South Carolina Department of Transportation

South Carolina’s SC 61 

Rural Road Safety Project 

SAFETY DATA CASE STUDY 

FHWA-SA-21-018 

Federal Highway Administration Office of Safety 

Roadway Safety Data Program 

http://safety.fhwa.dot.gov/rsdp
SAFETY DATA CASE STUDY FHWA-SA-21-018

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SAFETY DATA CASE STUDY FHWA-SA-21-018

1. Report No.

FHWA-SA-21-018

2. Government Accession 

No.

3. Recipient's Catalog No.

 

4. Title and Subtitle

South Carolina’s SC 61 Rural Road Safety Project

5. Report Date

February 2021

6. Performing Organization Code

7.Author(s) 

Ian Hamilton

8. Performing Organization Report No.

9. Performing Organization Name and Address

Vanasse Hangen Brustlin, Inc (VHB)

940 Main Campus Drive, Suite 500 

Raleigh, NC 27606 

10. Work Unit No.

11. Contract or Grant No.

DTFH61-16-D-00052

12. Sponsoring Agency Name and Address

Federal Highway Administration Office of Safety 

1200 New Jersey Ave., SE

Washington, DC 20590

13. Type of Report and Period

Case Study, January 2020-January 

2022

14. Sponsoring Agency Code

FHWA

15. Supplementary Notes

The contract manager for this report was Jerry Roche. Funding for this effort provided in part by the Highway Safety 

Manual Implementation Pooled Fund, TPF-5(255). 

16. Abstract

This purpose of this case study is to present an example application of the American Association of State Highway and 

Transportation Officials (AASHTO) Highway Safety Manual (HSM) to support a rural road corridor analysis. The South 

Carolina Department of Transportation (SCDOT) Rural Road Safety Program (RRSP) focuses on improving, “safety on SC’s 

rural roadways through engineering solutions identified to reduce the frequency of fatal and serious injury crashes occurring on 

these roadways” (South Carolina Department of Transportation 2020). SCDOT used HSM spreadsheets to apply HSM predictive 

methods from chapter 10, Rural Two-Lane, Two-Way Roads (AASHTO, 2010), to assess alternative designs. The spreadsheets 

allowed SCDOT to apply an Empirical-Bayes (EB) analysis that compared the observed crash frequency with the expected crash 

frequency based on the corridor’s characteristics. Context and the preference of the public and local stakeholders were the 

governing challenges of this analysis. The historical context, Scenic Byway designation, and other natural barriers required 

SCDOT to consider numerous, slightly different alternatives. The goal, as well as the key challenge of the public engagement, 

was to present the HSM analysis as clearly and concisely as possible. The general public appreciated seeing the differences in 

crash prediction associated with changes to the cross-section relative to the “No-Build” option, and it demonstrated that SCDOT 

had taken the time to consider stakeholder input. This helped make the case that the surrounding landscape should be gently 

modified to improve safety.

17. Key Words: 

Highway Safety Manual, HSM, Rural, Two-Lane, 

Undivided

18. Distribution Statement

No restrictions.

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this 

page) Unclassified

21. No. of Pages

 14 

22. Price

Technical Documentation Page
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SAFETY DATA CASE STUDY FHWA-SA-21-018

Acronyms

Acronym Description

AADT annual average daily traffic

AASHTO American Association of State Highway and Transportation Officials

CMF crash modification factor

EB Empirical-Bayes 

FHWA Federal Highway Administration 

FI fatal and injury

HSM Highway Safety Manual

ITMS Integrated Transportation Management System

RIMS Roadway Information Management System

RRSP Rural Road Safety Program 

SC South Carolina

SCDOT South Carolina Department of Transportation 
iii

SAFETY DATA CASE STUDY FHWA-SA-21-018

Table of Contents

Introduction............................................................................................................................................... 1

Safety Performance Analysis ................................................................................................................... 4

Challenges.................................................................................................................................................. 6

Conclusions and Lessons Learned ........................................................................................................... 6

References .................................................................................................................................................. 8

List of Figures

Figure 1. Graphic. SC 61 project location............................................................................................... 1

Figure 2. Graphic. SC 61 site photo of a completed section................................................................. 3

List of Tables

Table 1. SC 61 existing conditions........................................................................................................... 4

Table 2. SC 61 analysis assumptions. ...................................................................................................... 5

Table 3. SC 61 roadway design features and alternatives analysis....................................................... 5

Table 4. Alternatives analysis - public engagement example. .............................................................. 6
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SAFETY DATA CASE STUDY FHWA-SA-21-018

Executive Summary

This purpose of this case study is to present an example application of the American Association of State 

Highway and Transportation Officials (AASHTO) Highway Safety Manual (HSM) to support a rural road corridor 

analysis. The South Carolina Department of Transportation (SCDOT) Rural Road Safety Program (RRSP) 

focuses on improving, “safety on SC’s rural roadways through engineering solutions identified to reduce the 

frequency of fatal and serious injury crashes occurring on these roadways” (South Carolina Department of 

Transportation 2020). SCDOT used HSM spreadsheets to apply HSM predictive methods from chapter 10, Rural 

Two-Lane, Two-Way Roads (AASHTO, 2010), to assess alternative designs. The spreadsheets allowed SCDOT to 

apply an Empirical-Bayes (EB) analysis that compared the observed crash frequency with the expected crash 

frequency based on the corridor’s characteristics. Context and the preference of the public and local 

stakeholders were the governing challenges of this analysis. The historical context, Scenic Byway designation, 

and other natural barriers required SCDOT to consider numerous, slightly different alternatives. The goal, as 

well as the key challenge of the public engagement, was to present the HSM analysis as clearly and concisely as 

possible. The general public appreciated seeing the differences in crash prediction associated with changes to the 

cross-section relative to the “No-Build” option, and it demonstrated that SCDOT had taken the time to 

consider stakeholder input. This helped make the case that the surrounding landscape should be gently modified 

to improve safety. 
1 

SAFETY DATA CASE STUDY FHWA-SA-21-018

Introduction 

The Transportation Research Board’s Safety Performance Analysis (ACS20) User Liaison Subcommittee 

has an on-going initiative focused on practical application of the American Association of State Highway 

and Transportation Officials (AASHTO) Highway Safety Manual (HSM) (i.e., “using the HSM in the real 

world”). FHWA also administers the HSM Implementation Pooled Fund, which includes 22 States 

focused on projects to help further HSM implementation. Development of HSM case studies will assist 

practitioners in performing data-driven safety analysis using the advanced methods described in the 

HSM. The primary purpose of the HSM case studies is to highlight noteworthy applications of HSM 

methods, focus on common challenges, and feature agencies that overcame those challenges. These case 

studies serve as a source of lessons learned and noteworthy practices to help guide practitioners 

applying the HSM.

Background

This purpose of this case study is to present an example of a rural road corridor analysis. The South 

Carolina Department of Transportation (SCDOT) Rural Road Safety Program (RRSP) focuses on 

improving, “safety on SC’s rural roadways through engineering solutions identified to reduce the 

frequency of fatal and serious injury crashes occurring on these roadways” (SCDOT, 2020). To illustrate 

the magnitude of the issue, 5 percent of the worst performing rural roads in South Carolina account for 

30 percent of all fatalities and serious injuries. A 1.3-mile section of SC 61 in Dorchester County, SC is 

among the 55 rural non-interstate corridors identified for safety improvements through the RRSP (figure 

1).

Figure 1. Graphic. SC 61 project location.

©2021 Google® ©2021 Maxar Technologies. Modified by the authors. 

Note: The white location pins and white dashed line were added by the authors to delineate the project bounds.
2 

SAFETY DATA CASE STUDY FHWA-SA-21-018

Purpose and Need

The purpose of the RRSP is to reduce roadway departure crashes on rural roads in South Carolina; 

these crashes account for more than half of all fatalities and serious injuries in the State. The projects 

developed through the RRSP focus on three objectives (consistent with FHWA’s roadway departure 

objectives): 

1. Keep vehicles on the roadway. 

2. Provide for safe recovery. 

3. Reduce the frequency of fatal and serious injury crashes. 

To accomplish these objectives, SCDOT explored relevant countermeasures that address roadway 

departure crashes, including:

• Rumble strips. 

• Wider and brighter pavement markings. 

• Brighter and more reflective signs. 

• Wider/paved shoulders. 

• Wider clear zones. 

• Guardrails. 

• Smoother pavement edges. 

• Beveling driveway pipes. 

The SC 61 project was divided into two sections, 1) a 0.6-mile section north of the main entrance to 

Middleton Plantation, and 2) a 0.7-mile section that continues southeast and ends at the Dorchester and 

Charleston county line. As a scenic rural corridor in southeast South Carolina, SC 61 presented several 

challenges that limited the applicability of some of the roadway departure countermeasures. For 

instance, local officials and the general public wanted to preserve the historic and natural aesthetics of 

the corridor (the corridor is a designated SC Scenic Byway), and they were generally unwilling to 

remove trees and other vegetation as some countermeasure implementations would require (figure 2). 

Environmental constraints and project bottlenecks (e.g., a bridge over a river tributary) limited the 

applicability of other roadway departure countermeasures. SCDOT’s engineers documented the 

impacts, trade-offs, and challenges associated with various treatments as part of the alternatives analysis.
3 

SAFETY DATA CASE STUDY FHWA-SA-21-018

Project Description

• Sponsoring agency: SCDOT. 

• Project location: Dorchester County, SC. 

• Project bounds and length of project: SC 61 between milepoint 18.471 - 19.773 (1.3 miles).

• Facility type(s): 2-lane undivided, two-way minor arterial.

• Area type: Rural.

• Project status (winter 2021): Constructed.

Figure 2. Graphic. SC 61 site photo of a completed section.

©SCDOT
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SAFETY DATA CASE STUDY FHWA-SA-21-018

Safety Performance Analysis

This section provides an overview of the analysis methods, proposed alternatives, and results. 

Analysis Overview 

The corridor’s existing cross-section consists of two, 10.5-ft lanes with no paved shoulders. SCDOT 

assessed a roadside hazard rating of 6 (out of maximum 7) before any improvements or vegetation 

clearing occurred. Given the coastal plain nature of the area, the road is generally straight and level 

throughout its length. The corridor experienced a moderate level of daily traffic, with an annual average 

daily traffic (AADT) volume of 9,700. SCDOT assumed no change in AADT over the five years of crash 

history used in the analysis. 

SCDOT used HSM spreadsheets to apply HSM predictive methods from Chapter 10, Rural Two-Lane, 

Two-Way Roads (AASHTO, 2010), to assess 11 preliminary alternative designs. SCDOT engineers 

obtained road attributes from the State’s Integrated Transportation Management System (ITMS), as well 

as historic crash data from the Roadway Information Management System (RIMS). 

The spreadsheets allowed SCDOT to apply an Empirical-Bayes (EB) analysis that compared the observed 

crash frequency with the expected crash frequency based on the corridor’s characteristics. SCDOT 

applied a calibration factor of 0.99 for the EB analysis. All design alternatives included crash modification 

factors (CMFs) for shoulder and centerline rumble strips (0.87 and 0.94, respectively).

Analysis Details

Table 1 summarizes the existing conditions on the corridor, and table 2 summarizes the analysis 

assumptions.

Table 1. SC 61 existing conditions.

Category Type SC 61 Characteristics

Cross-Section

Number of Lanes 2 

Median Presence Undivided

Lane Widths 10.5 feet

Shoulder Type/Widths None/Negligible

Alignment Curve/Grade Straight/Level 

Traffic AADT 9,700

Crash History 5-Year Statistics

44 Total Crashes

22 Injuries

3 Fatalities
5 

SAFETY DATA CASE STUDY FHWA-SA-21-018

Table 2. SC 61 analysis assumptions. 

Category Assumptions

HSM Chapter(s) 10

Calibration Factor(s) 0.99

CMF(s)

Shoulder rumble strips:

0.87

(CMF Clearinghouse ID #1195)

Centerline rumble strips:

0.94 (HSM Chapter 10.7.1)

Results

A detailed alternatives analysis revealed minor crash reductions across the spectrum of future “Build” 

options, despite major proposed alterations to the clear zone. Local stakeholders requested minor 

alterations to the paved cross-section (1-2 ft) to assess the granular impacts of any proposed widening. 

Based on the small differences in fatal and injury (FI) and total crash reduction, local stakeholders asked 

that SCDOT only make limited alterations to the cross-section and roadside environment. Table 3

details each alternative, as well as the associated crash prediction with each alternative.

Table 3. SC 61 roadway design features and alternatives analysis.

Design 

Features

Alternatives

1 2A 2B 3 4 5 6 7 8 9 10 

Lane Width (feet) 12 11 11 11 10 12 11 11 11 11 11

Total Shoulder 

Width (feet) 10 4 2 2 4 2 2 2 4 3 3 

Paved Shoulder

(feet) 4 4 2 2 2 2 2 2 2 3 3 

Added Pavement 

Width (feet per 

side)

5.5 4.5 2.5 2.5 1.5 3.5 2.5 2.5 2.5 3.5 3.5

Clear Zone (feet) 20+ 12 12 <5 5-10 5-10 2 5-10 12 12 5-10

Predicted FI 

Crashes - 10 years 9 11 12 14 13 12 14 12 11 11 12

Reduction in FI 

Crashes from NoBuild

65.4% 57.7% 53.8% 46.2% 50.0% 53.8% 46.2% 53.8% 57.7% 57.7% 53.8%

Predicted Total

Crashes - 10 years 25 30 32 39 36 33 39 34 30 31 33

Reduction in 

Total Crashes 

from No-Build

64.8% 57.7% 54.9% 45.1% 49.3% 53.5% 45.1% 52.1% 57.7% 56.3% 53.5%

Documentation and Use of Analysis Results

While the formal analysis report is not available to the public, presentation materials developed for 

public engagement explained the tradeoffs associated with specific alternatives. Table 4 is an example of 

the design tradeoffs in terms of tree removal relative to crash reductions.
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SAFETY DATA CASE STUDY FHWA-SA-21-018

Table 4. Alternatives analysis - public engagement example.

Roadway Design Features Alternative 1

Standard Typical

Alternative 2

Contextually Sensitive 

Design 

Lane Width (feet) 12 11

Shoulder Width (feet) 10 (4 paved; 6 earth) 4 (paved)

Clear Zone (feet) 20+ 12

Trees Impacted 283 58

Traffic Injury Reductions 65% 58%

The goal, as well as the key challenge of the public engagement, was to present the HSM analysis as 

clearly and concisely as possible while also conveying the high level of thought and consideration that 

went into each alternative. While most attendees opposed any alternative that removed trees from the 

corridor, SCDOT noted that the public appreciated the clear illustration of the alternative impacts and 

potential crash reductions. Ultimately, SCDOT selected an alternative of 11-ft travel-lane widths, a 3-ft 

paved shoulder, and no additional tree clearing.

Challenges

Context and the preference of the public and local stakeholders were the governing challenges of this 

analysis. The historical context, Scenic Byway designation, and other natural barriers required SCDOT 

to consider numerous, slightly different alternatives. SCDOT also noted several issues with data

collection and methods for quantifying existing conditions. For instance, SCDOT received conflicting 

posted speed limit sign placement reports on the corridor, and the agency had some difficulty defining 

the roadside hazard rating for the area; this assessment varied according to certain proposed 

alternatives, and the exact rating may vary by practitioner. Finally, without a clear and obvious 

alternative based on predicted crash reductions alone, there was considerable desire from stakeholders 

to move forward with a less intrusive, more contextually sensitive alternative. 

Limitations of the HSM method

According to SCDOT, HSM analysis methods have their limitations, and it is important to communicate 

that to all decision makers. As noted in the HSM’s preface (AASHTO, 2010):

“The information in the HSM is provided to assist agencies in their effort to integrate safety into 

their decision-making processes. The HSM is not intended to be a substitute for the exercise of 

sound engineering judgment” (Considerations and Cautions When Using the HSM).

The HSM should not be used as a tool to precisely predict the change in crashes over a 5-year period 

given the addition or removal of a single foot of pavement. Rather, it is a guide for practitioners to 

better understand the tradeoffs associated with major design decisions.

Conclusions and Lessons Learned

SCDOT’s original scope was to analyze two or three alternatives for safety improvements along SC 61; 

however, public and local organization feedback encouraged SCDOT to analyze a suite of slightly 
7 

SAFETY DATA CASE STUDY FHWA-SA-21-018

modified alternatives (table 3). Although there was considerable pressure from the public and local 

stakeholders to assess very minor changes to different alternatives, SCDOT noted that this should be 

discouraged whenever possible. Minor changes in design specifications, especially on short analysis 

corridors, yield minor changes in predicted crash reductions, and the practitioner may lose confidence 

in meaningful differences between alternative results. Still, the HSM analysis produces useful insights that 

allow safety practitioners to communicate and champion safety to non-technical stakeholders. The 

general public appreciated seeing the differences in crash prediction associated with changes to the 

cross-section relative to the “No-Build” option, and it demonstrated that SCDOT had taken the time to 

consider stakeholder input. This helped make the case that the surrounding landscape should be gently 

modified to improve safety. By combining public concern with the predicted outcomes based on the 

HSM analysis, SCDOT was able to achieve a context-sensitive solution that will yield safety benefits for 

rural road users.
8 

Contact

South Carolina Department of Transportation

Brian L. Jones, Rural Road Safety Program Manager

jonesbl@scdot.org

Duncan Smith, State Traffic Safety Engineer

SmithD@scdot.org

References

 American Association of State Highway and Transportation Officials. (2010). Highway Safety 

Manual: 1st Edition, Washington, D.C. Spreadsheet tools available online: 

http://www.highwaysafetymanual.org/Pages/Tools.aspx.

 Federal Highway Administration. (2020). “CMF Clearinghouse” (website). Available online: 

http://www.cmfclearinghouse.org/index.cfm. 

 Phone interview on June 17, 2020, with Duncan Smith.

 South Carolina Department of Transportation. (2019). Data Driven Safety Analysis – Integrating 

Safety into All Projects [PowerPoint Slides].

 South Carolina Department of Transportation. (2020). “South Carolina Department of 

Transportation Rural Roads Safety Program” (website). Available online: 

https://scdot.maps.arcgis.com/apps/MapSeries/index.html?appid=df937e9b63ce4377a3705feb9fb

05466.