day).

• xst_aadt: AADT on the cross street (vehicles/day).

• ratio1: ln(xst_aadt)/ln(ml_aadt), with ln(xst_aadt) being the natural logarithm of AADT 

on the cross street and ln(ml_aadt) being the natural logarithm of AADT on the mainline.

• ratio2: xst_aadt/ml_aadt, with xst_aadt and ml_aadt being the AADT on cross street and 

mainline, respectively.

• ratio3: xst_aadt/(xst_aadt + ml_aadt), with xst_aadt and ml_aadt being the AADT on the

cross street and mainline, respectively.

• ratio4: ln(xst_aadt)/ln(xst_aadt + ml_aadt), with ln(xst_aadt) being the natural logarithm 

of AADT on the cross street and ln(xst_aadt + ml_aadt) being the natural logarithm of 

total traffic at the intersection.

• Urban: Urban/rural indicator for the intersection (1 for urban, 0 otherwise).

• 1, 2, 3, 4: Parameters estimated in the SPF development process using maximum 

likelihood method.

• k: Overdispersion parameter.

SPFs FOR 3 X 22 INTERSECTIONS

The SPF for total crashes at three-legged intersections with two lanes on the mainline and two 

lanes on the cross street has the form shown in figure 8.

β β β β
33

Figure 8. Equation. Total crash SPF for 3 x 22 intersections.

Table 15 presents the total crash SPF parameters for three-legged intersections with two lanes on 

the mainline and two lanes on the cross street.

Table 15. SPF parameters for total crashes at 3 x 22 intersections.

Parameter Description Estimated Value Standard Error

1 Coefficient for mainline AADT 0.372 0.111

2 Coefficient for cross-street AADT 0.141 0.053

3 Coefficient for urban/rural indicator 0.304 0.149

4 Intercept term −3.887 0.986

k Overdispersion parameter 0.166 0.042

The research team could not develop a statistically significant model for fatal and injury crashes. 

The SPF for total crashes was used with an adjustment factor to predict fatal and injury crashes 

for three-legged intersections with two lanes on the mainline and two lanes on cross street.

The SPF for rear-end crashes at three-legged intersections with two lanes on the mainline and two 

lanes on the cross street has the form shown in figure 9.

Figure 9. Equation. Rear-end crash SPF for 3 x 22 intersections.

Table 16 presents the rear-end crash SPF parameters for three-legged intersections with two lanes 

on the mainline and two lanes on the cross street.

Table 16. SPF parameters for rear-end crashes at 3 x 22 intersections.

Parameter Description Estimated Value Standard Error

1 Coefficient for mainline AADT 0.687 0.157

3 Coefficient for urban/rural indicator 0.265 0.208

4 Intercept term −6.403 1.409

k Overdispersion parameter 0.367 0.096

The SPF for right-angle crashes at three-legged intersections with two lanes on the mainline and 

two lanes on the cross street has the form shown in figure 10.

Figure 10. Equation. Right-angle crash SPF for 3 x 22 intersections.

Table 17 presents the right-angle crash SPF parameters for three-legged intersections with two 

lanes on the mainline and two lanes on the cross street.

Total3×22 = ml_aadt

β1 × xst_aadtβ2 × e

 β3×urban +β4 

β

β

β

β

Rear End3×22 = ml_aadt

β1 × e

 β3×urban +β4

 

β

β

β

Right Angle3×22 = ml_aadt

β1 × e

 β2×ratio 4+β3×urban +β4

 
34

Table 17. SPF parameters for right-angle crashes at 3 x 22 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1 Coefficient for mainline AADT 0.512 0.197

2

Coefficient for 

ratio4 = ln(xst_aadt)/ln(xst_aadt + ml_aadt)

3.898 1.249

3 Coefficient for urban/rural indicator 0.476 0.291

4 Intercept term −8.863 2.198

k Overdispersion parameter 0.198 0.123

The SPF for nighttime crashes at three-legged intersections with two lanes on the mainline and 

two lanes on the cross street has the form shown in figure 11.

Figure 11. Equation. Nighttime crash SPF for 3 x 22 intersections.

Table 18 presents the nighttime crash SPF parameters for three-legged intersections with two lanes 

on the mainline and two lanes on the cross street.

Table 18. SPF parameters for nighttime crashes at 3 x 22 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1 Coefficient for mainline AADT 0.675 0.194

2

Coefficient for 

ratio4 = ln(xst_aadt)/ln(xst_aadt + ml_aadt)

4.355 1.340

4 Intercept term −10.501 2.272

k Overdispersion parameter 0.215 0.157

SPFs FOR 4 X 22 INTERSECTIONS

The SPF for total crashes at four-legged intersections with two lanes on the mainline and two lanes 

on the cross street has the form shown in figure 12.

Figure 12. Equation. Total crash SPF for 4 x 22 intersections.

Table 19 presents the total crash SPF parameters for four-legged intersections with two lanes on 

the mainline and two lanes on the cross street.

β

β

β

β

Night3×22 = ml_aadt

β1 × e

 β2×ratio 4+β4 

β

β

β

Total4×22 = ml_aadt

β1 × xst_aadtβ2 × e

 β3×urban +β4 
35

Table 19. SPF parameters for total crashes at 4 x 22 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1 Coefficient for mainline AADT 0.556 0.049

2 Coefficient for cross-street AADT 0.168 0.032

3 Coefficient for urban/rural indicator −0.154 0.068

4 Intercept term −5.301 0.497

k Overdispersion parameter 0.234 0.029

The SPF for fatal and injury crashes at four-legged intersections with two lanes on the mainline 

and two lanes on the cross street has the form shown in figure 13.

Figure 13. Equation. Fatal and injury crash SPF for 4 x 22 intersections.

Table 20 presents the fatal and injury crash SPF parameters for four-legged intersections with two 

lanes on the mainline and two lanes on the cross street.

Table 20. SPF parameters for fatal and injury crashes at 4 x 22 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1 Coefficient for total intersection AADT 0.242 0.091

2

Coefficient for 

ratio4 = ln(xst_aadt)/ln(xst_aadt + ml_aadt)

1.197 0.584

4 Intercept term −3.683 1.015

k Overdispersion parameter 0.242 0.081

The SPF for rear-end crashes at four-legged intersections with two lanes on the mainline and two 

lanes on the cross street has the form shown in figure 14.

Figure 14. Equation. Rear-end crash SPF for 4 x 22 intersections.

Table 21 presents the rear-end crash SPF parameters for four-legged intersections with two lanes 

on the mainline and two lanes on the cross street.

β

β

β

β

FI4×22 = aadt

β1 × e

 β2×ratio 4+β4 

β

β

β

Rear End4×22 = ml_aadt

β1 × xst_aadtβ2 × e

 β3×urban +β4

 
36

Table 21. SPF parameters for rear-end crashes at 4 x 22 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1 Coefficient for mainline AADT 0.855 0.085

2 Coefficient for cross-street AADT 0.242 0.054

3 Coefficient for urban/rural indicator −0.157 0.116

4 Intercept term −9.583 0.846

k Overdispersion parameter 0.629 0.089

The SPF for right-angle crashes at four-legged intersections with two lanes on the mainline and 

two lanes on the cross street has the form shown in figure 15.

Figure 15. Equation. Right-angle crash SPF for 4 x 22 intersections.

Table 22 presents the right-angle crash SPF parameters for four-legged intersections with two lanes 

on the mainline and two lanes on the cross street.

Table 22. SPF parameters for right-angle crashes at 4 x 22 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1 Coefficient for mainline AADT 0.385 0.071

2 Coefficient for cross-street AADT 0.153 0.048

3 Coefficient for urban/rural indicator −0.154 0.097

4 Intercept term −4.547 0.719

k Overdispersion parameter 0.357 0.064

The SPF for nighttime crashes at four-legged intersections with two lanes on the mainline and 

two lanes on the cross street has the form shown in figure 16.

Figure 16. Equation. Nighttime crash SPF for 4 x 22 intersections.

Table 23 presents the nighttime crash SPF parameters for four-legged intersections with two lanes 

on the mainline and two lanes on the cross street.

β

β

β

β

Right Angle4×22 = ml_aadt

β1 × xst_aadtβ2 × e

 β3×urban +β4 

β

β

β

β

Night4×22 = ml_aadt

β1 × xst_aadt

β2 × eβ4
37

Table 23. SPF parameters for nighttime crashes at 4 x 22 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

β1 Coefficient for mainline AADT 0.541 0.091

β2 Coefficient for cross-street AADT 0.117 0.060

β4 Intercept term −6.417 0.925

k Overdispersion parameter 0.364 0.109

SPFs FOR 3 X 42 INTERSECTIONS

The SPF for total crashes at three-legged intersections with four lanes on the mainline and two lanes 

on the cross street has the form shown in figure 17.

Figure 17. Equation. Total crash SPF for 3 x 42 intersections.

Table 24 presents the total crash SPF parameters for three-legged intersections with four lanes on 

the mainline and two lanes on the cross street.

Table 24. SPF parameters for total crashes at 3 x 42 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1

Coefficient for total intersection AADT 

(= mainline AADT + cross-street AADT) 0.463 0.123

2

Coefficient for 

ratio3 = xst_aadt/(xst_aadt + ml_aadt)

1.550 0.287

3 Coefficient for urban/rural indicator −0.243 0.116

4 Intercept −3.491 1.215

k Overdispersion parameter 0.202 0.035

The SPF for fatal & injury crashes at three-legged intersections with four lanes on the mainline 

and two lanes on the cross street has the form shown in figure 18.

Figure 18. Equation. Fatal & injury crash SPF for 3 x 42 intersections.

Table 25 presents the fatal and injury crash SPF parameters for three-legged intersections with 

four lanes on the mainline and two lanes on the cross street.

Total3×42 = aadt

β1 × e

 β2×ratio 3+β3×urban +β4 

β

β

β

β

FI3×42 = aadt

β1 × e

 β2×ratio 3+β3×urban +β4 
38

Table 25. SPF parameters for fatal and injury crashes at 3 x 42 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1

Coefficient for total intersection AADT 

(= mainline AADT + cross-street AADT) 0.350 0.195

2

Coefficient for 

ratio3 = xst_aadt/(xst_aadt + ml_aadt)

1.364 0.461

3 Coefficient for urban/rural indicator −0.293 0.180

4 Intercept term −3.534 1.929

k Overdispersion parameter 0.247 0.093

The SPF for rear-end crashes at three-legged intersections with four lanes on the mainline and 

two lanes on the cross street has the form shown in figure 19.

Figure 19. Equation. Rear-end crash SPF for 3 x 42 intersections. 

Table 26 presents the rear-end crash SPF parameters for three-legged intersections with four lanes 

on the mainline and two lanes on the cross street.

Table 26. SPF parameters for rear-end crashes at 3 x 42 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1

Coefficient for total intersection AADT 

(= mainline AADT + cross-street AADT) 0.563 0.164

2

Coefficient for 

ratio4 = ln(xst_aadt)/ln(xst_aadt + ml_aadt)

1.538 0.557

3 Coefficient for urban/rural indicator −0.453 0.154

4 Intercept term −6.156 1.770

k Overdispersion parameter 0.230 0.075

The SPF for right-angle crashes at three-legged intersections with four lanes on the mainline and 

two lanes on the cross street has the form shown in figure 20.

Figure 20. Equation. Right-angle crash SPF for 3 x 42 intersections.

Table 27 presents the right-angle crash SPF parameters for three-legged intersections with four lanes 

on the mainline and two lanes on the cross street.

β

β

β

β

Rear End3×42 = aadt

β1 × e

 β2×ratio 4+β3×urban +β4 

β

β

β

β

Right Angle3×42 = aadt

β1 × e

 β2×ratio 2+β4 
39

Table 27. SPF parameters for right-angle crashes at 3 x 42 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1

Coefficient for total intersection AADT 

(= mainline AADT + cross-street AADT) 0.283 0.201

2 Coefficient for ratio2 = xst_aadt/ml_aadt 1.049 0.254

4 Intercept term −2.970 1.998

k Overdispersion parameter 0.487 0.099

The SPF for nighttime crashes at three-legged intersections with four lanes on the mainline and 

two lanes on the cross street has the form shown in figure 21.

Figure 21. Equation. Nighttime crash SPF for 3 x 42 intersections. 

Table 28 presents the nighttime crash SPF parameters for three-legged intersections with four lanes 

on the mainline and two lanes on the cross street.

Table 28. SPF parameters for nighttime crashes at 3 x 42 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1

Coefficient for total intersection AADT 

(= mainline AADT + cross-street AADT) 0.277 0.231

2

Coefficient for

ratio3 = ln(xst_aadt)/ln(xst_aadt + ml_aadt)

1.467 0.549

3 Coefficient for urban/rural indicator −0.340 0.209

4 Intercept term −3.080 2.293

k Overdispersion parameter 0.445 0.140

SPFs FOR 4 X 42 INTERSECTIONS

The SPF for total crashes at four-legged intersections with four lanes on the mainline and two 

lanes on the cross street has the form shown in figure 22.

Figure 22. Equation. Total crash SPF for 4 x 42 intersections.

Table 29 presents the total crash SPF parameters for four-legged intersections with four lanes on 

the mainline and two lanes on the cross street.

β

β

β

Night3×42 = aadt

β1 × e

 β2×ratio 3+β3×urban +β4 

β

β

β

β

Total4×42 = aadt

β1 × e

 β2×ratio 2+β3×urban +β4 
40

Table 29. SPF parameters for total crashes at 4 x 42 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1

Coefficient for total intersection AADT 

(= mainline AADT + cross street-AADT) 0.932 0.043

2 Coefficient for ratio2 = xst_aadt/ml_aadt 0.822 0.089

3 Coefficient for urban/rural indicator −0.169 0.067

4 Intercept term −7.880 0.409

k Overdispersion parameter 0.345 0.022

The SPF for fatal and injury crashes at four-legged intersections with four lanes on the mainline 

and two lanes on the cross street has the form shown in figure 23.

Figure 23. Equation. Fatal and injury crash SPF for 4 x 42 intersections.

Table 30 presents the fatal and injury crash SPF parameters for four-legged intersections with 

four lanes on the mainline and two lanes on the cross street.

Table 30. SPF parameters for fatal and injury crashes at 4 x 42 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1

Coefficient for total intersection AADT 

(= mainline AADT + cross-street AADT) 0.892 0.063

2 Coefficient for ratio2 = xst_aadt/ml_aadt 1.112 0.128

3 Coefficient for urban/rural indicator −0.235 0.099

4 Intercept term −8.641 0.599

k Overdispersion parameter 0.471 0.049

The SPF for rear-end crashes at four-legged intersections with four lanes on the mainline and two 

lanes on the cross street has the form shown in figure 24.

Figure 24. Equation. Rear-end crash SPF for 4 x 42 intersections. 

Table 31 presents the rear-end crash SPF parameters for four-legged intersections with four lanes 

on the mainline and two lanes on the cross street.

β

β

β

β

FI4×42 = aadt

β1 × e

 β2×ratio 2+β3×urban +β4 

β

β

β

β

Rear End4×42 = aadt

β1 × e

 β2×ratio 2+β4 
41

Table 31. SPF parameters for rear-end crashes at 4 x 42 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1

Coefficient for total intersection AADT 

(= mainline AADT + cross-street AADT) 1.287 0.065

2 Coefficient for ratio2 = xst_aadt/ml_aadt 0.447 0.136

4 Intercept term −12.538 0.642

k Overdispersion parameter 0.578 0.054

The SPF for right-angle crashes at four-legged intersections with four lanes on the mainline and 

two lanes on the cross street has the form shown in figure 25.

Figure 25. Equation. Right-angle crash SPF for 4 x 42 intersections.

Table 32 presents the right-angle crash SPF parameters for four-legged intersections with four lanes 

on the mainline and two lanes on the cross street.

Table 32. SPF parameters for right-angle crashes at 4 x 42 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1

Coefficient for total intersection AADT 

(= mainline AADT + cross-street AADT) 0.747 0.056

2 Coefficient for ratio2 = xst_aadt/ml_aadt 1.096 0.116

3 Coefficient for urban/rural indicator −0.242 0.088

4 Intercept term −6.933 0.535

k Overdispersion parameter 0.439 0.040

The SPF for nighttime crashes at four-legged intersections with four lanes on the mainline and 

two lanes on the cross street has the form shown in figure 26.

Figure 26. Equation. Nighttime crash SPF for 4 x 42 intersections.

Table 33 presents the nighttime crash SPF parameters for four-legged intersections with four lanes 

on the mainline and two lanes on the cross street.

β

β

β

Right Angle4×42 = aadt

β1 × e

 β2×ratio 2+β3×urban +β4 

β

β

β

β

Night4×42 = aadt

β1 × e

 β2×ratio 2+β3×urban +β4

 
42

Table 33. SPF parameters for nighttime crashes at 4 x 42 intersections.

Parameter Description

Estimated 

Value

Standard 

Error

1

Coefficient for total intersection AADT 

(= mainline AADT + cross-street AADT) 1.123 0.074

2

Coefficient for

ratio3 = xst_aadt/(xst_aadt + ml_aadt)

1.106 0.151

3 Coefficient for urban/rural indicator −0.211 0.123

4 Intercept term −11.341 0.712

k Overdispersion parameter 0.560 0.071

BEFORE–AFTER ADJUSTMENT FACTORS

SPFs often account for time trend, as discussed in the first section of chapter 5. In this study, 

however, the SPFs did not include yearly indicator variables because, after numerous attempts, the 

research team could not achieve a reasonable level of statistical significance for these individual 

variables. The research team decided to account for the time trend by using a before–after 

adjustment factor. Instead of using one annual adjustment factor for each year, the research team 

made the decision to use adjustment factors to account for the difference (i.e., crash trend) between 

the before and after periods. These factors were calculated based on the observed and predicted 

crashes at the reference sites. Because SCDOT did not install the treatment at all sites in the same 

year, the installation periods varied. For this reason, the team calculated one adjustment factor for 

each installation period. The before–after adjustment factor was calculated as shown in figure 27.

Figure 27. Equation. Before–after adjustment factor calculation.

Where:

Adj_Factor = factor for adjusting the difference between the before and after period.

Obs_before = observed number of crashes at reference sites during before period.

Predbefore = predicted number of crashes at reference sites during before period (calculated by 

SPF).

Obs_after = observed number of crashes at reference sites during after period.

Predafter = predicted number of crashes at reference sites during after period (calculated by 

SPF).

Table 34 through table 38 present the before–after adjustment factors for each installation time 

frame and crash type.

β

β

β

β

Adj_Factor =

Obsafter

Predafter

Obsbefore

Predbefore
43

Table 34. Before–after adjustment factor for total crashes.

Installation 

Year(s)

Observed 

Crashes—

Before

Observed 

Crashes—

After

Predicted 

Crashes

—Before

Predicted 

Crashes—After

Adjustment 

Factor

2009–2011 4,542 3,619 4,560 3,389 1.072

2010–2011 5,672 3,619 5,679 3,389 1.069

2010–2012 5,672 2,519 5,679 2,251 1.120

2011 6,774 3,619 6,801 3,389 1.072

Table 35. Before–after adjustment factor for fatal and injury crashes.

Installation 

Year(s)

Observed 

Crashes—

Before

Observed 

Crashes—

After

Predicted 

Crashes—

Before

Predicted 

Crashes—After

Adjustment 

Factor

2009–2011 1,448 1,035 1,388 1,032 0.962

2010–2011 1,794 1,035 1,730 1,032 0.967

2010–2012 1,794 701 1,730 686 0.986

2011 2,149 1,035 2,072 1,032 0.967

Table 36. Before–after adjustment factor for rear-end crashes.

Installation 

Year(s)

Observed 

Crashes—

Before

Observed 

Crashes—

After

Predicted 

Crashes—

Before

Predicted 

Crashes—After

Adjustment 

Factor

2009–2011 1,534 1,431 1,652 1,225 1.259

2010–2011 1,895 1,431 2,055 1,225 1.267

2010–2012 1,895 1,000 2,055 812 1.335

2011 2,260 1,431 2,459 1,225 1.271

Table 37. Before–after adjustment factor for right-angle crashes.

Installation 

Year(s)

Observed 

Crashes—

Before

Observed 

Crashes—

After

Predicted 

Crashes—

Before

Predicted 

Crashes—After

Adjustment 

Factor

2009–2011 1,928 1,397 1,819 1,354 0.973

2010–2011 2,355 1,397 2,267 1,354 0.993

2010–2012 2,355 979 2,267 900 1.047

2011 2,787 1,397 2,717 1,354 1.006

Table 38. Before–after adjustment factor for nighttime crashes.

Installation 

Year(s)

Observed 

Crashes—

Before

Observed 

Crashes—

After

Predicted 

Crashes—

Before

Predicted 

Crashes—After

Adjustment 

Factor

2009–2011 1,034 741 992 736 0.966

2010–2011 1,313 741 1,235 736 0.947

2010–2012 1,313 507 1,235 489 0.976

2011 1,535 741 1,479 736 0.970

45

CHAPTER 8. BEFORE–AFTER EVALUATION RESULTS

This chapter presents the evaluation results, both the aggregate results for all intersections and the 

results disaggregated by area type, intersection configuration, level of traffic, and the expected 

number of crashes in the before period.

AGGREGATE ANALYSIS

Table 39 provides the estimates of expected crashes in the after period without treatment, the 

observed crashes in the after period, and the estimated crash modification factor (CMF) and its 

standard error for each crash type considered in this study. The results in table 39 indicate 

reductions for all crash types analyzed in this study. The CMFs for fatal and injury and rightangle crashes were 0.893 and 0.883, respectively, which were statistically significant at the 95-

percent confidence level. The CMFs for total, rear-end, and nighttime crashes were 0.955, 0.974,

and 0.969, respectively. The CMFs for total, rear-end, and nighttime crashes were not statistically 

significant at the 95-percent confidence level, but the CMF for total crashes was statistically 

significant at the 90-percent confidence level.

Table 39. Aggregate results for EB before-after study.

Statistic Total

Fatal 

and

Injury Rear-End Right-Angle Nighttime

EB estimate of crashes expected 

in the after period without the 

systemic improvement

2,801 617 1,385 1,042 599

Count of crashes observed in the 

after period 2,675 551 1,349 921 581

Estimated CMF 0.955 0.893 0.974 0.883 0.969

Standard error of the estimated 

CMF 0.023 0.045 0.034 0.035 0.048

Note: Boldface indicates statistically significant results at the 95-percent confidence level.

DISAGGREGATE ANALYSIS

The objective of disaggregate analyses was to identify specific CMFs by crash type and different 

conditions. The analysis also revealed those conditions under which the studied combination of 

multiple low-cost treatments were more effective. The research team identified several variables 

of interest, including area type (urban or rural), number of legs (three or four), lane configuration 

of the mainline and the cross street, and traffic volumes. All of these variables are likely 

correlated, and caution should be exercised in interpreting and applying the disaggregate analysis 

results.

Table 40 presents the disaggregate results by area type (urban or rural), indicating the sample size 

(number of sites), CMF, and standard error of the CMF (in parentheses) by group for each crash 

type considered in this study. For the 58 urban intersections, the results in table 40 indicate 
46

reductions in all crash types analyzed in this study. Specifically, the CMFs for total, fatal and

injury, and right-angle crashes were 0.949, 0.854, and 0.884, respectively, and statistically 

significant at the 95-percent confidence level. The CMFs for rear-end and nighttime crashes were 

0.960 and 0.974, respectively, and were not statistically significant at the 95-percent confidence 

level. For the 26 rural intersections, the results in table 40 indicate reductions for total, rightangle, and nighttime crashes. The results indicate increases in fatal and injury and rear-end 

crashes. Note the results for rural intersections were not statistically significant at the 95-percent 

confidence level. Based on the disaggregate analysis by area type, it appears the combination of 

these multiple treatments was more effective at urban intersections than rural intersections.

However, as noted above, this effect may be due to other correlated variables.

Table 40. Disaggregate results by area type.

Statistic Urban Rural

Number of intersections 58 26

Total crash CMF (standard error) 0.949 (0.025) 0.983 (0.058)

Fatal and injury crash CMF (standard error) 0.854 (0.048) 1.054 (0.113)

Rear-end CMF (standard error) 0.960 (0.036) 1.060 (0.097)

Right-angle CMF (standard error) 0.884 (0.038) 0.878 (0.082)

Nighttime CMF (standard error) 0.974 (0.053) 0.938 (0.112)

Note: Boldface indicates statistically significant results at the 95-percent confidence level.

Table 41 presents the disaggregate results by number of legs, indicating the sample size (number 

of sites), CMF, and standard error of the CMF (in parentheses) by group for each crash type

considered in this study. For the 14 three-legged intersections, the results in table 41 indicate 

reductions in total, fatal and injury, rear-end, and right-angle crashes. The CMFs for total and

rear-end crashes were 0.860 and 0.822, respectively, and statistically significant at the 95-percent 

confidence level. The CMF for fatal and injury crashes was 0.810 and statistically significant at 

the 90-percent confidence level. The CMFs for right-angle and nighttime crashes were not 

statistically significant at the 95-percent confidence level. For the 70 four-legged intersections, 

the results in table 41 indicate reductions in total, fatal and injury, right angle, and nighttime 

crashes. The CMF for right-angle crashes was 0.881 and statistically significant at the 95-percent 

confidence level. The CMF for fatal and injury crashes was 0.908 and statistically significant at 

the 90-percent confidence level. The CMFs for total, rear-end, and nighttime crashes were not 

statistically significant at the 95-percent confidence level. Based on the disaggregate analysis by 

number of legs, it appears this strategy was slightly more effective at three-legged intersections 

than four-legged intersections for total, fatal and injury, and rear-end crashes. However, as noted 

above, this effect may be due to other correlated variables.
47

Table 41. Disaggregate results by number of legs.

Statistic Three-Legged Four-Legged

Number of intersections 14 70

Total crash CMF (standard error) 0.860 (0.053) 0.972 (0.025)

Fatal and injury crash CMF (standard error) 0.810 (0.103) 0.908 (0.049)

Rear-end CMF (standard error) 0.822 (0.075) 1.000 (0.037)

Right-angle CMF (standard error) 0.895 (0.091) 0.881 (0.037)

Nighttime CMF (standard error) 1.044 (0.126) 0.955 (0.052)

Note: Boldface indicates statistically significant results at the 95-percent confidence level.

Table 42 presents the disaggregate results by number of lanes, indicating the sample size (number 

of sites), CMF, and standard error of the CMF (in parentheses) by group for each crash type 

considered in this study. For the 20 intersections with two-lane major roads, the results in table 42

indicate reductions in all crash types analyzed in this study. The CMFs for total, fatal and injury, 

and right-angle crashes were 0.784, 0.756, and 0.654, respectively, and statistically significant at 

the 95-percent confidence level. For the 64 intersections with four-lane major roads, the results in 

table 42 indicate reductions in all crash types analyzed in this study. The CMF for right-angle 

crashes was 0.917 and statistically significant at the 95-percent confidence level. The CMF for 

fatal and injury crashes was 0.911 and statistically significant at the 90-percent confidence level. 

The CMFs for total, rear-end, and nighttime crashes were not statistically significant at the 95-

percent confidence level. Based on the disaggregate analysis by number of lanes, it appears this 

strategy was more effective at two-lane major road intersections than four-lane major road 

intersections. However, as noted above, this effect may be due to other correlated variables.

Table 42. Disaggregate results by number of lanes.

Statistic

Two Mainline Lanes 

and Two Cross-Street 

Lanes

Four Mainline Lanes 

and Two Cross-Street 

Lanes

Number of intersections 20 64

Total crash CMF (standard error) 0.784 (0.057) 0.978 (0.025)

Fatal and injury crash CMF (standard 

error) 0.756 (0.112) 0.911 (0.049)

Rear-end CMF (standard error) 0.866 (0.091) 0.988 (0.036)

Right-angle CMF (standard error) 0.654 (0.079) 0.917 (0.038)

Nighttime CMF (standard error) 0.884 (0.124) 0.980 (0.052)

Note: Boldface indicates statistically significant results at the 95-percent confidence level.

Table 43 presents the disaggregate results by number of legs and number of lanes, indicating the 

sample size (number of sites), CMF, and standard error of the CMF (in parentheses) by group for 

each crash type considered in this study. For the five three-legged intersections with two-lane 

major roads, none of the CMFs were statistically significant at the 95-percent confidence level.

For the nine three-legged intersections with four-lane major roads, the results in table 43 indicate 

reductions in total, fatal and injury, rear-end, and right-angle crashes. The CMFs for total and 

rear-end crashes were 0.846 and 0.829, respectively, and statistically significant at the 95-percent 

confidence level. The CMF for right-angle crashes was 0.833 and statistically significant at the 
48

90-percent confidence level. The CMFs for fatal and injury and nighttime crashes were not 

statistically significant at the 95-percent confidence level.

For the 15 four-legged intersections with two-lane major roads, the results in table 43 indicate 

reductions in all crash types analyzed in this study. The CMFs for total, fatal and injury, and 

right-angle crashes were 0.732, 0.746, and 0.509, respectively, and statistically significant at the 

95-percent confidence level. The CMFs for rear-end and nighttime crashes were not statistically 

significant at the 95-percent confidence level.

For the 55 four-legged intersections with four-lane major roads, the results in table 43 indicate 

reductions in total, fatal and injury, right-angle, and nighttime crashes. The CMF for right-angle 

crashes was 0.928 and statistically significant at the 90-percent confidence level. The CMFs for 

total, fatal and injury, rear-end, and nighttime crashes were not statistically significant at the 

95-percent confidence level.

The disaggregate analysis by number of legs and number of lanes indicates that this strategy was

effective for most combinations of legs and lanes. Based on the limited sample sizes for the 

individual categories, it appears the strategy was most effective at four-legged intersections with 

two-lane major roads. However, as noted above, this may be due to other correlated variables.

Table 43. Disaggregate results by number of legs and number of lanes.

Statistic

Three-Legged 

With Two 

Mainline 

Lanes and 

Two CrossStreet Lanes

Three-Legged 

With Four 

Mainline 

Lanes and 

Two CrossStreet Lanes

Four-Legged 

With Two 

Mainline 

Lanes and 

Two CrossStreet Lanes

Four-Legged 

With Four 

Mainline 

Lanes and 

Two CrossStreet Lanes

Number of intersections 5 9 15 55

Total crash CMF 

(standard error) 0.901 (0.113) 0.846 (0.060) 0.732 (0.065) 0.998 (0.027)

Fatal and injury crash 

CMF (standard error) 0.771 (0.202) 0.819 (0.118) 0.746 (0.132) 0.926 (0.053)

Rear-end CMF (standard 

error) 0.794 (0.148) 0.829 (0.086) 0.895 (0.114) 1.011 (0.039)

Right-angle CMF 

(standard error) 1.104 (0.217) 0.833 (0.099) 0.509 (0.078) 0.928 (0.041)

Nighttime CMF 

(standard error) 0.809 (0.203) 1.128 (0.156) 0.914 (0.154) 0.958 (0.055)

Note: Boldface indicates Statistically significant results at the 95-percent confidence level.

Figure 28 and figure 29 show the individual CMFs for total crashes and fatal and injury crashes, 

respectively, for each treatment site compared with the total entering traffic volume associated 

with the intersection. The linear trend line suggests the multiple low-cost treatments were slightly 

more effective on average for intersections with lower traffic volumes, and the effectiveness 

decreased (i.e., CMF increased) as traffic volume increased. The trend was more apparent for 

total crashes and nearly flat for fatal and injury crashes. The CMF in figure 28 appears to cross 

1.0 when AADT is about 35,000. This suggests that the reduction potential for total crashes was 
49

better for intersections with total entering AADT under 35,000. Similarly, figure 29 suggests that 

the reduction potential for fatal and injury crashes was better for locations with total entering 

AADT under 50,000. Again, the perceived relationship may also be due to correlations with other 

variables.

Source: FHWA.

Figure 28. Chart. The relationship between CMF (total crashes) and total intersection 

AADT (2014).

Source: FHWA.

Figure 29. Chart. The relationship between CMF (fatal and injury crashes) and total 

intersection AADT (2014).

Figure 30 and figure 31 show the individual CMFs for total crashes and fatal and injury crashes, 

respectively, for each treatment site compared with the expected crashes per year in the before 
50

period. The linear trend line is nearly flat in both cases, suggesting the multiple low-cost 

treatments were approximately equally effective on average for intersections across the range of 

expected crashes per year in the before period. This confirms the need for caution when 

interpreting the results of the univariate analyses. Specifically, the net effect of the multiple 

correlations among variables investigated is a negligible effect on the expected number of 

crashes, which collectively captures the effects of those variables.

Source: FHWA.

Figure 30. Chart. The relationship between CMF (total crashes) and expected total crashes 

during the before period.

Source. FHWA.

Figure 31. Chart. The relationship between CMF (fatal and injury crashes) and expected 

fatal and injury crashes during the before period.
51

CHAPTER 9. ECONOMIC ANALYSIS

The research team conducted an economic analysis to estimate the B/C ratio for implementing 

various pavement marking, signing, signal, and pedestrian improvements at signalized 

intersections. The statistically significant aggregate reduction in total crashes was used to 

calculate the benefits for an average intersection. The research team performed the economic 

analysis of total crashes as a conservative estimate of the economic benefit.

Based on work order cost data for signalized intersections provided by SCDOT, the economic 

analysis assumed an average pavement marking and signing construction cost of approximately 

$7,000. Typical costs were roughly $13,300 for signal enhancements, $18,300 for installing 

pedestrian signal assemblies, and $2,500 each for curb ramps. Annual maintenance and 

operations costs were not available except for pedestrian signal head maintenance and were

otherwise assumed to be zero (i.e., these costs will not be incurred within the service life).

PE, project management, and other general costs were not provided; however, a large portion of 

project planning was completed by State forces, and other costs for two contractors would have 

been split across all intersections if the costs were available. In future economic analyses of 

similar projects, all of these preliminary costs should be added to the construction costs.

The analysis assumed the useful service life for safety benefits was 7 years. Pavement markings 

were assumed to last roughly 4 years and all other treatments roughly 7 to 10 years with minimal 

maintenance. A conservative average of 7 years was used for the overall project. 

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

Management and Budget Circular A-4 as a resource for the real discount rate of 7 percent to 

calculate the present value benefits and costs of the multiple low-cost treatments over the service 

life.(19) With this information, the capital recovery factor was computed for all intersection types 

as 5.39 for a service life of 7 years.

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

disaggregated by crash severity and location type were used as a base.(20) These costs were 

developed based on 2001 crash costs, and the unit cost (in 2001 U.S. dollars (USD)) was 

$158,177 for fatal and injury crashes and $7,428 for PDO crashes. This was updated to 2015

USD by applying the ratio of the USDOT 2015 value of a statistical life of $9.4 million to the 

2001 value of $3.8 million.(20,21) Applying this ratio of 2.474 to the unit costs for fatal and

injury and PDO crashes yielded values of $391,280 and $18,375, respectively. The research 

team then weighted the values at approximately 20 percent fatal and injury crashes in the after 

period, which resulted in a total crash cost of $95,186 in 2015 USD. 

When the analysis was preformed, all project costs were brought forward to 2015 USD for 

consistency with crash cost values based on the same 7-percent discount rate, assuming original 

project costs were in 2011 USD.

The total crash reduction was calculated by subtracting the actual crashes in the after period from 

the expected crashes in the after period had the intersection treatments not been implemented. 

The total crash reduction was then divided by the average number of after period years per site to 
52

compute the total crashes saved per year. The treatments saved 50.6 total crashes per year for the 

sample sites, or an average reduction of 0.6 crashes per site per year across the 84 treatment sites.

Similarly, the treatments resulted in a reduction of 26.5 fatal and injury crashes, or approximately 

0.3 fatal and injury crashes fewer per site per year across all 84 sites.

The annual economic benefits were calculated by multiplying the crash reduction per site per year 

by the cost of a crash. Total crash reduction and total crash cost were used in the calculation. The 

B/C ratio was calculated as the ratio of the present value of benefits to the present value of all 

costs. USDOT recommended a sensitivity analysis be conducted assuming values of a statistical 

life of 0.57 and 1.41 times the recommended 2015 value.(21) These factors were applied directly 

to the estimated B/C ratios to obtain a lower and upper bound of the B/C ratios. Table 44 presents

the resulting B/C ratios for two scenarios: (1) assuming signing, marking, and signal 

improvements and (2) assuming scenario 1 plus pedestrian-related improvements.

Table 44. B/C ratios.

Treatments Lower Bound Average B/C Upper Bound

Signing, marking, and signal hardware 

improvements 6.6 11.7 16.4

Signing, marking, signal head 

replacements, and pedestrian signal 

installation with curb ramps

2.3 4.1 5.8

These results suggest that implementation of the various intersection treatments, even with 

conservative assumptions of service life and the value of a statistical life, can be cost effective in 

reducing crashes at signalized intersections.
53

CHAPTER 10. SUMMARY AND CONCLUSIONS

The objective of this study was to undertake a rigorous before–after evaluation of the safety 

effectiveness, as measured by crash frequency, of multistrategy, low-cost improvements at 

signalized intersections. The study used data from a systemic intersection improvement program 

in South Carolina to examine the effects for the following specific crash types: total, fatal and

injury, rear-end, right-angle, and nighttime. Based on the aggregate results, table 45 presents the 

recommended CMFs for the various crash types. 

Table 45. Recommended CMFs.

Variable Total

Fatal and

Injury Rear-End Right-Angle Nighttime

CMF 0.955 0.893 0.974 0.883 0.969

Standard error 0.023 0.045 0.034 0.035 0.048

Note: Boldface indicates statistically significant results at the 95-percent confidence level.

The disaggregate analysis identified specific CMFs by crash type and different conditions. The 

process also revealed the conditions under which the multiple low-cost treatments were more

effective. Variables of interest included area type (urban or rural), number of legs (three or four), 

lane configuration of the mainline and the cross street, traffic volumes, and expected crashes 

without treatment. The disaggregate analysis indicated larger reductions of most crash types for 

intersections in urban areas, three-legged intersections, and intersections with two-lane major 

roads. For total entering volume, the disaggregate analysis indicated the strategy was slightly 

more effective on average for intersections with lower traffic volumes. The strategy was

approximately equally effective across the range of expected crashes before treatment, suggesting

the need for caution in interpreting and applying the results of the other univariate comparisons, 

which are likely confounded by multiple correlative effects.

The B/C ratio for intersections with pavement marking, signing, signal hardware, and pedestrian 

infrastructure, estimated with conservative cost and service life assumptions and considering the 

benefits for total crashes, was 4.1:1. With the USDOT recommended sensitivity analysis, these 

values could range from 2.3:1 up to 5.8:1. The B/C ratio when excluding the cost of pedestrian 

improvements was 11.7:1. These results suggest that the implementation of multiple low-cost 

treatments, even with conservative assumptions on cost, service life, and the value of a statistical 

life, can be cost effective in reducing crashes at signalized intersections.

This research demonstrates the potential effect of a systemic intersection improvement program 

by evaluating a program in which each site received an individualized version of a package of 

intersection treatments with some differences in application at each individual intersection. 

Although information regarding the exact treatments installed at each site was not available to the 

research team, such data would have value in future evaluations of multiple-strategy 

improvement projects. Agencies should consider how to best document and track the 

improvements at each site to facilitate more complete and rigorous disaggregate analyses. 

However, the approach used in this research was able to quantify the overall effects of an 

improvement program and to suggest the expected effectiveness of similar future programs.

55

APPENDIX. ADDITIONAL INSTALLATION DETAILS

This appendix provides a description and examples of the general work completed by SCDOT to 

implement the multiple-strategy improvements at signalized intersections and illustrations of the 

SCDOT Standard Drawings used in the project. The appendix concludes with a list of general 

notes related to standard review guidelines, field notes, final plans, and submissions. Most of the 

following text is excerpted from SCDOT project guidelines. For explanatory purposes, the 

authors of this report added the text in brackets.

EXAMPLE OF DOCUMENTS USED DURING THE LOW-COST INTERSECTION 

IMPROVEMENT PROJECT(22)

[SCDOT used the following documents during the project:]

• General Signing and Pavement Marking Notes for all Intersections. SCDOT included 

this document in each work order sent to the contractors. It contains general notes and 

instructions that pertain to all intersections.

• SCDOT Standard Drawing 625-305-00. This document shows the standard pavement 

markings for intersections.

• SCDOT Standard Drawing 625-310-00. This document shows the standard pavement 

markings for turn lanes.

• SCDOT Standard Drawing 625-410-00. This document shows the standard pavement 

markings for arrows and the word message “Only.”

• Additional Sign Inventory for Replacement. SCDOT decided to replace additional 

warning and regulatory signs (in addition to the typical) from this table to include signs 

near the intersection that were considered to have notable safety impacts.

• Unsignalized Intersection Design. This document shows general pavement marking and 

sign installation information for unsignalized intersections.

• SCDOT Traffic Engineering Guideline 20. SCDOT Traffic Engineering designed this 

document to provide information on the installation of retroreflective sign post panels.

• SCDOT Guidelines for Advance Placement of Warning Signs. SCDOT revised this 

document from the Table 2C-4 from the 2009 MUTCD to show suggested placement of 

advanced warning signs. Proper staking has been one of the biggest issues to date so this 

document was used to serve as a guideline. 100 feet was added to Condition B (0) to 

provide additional advance notice needed for the added street name sign.

• Intersection Typicals. These documents are examples of intersection typicals provided to 

the contractor by SCDOT. They include typicals for a signalized intersection, a four-way 

stop controlled intersection, a cross-type stop controlled intersection, and a t-type stop 
56

controlled intersection. These typicals are revised after field inspection to create a final 

plan.

• Street Name Sign Typical. This document is an example of the SignCADD layout 

provided with each intersection typical.

• Placement Dimensions for Stop Ahead. [This document shows dimensions for 

placement of rumble strips preceding a Stop sign.]

GENERAL SIGNING AND PAVEMENT MARKING NOTES FOR ALL 

INTERSECTIONS(22)

[SCDOT used the following guidance for signing and pavement marking for all intersections.]

Remark all existing stop lines, crosswalks, arrows and word messages unless:

• The roadway has been resurfaced within 1 calendar year and new thermoplastic markings 

have been applied. 

• Existing markings are uniformly reflective and above ground thickness is ≥ 90 mils.

• Otherwise directed by a district representative

Individual typicals in work orders may not show all desired markings; therefore, all turn lanes 

shall be marked to include the pattern of lane arrows and accompanying word message “ONLY” 

based on the turn lane length, in accordance with Standard Drawing 625-410-00.

As referenced in Standard Drawing 625-410-00 for signalized intersections, combination Straight 

and Left or Right Turn arrows shall be added on all shared usage lanes where there are two or 

more through lanes (exclusive or shared). For example, if an approach has an exclusive through 

lane AND a shared through/right turn lane, the shared lane shall have two through/right turn 

arrows installed in accordance with Standard Drawing 625-410-00.

Additional pavement marking details for intersections shall be followed in accordance with 

Standard Drawing 625-305-00 and 625-310-00. Note that all turn bays should be delineated with 

an extended dashed edgeline as shown in the standard drawings.

If existing lane markings and word messages are in good condition but not compliant with the 

typical, retain the existing marking scheme and do not install the typical layout.

For fabrication of D series signs, utilize appendix C of the blue MUTCD with 8” capital letters 

for 4-lane divided roadways and 6” capital letters for all other roadways.

Opposite side signs should be placed adjacent to the existing sign within a 30’ tolerance.

If “STOP” pavement marking is used, place 8’ letters approximately 10’ in advance of stop limit 

line.
57

Install retroreflective sign post panels only on signs as indicated on Traffic Engineering Guideline 

TG-20 that are shown on the original typicals. Additional signs will not require sign post panels.

Do not replace Junction signs with blue border and lettering.

For electric sign mounted flashers, contact the RCE for disconnect of electric power to convert to 

solar flasher.

Replace all other existing signs within 500’ of the intersection that are included in the attached 

table entitled “Additional Sign Inventory for Replacement.”

Reinstall all pavement markings to match the existing field markings unless otherwise noted, i.e., 

TWLT markings should not be remarked as double yellow, dashed edge lines should not be 

installed for single turn lanes, etc.

STANDARD MARKINGS FOR INTERSECTIONS(22)

[The following text is transcribed from SCDOT Standard Drawing 625-305-00. Excerpts of 

details from the standard drawing are included as figures.]

Application of Markings at Intersections

1. Stop lines are to be applied at all signalized intersections. 

2. At non-signalized intersections, the roadways which must stop are to have stoplines if 

centerlines are present. 

3. Where stoplines are used, lane lines and center lines will terminate at the stopline. They do 

not extend across stoplines nor do they terminate prior to stoplines. Location of stoplines 

should be determined prior to marking longitudinal lines. 

4. Lane lines terminating at a stopline should not be less than 10 ft in length, however they may 

be longer. The last lane line will be 10 to 40 ft long. The following procedure will aid in this 

determination: 

a. Mark a spot 50 ft in advance of stopline of each lane line approach. 

b. If a line is being applied when the spot is crossed, the striper operator permits 

automatic cut-off and the following 30 gap. When the next line begins, the striper 

operator will manually override the automatic cut-off and will extend the line to 

the stopline. 

c. If a line is not being applied when the spot is crossed, when the next line begins 

the striper operator will manually override the automatic cut-off and will extend 

the line to the stopline. 

5. At all intersections, lane lines will normally be omitted within the intersection area where 

turning vehicles must maneuver.
58

[Figure 32 shows a detail of standard markings for intersections.]

©SCDOT

Figure 32. Illustration. SCDOT Standard Drawing 625-305-00 excerpts for application of 

markings at intersections.
59

Arrows and Word Messages

Arrows and word messages are not typical at all turn lanes and will be placed only at locations 

shown on the plans or where directed by the engineer. In the absence of a marked crosswalk, the 

stopline should be placed at a distance of no less than 4 ft and no more than 30 ft from the where 

arrows supplement signs to prohibit a movement that would otherwise be legal from that lane; the 

arrow must be accompanied by the word “only.” All arrows and word messages shall be as 

indicated on standard drawings 625-410-00.

Additional Guidance through Intersections

[Figure 33 shows guidelines for applying dashed-line pavement markings through intersections.]

© SCDOT

Figure 33. Illustration. SCDOT Standard Drawing 625-305-00 excerpt for guidance through 

intersections.

Crosswalks

All crosswalks are to be marked with 8” solid white lines. Crosswalk lines are to be spaced not 

less than 6 feet apart. [Figure 34 shows standards for an unsignalized school crosswalk.]
60

© SCDOT

Figure 34. Illustration. SCDOT Standard Drawing 625-305-00 excerpts for crosswalk 

markings.

TYPICAL MARKINGS FOR TURN LANE INSTALLATIONS(22)

[The following text is transcribed from SCDOT Standard Drawing 625-310-00. Excerpts of 

details from the standard drawing are included as figures.]

Notes

[The following notes relate to installing typical markings for turn lanes:]

1. Length of tapers and chevrons vary. See plan sheets for dimensions. 

2. Apply arrows, see Standard Drawing number 625-410-00. 

3. Apply ‘only’ copy, see Standard Drawing number 625-410-00. 
61

4. No raised markers are to be applied on chevrons. 

5. Stoplines shown on mainline are to be applied only at signalized intersections.

[Figure 35 through figure 38 show details for turn lane markings from SCDOT Standard Drawing 

625-310-00.]

©SCDOT

Figure 35. Illustration. SCDOT Standard Drawing 625-310-00 excerpts for turn lane 

installations (part 1).
62

© SCDOT

Figure 36. Illustration. SCDOT Standard Drawing 625-310-00 excerpts for turn lane 

installations (part 2).
63

© SCDOT

Figure 37. Illustration. SCDOT Standard Drawing 625-310-00 excerpts for turn lane 

installations (part 3).
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©SCDOT

Figure 38. Illustration. SCDOT Standard Drawing 625-310-00 excerpts for chevron 

marking details.

STANDARD PAVEMENT MARKINGS(22)

[Figure 39 through figure 43 are excerpts of details from SCDOT Standard Drawing 625-410-

00.]
65

©SCDOT

Figure 39. Illustration. SCDOT Standard Drawing 625-410-00 excerpt for straight arrow 

standard pavement marking.
66

© SCDOT

Figure 40. Illustration. SCDOT Standard Drawing 625-410-00 excerpt for right or left turn arrow and combination straight 

and left or right turn arrow standard pavement marking.

66
67

 © SCDOT

Figure 41. Illustration. SCDOT Standard Drawing 625-410-00 excerpt for right lane drop arrow and left lane drop arrow 

standard pavement marking.

67
68

©SCDOT

Figure 42. Illustration. SCDOT Standard Drawing 625-410-00 excerpt for “ONLY” 

standard pavement marking.
69

©SCDOT

Figure 43. Illustration. SCDOT Standard Drawing 625-410-00 excerpt for right or left turn markings application.

69
70

ADDITIONAL SIGN INVENTORY FOR REPLACEMENT(22)

[SCDOT replaced additional warning and regulatory signs (in addition to the typical) shown in 

figure 44 through figure 47, including signs near the intersection that were considered to have 

notable safety impacts.]

©SCDOT

Figure 44. Illustration. Additional sign inventory for replacement (part 1).
71

 © SCDOT

Figure 45. Illustration. Additional sign inventory for replacement (part 2).
72

 © SCDOT

Figure 46. Illustration. Additional sign inventory for replacement (part 3).
73

©SCDOT

Figure 47. Illustration. Additional sign inventory for replacement (part 4).

[Table 46 shows the advance placement distance at different posted speed limits or 85-precentile 

speeds.] 

This chart is intended as a reference with adjustments expected due to field conditions and 

engineering judgment.
74

Table 46. Advance placement distance for signal ahead, stop, or intersection warning signs.

Posted or 85th-Percentile 

Speed (mi/hr) Multilane Approach* (ft) Single Lane Approach** (ft)

20 225 200

25 325 200

30 460 200

35 565 200

40 670 225

45 775 275

50 885 350

55 990 425

60 1,100 500

65 1,200 575

70 1,250 650

75 1,350 750

Note: * These values reflect condition A from Table 2C-4 of the 2009 MUTCD and should be used as a reference for 

designated signs on multilane approaches.

** These values reflect condition B from Table 2C-4 of the 2009 MUTCD plus 100 ft due to the chart representing 

minimum guidelines and the additional advance notice needed due to the supplemental street name signs added to 

these sign assemblies.

RETROREFLECTIVE SIGN POST PANELS(22)

[The following guidelines on the use of retroreflective signpost panels were signed and approved 

by South Carolina’s Director of Traffic Engineering on June 24, 2008:]

Number: TG-20

Subject: Retroreflective Sign Post Panels

Background: Section 2A.21 of the MUTCD provides guidance on the use of 

Retroreflective Sign Post Panels. This section states that these panels can be 

applied to regulatory and warning signs where engineering judgment 

indicates a need for additional target enhancement during nighttime 

conditions. Therefore, these panels will generally be applied where crash 

history indicates a relatively high percentage of nighttime crashes.

Guideline: The panels shall be constructed of a nonmetallic composite or 3mm 

aluminum composite material approved by the SCDOT covered with a 3-inch 

wide type III sheeting. The panel shall be placed for the full length of the 

support from the sign except that the color for the “Yield” and “Do Not 

Enter” signs shall be red. If there are two posts supporting the sign, panels 

should be added to both posts.
75

To avoid excessive use of the Retroreflective Sign Post Panel, it is suggested 

that panels only be applied when needed to the regulatory signs below:

• Red Regulatory Signs. Stop, Yield, Do Not Enter, and Wrong Way 

signs—Red Panels.

• Horizontal Alignment Signs. Chevrons, Curve, Turn, and Large 

Arrow signs—Yellow Panels.

• Advance Traffic Control Signs. Stop Ahead, Yield Ahead, and 

Signal Ahead signs—Yellow Panels.

• Intersection Warning Signs. Cross Road, Side Road, and TwoDirection Large Arrow signs—Yellow Panels.

• Pedestrian Signs and School Area Signs. W11-2 and S1 Series—

Fluorescent Yellow Green Panels.

[Figure 48 and figure 49 show standards for pavement marking and rumble strip placement at 

unsignalized intersections.]
76

©SCDOT

Figure 48. Illustration. SCDOT nonsignalized intersection design for pavement marking and sign installations.

76
77

©SCDOT

Figure 49. Illustration. SCDOT traffic engineering rumble strip typical.

[Figure 50 through figure 53 are examples of intersection typicals that SCDOT provided to the 

contractor, including a signalized intersection, a four-way stop-controlled intersection, a crosstype controlled intersection, and a t-type strop-controlled intersection.]
78

©SCDOT

Figure 50. Illustration. SCDOT typical for a signalized intersection.
79

© SCDOT

Figure 51. Illustration. SCDOT typical for a four-way stop controlled intersection.
80

© SCDOT

Figure 52. Illustration. SCDOT typical for a cross-type stop controlled intersection.
81

©SCDOT

Figure 53. Illustration. SCDOT typical for a t-type stop controlled intersection.

[Figure 54 shows a street name sign typical layout.]
82

©SCDOT

Figure 54. Illustration. SCDOT street name sign typical.

STANDARD REVIEW GUIDELINES(23)

[The following is an excerpt from an internal SCDOT document containing standard review 

guidelines for reviewing installations of the treatments in this project.]

General Notes

• Speed limit signs are not to be replaced as part of this project. 

• Additional Advisory Speed plaques (such as speed plaques on “Trucks Entering Hwy” 

sign, etc.) will not be addressed as a part of this project. 

• Do not list info for retroreflective sign post panels on the plans. 

• Left Turn arrow Pavement Markings shall be installed in TWLTL’s. 

• Show all signs as proposed; do not shade anything to represent existing conditions. 

• Any non‐standard intersections, i.e., free flow interchanges or roundabouts should be sent 

to SCDOT Safety Office for verification and instruction. 

• No Pavement Markings or signs shall be applied to routes that are not state maintained. 

• All Illumination shall be upgraded to LED. 

• Yield conditions shall receive yield line and skip line pavement markings. 

• Place note “retain existing” for all non‐MUTCD signs. 
83

• A photo of the current street name signs in the field trumps all spelling of the street name. 

• No signs should be placed in paved radii.

• Limit sign placement for dead ends, short routes, grid systems, etc. 

• Engineering judgment should be used when placing all warning signs.

o Under urban grid conditions cross road warning signs (or signal ahead signs) should 

not be placed.

o Signs should not be placed near driveways (where it would obstruct sight distance) or 

in citizen’s front yard.

o Signs should not be placed when an object would obscure their view (i.e., a large tree, 

shrubs, bridge columns, etc.).

o Use caution when placing signs in historic districts.

o Use caution when placing signs on interchange entrance and exit ramps.

• Guidelines for placing opposite side intersections (or signal) warning signs.

o Do not place on roadways with more than three‐lanes (three‐lanes meaning two 

through lanes and a paved median).

o On four‐lane divided highways signs should be placed in the median.

• We will not be making upgrades to existing ramps at crosswalks.

Non‐Signalized Locations

• Include estimated quantities for crosswalk, stop lines, yield lines, and skip lines on final 

sheet for non‐signalized locations. 

• Left turn Arrows and ONLY’s: 

o Less than 250’ – arrow ONLY arrow.

o 250’ or more – arrow ONLY arrow ONLY. 

• D‐Signs – Make a note of the size of the letters on the signs: 

o Only need to note of existing 8” letters on 4‐lane divided. 

• Do not show junction signs unless they are attached to a D‐sign that is being moved or 

replaced. 

• Skip lines and yield lines must be shown at all yields. 

• Overhead Flashers at a stop intersection should be treated as a signalized location and 

have plans made for both pavement markings/signs and signals. All flashers will be 

replaced with LED casings and modules. NOTE: this does not include pole‐mounted 

flashers on a Stop sign, Stop Ahead sign or intersection warning sign. 
84

• No NEW Flashers will be installed on Stop controlled intersections.

• All Signal Ahead, Stop Ahead, Yield and Stop signs shall be 48”. Stop and Yield signs in 

the median or in exclusive turn lanes can be 30”. Use engineering judgment to determine. 

• Intersection warning signs shall be 36”.

• Guidelines for placing street name signs on intersection warning signs: 

o On all undivided roadways street name signs should be placed on only the right side 

intersection warning sign (no opposite side sign placement).

o On four‐lane divided highways street name signs should be placed on both the right 

side and opposite side intersection warning signs (if they are both used).

o Word messages (such as “STOP” and “STOP AHEAD”) should be used sparingly. 

Only when currently in the field or engineering judgment warrants their placement 

(i.e., no warning signs or opposite side signs can be placed).

Signalized Locations

• All signal ahead signs must have street names. 

• Yield and stop lines must be behind crosswalk. Indicate on plans that the stop lines/yield 

lines need to be eradicated and new ones installed to accommodate the crosswalk. 

• Show piano lines in crosswalk only if they currently exist. 

• Ramps will be counted as 2 if crosswalks don’t connect at the corner and 1 if they do. 

• Left Turn Yield on Green (ball) sign ‐ only installed when protected/permissive left turn 

(5‐signal face, dog‐house style head). 

• There should be one 3‐signal face head located in the center of each thru lane, as a 

minimum. A 5‐signal face PT/PM head counts as one thru lane signal. 

• Skip lines and yield lines must be shown at all yields. 

• No NEW flashers will be installed at signalized locations. 

• At intersections where Ped Heads are currently installed, if the “Walk” symbol appears 

automatically, then no Push Buttons are required. 

• All Ped Heads shall be Countdown. 

• If Ped Heads are present, propose crosswalks. 

• If Push Buttons are present a crosswalk is not required. 
85

• If a crosswalk is required, show ramps. If ramps cannot be installed mark on field notes 

why (i.e., catch basin). 

• If ped BUTTONS are present (or proposed) = cross walks are not required.

• If ped HEADS are present (or proposed) = cross walks are required.

• If cross walks are present (or proposed) = ped heads should be present. 

• Quantities for pedestrian equipment will be estimated based on the number of NEW 

pedestrian equipment installed. 

• Number of signal heads: With permitted/protected left ‐ 1 signal head per thru lane (5 

signal face, dog‐house style counts as one). With protected only left ‐ 4 signal face, red 

arrows for left lane + 1 signal face per thru lane.

Field Notes

• Location information (Street names, county, etc.) 

• Indicate reasons for not following regular guidelines so we know that it was not just 

overlooked... ped treatments, crosswalk, ramps, signs, etc. 

• Any information or recommendations that may be helpful that you happen to notice while 

you are in the field. 

• If “Signal Ahead” or “Stop Ahead” signs are determined not to be necessary, put note on 

field sheet as to why. 

• On field notes, make mention of conflicting signs. For example, a Stop controlled 

intersection located between a signalized intersection and its coordinating “Signal Ahead” 

sign. Locations of the proposed intersection warning signs should be discussed with 

SCDOT. 

• On field notes, note if JCT signs are black or blue. 

• Note on field sheets any landscaped areas where proposed signs are to be located as well 

as any historic districts. 

• Note on field sheets if medians or islands are pavement markings or raised. If raised, note 

if it is earthen or concrete. 

• Street name signs on Mast Arms and Span Wire to be noted in field notes but not to be 

replaced. 

Final Plans

• Name the intersections as they are in the list given by SCDOT. 
86

• Consultant Company logos.

• Note NOT TO SCALE.

• Speeds from each approach. 

• North arrow.

• Any changes that are made to a signalized location must be called out with an arrow 

pointing and a note indicating a change, i.e., Install new ped treatments (with arrows to 

new ped treatments only), Install NEW near‐side head (with arrows to new near‐side 

heads only), Install NEW ramp (...), Install NEW overhead signs (...), Install NEW red 

arrow LED head (...), etc.).

• These changes may or may not require a new PE’d plan. At a minimum, they will require 

an update to the signal plan if one exists. Please supply a list of signalized locations that 

will require a new plan to be drawn and which additions there were to the plan. (See 

checklist for submitting packets.)

• Example list: 

o US1 @ SC12 – nearside head, ped treatments. 

o US1 @ S‐35 – ped treatments, ramps.

o US1 @ S‐1298 – additional thru lane head, ramps.

• Small maps are not necessary on Final plans (per example plan set for LCSI letting.pdf).

• Right of way does not need to be shown on plans. 

• Signal Equipment box is not necessary on the Final plans.

Submissions

• Round 2 and 3 signals and stops can be submitted at the same time to cover the area all at 

once. 

• Submit packets of approximately 50 locations at a time. 

• If at all possible, do not split up a single county into two different submittals; it’s best to 

have all locations in each county together. 

• Submittals should include two packets: 

o Signal group – this packet will go to the signal group for review and contain all 

necessary documents spelled out below. 

o Safety group – this packet will go to the safety group for review and contain all 

necessary documents spelled out below. 
87

Checklist for Submitting Packets

Initial Signal Group: 

_____ Final plan – electronic version printout, may have pavement markings and signs on them 

_____ Field notes plan – can be hand drawn plan or notes handwritten on electronic print, make 

notes for all decisions that are not following the standard recommendations (i.e., no signal 

ahead sign because signal nearby, no sidewalk ramp because gutter under curb, no double 

up on signal ahead because of somebody’s beautiful garden, etc.) 

_____ Quantities form – can be hand written from field as long as legible, include color of signal 

head/ped head casing, whether there are mast arms or not 

_____ Electronic photos or ftp 

_____ Electronic plans on disk or ftp 

_____ Electronic list of locations needing updated signal plan 

Final Signal Group Construction Packet: 

_____ Coversheet 

_____ Quantity sheets 

_____ Drawing for each location 

_____ Construction specifications with specific location information for the district 

Safety Group: 

_____ Electronic Documents (submitted on CD is fine) ‐ Microstation file for each location, 

PDF of field notes and quantities sheet, PDF of final plan and any photos taken during the 

site review 

_____ Cover sheet – this should include a list of all locations included in the packet (along with 

their signalized or stop controlled status) and all locations omitted from the packet along 

with the reason for omission (current project, interchange, etc.). Please also note the 

locations where overhead flashers (mounted on span wire or mast arms) are present. 

These will need to be included in both the signalized and safety packets. 

_____ For signalized locations: include a hard copy of the final signing and marking plan 

(without signal information), a copy of the field notes and a copy of the quantities sheet. 

_____ For stop controlled locations: include a hard copy of the final signing and marking plan, a 

copy of the field notes and a copy of the quantities sheet. 

_____ For stop controlled locations with overhead flashers: include a hard copy of the final 

signing and marking plan (include flasher information on both the safety and signal 

copies for overhead flashers only), a copy of the field notes and a copy of the quantities 

sheet.

89

ACKNOWLEDGMENTS

This report was prepared for the Federal Highway Administration, Office of Safety Research and 

Development under Contract DTFH61-13-D-00001. The FHWA Program and Task Manager for 

this project was Roya Amjadi. 

The project team gratefully acknowledges the participation of the staff of the SCDOT for their 

assistance in this study.

91

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HRDS-20/05-18(200)E