Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

PUBLICATION NO. FHWA-HRT-23-041 MARCH 2023

Interchange Comparison Safety Tool 

User Guide
FOREWORD

The Highway Safety Manual has made predictive safety analysis feasible for many basic 

roadway elements, including freeway and ramp segments and ramp terminals. This capability 

raises the expectation for predicting the safety performance of more complex roadway facilities, 

such as interchanges. Although interchanges can be decomposed into basic road elements, the 

safety performance of interchanges cannot be derived by simply adding predictions from 

individual components. 

The Federal Highway Administration (FHWA) supported a project to explore planning-level 

analysis of interchange configurations during alternatives analysis or Interchange Access 

Requests (IARs). FHWA sought to identify the most commonly considered configurations in 

IARs and develop a predictive model and implementation tool. The purpose of this predictive 

model and associated implementation tool is to use an approach with more robust considerations 

than a single crash modification factor and provide reliable predictions using information 

commonly known during interchange project planning. This tool can be used to evaluate the 

predicted crash frequency and severity for interchange configurations under consideration using 

basic inputs for the entire interchange area.

This user guide assists safety professionals in applying the predictive methodology and using the 

accompanying implementation tool. This guide provides details on applying the predictive 

method to an interchange area and how that fits into the context of the overall interchange 

influence area. Additionally, this user guide details where the predictive method is applicable 

and the required inputs and supporting calculations. Finally, this guide provides instructions on 

using the spreadsheet implementation tool and interpreting the spreadsheet inputs and outputs. 

Brian P. Cronin, P.E. 

Director, Office of Safety and Operations 

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-23-041

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

4. Title and Subtitle

Interchange Comparison Safety Tool User Guide

5. Report Date

March 2023 

6. Performing Organization Code:

7. Author(s)

Scott Himes (ORCID: 0000-0003-2932-7691), Samantha 

Arnold (ORCID: 0000-0002-7642-3937), Vikash Gayah

(ORCID: 0000-0002-0648-3360), and Ginny O’Connor

(ORCID: 0000-0002-4668-6832) 

8. Performing Organization Report No.

9. Performing Organization Name and Address

VHB

940 Main Campus Drive

Suite 500

Raleigh, NC 27606

10. Work Unit No.

11. Contract or Grant No.

DTFH6116D00040

12. Sponsoring Agency Name and Address

Office of Safety and Operations Research and Development

Federal Highway Administration

6300 Georgetown Pike

McLean, VA 22101-2296

13. Type of Report and Period Covered

User Manual; 

September 2020–November 2022

14. Sponsoring Agency Code

HRSO-10

15. Supplementary Notes

The contract manager for this project was Wei Zhang (HRSO-10; ORCID: 0000-0002-5175-7527). The technical 

advisory committee included Mark Doctor, Jeff Shaw, Tim Taylor, and Dave Petrucci.

16. Abstract

This user guide is intended for use alongside the Federal Highway Administration (FHWA) report Safety 

Comparisons Between Interchange Types (forthcoming) and the spreadsheet tool FHWA Interchange 

Configuration Safety Comparison Tool.

(1,2) This user guide provides an overview of the data needs and workflow 

for using the spreadsheet tool. Additionally, it provides information on the interchange configurations for which 

this tool can be used and the ranges of characteristics to which it applies. Further, this guide provides an overview 

for finding results within the tool. 

17. Key Words

Interchange, configuration, safety, crash prediction, 

predictive model, safety comparison tool, 

spreadsheet tool

18. Distribution Statement

No restrictions. This document is available to the public 

through the National Technical Information Service, 

Springfield, VA 22161.

https://www.ntis.gov

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

22 

22. Price

N/A

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
iii

TABLE OF CONTENTS

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

Objective ..................................................................................................................................... 1

Background ................................................................................................................................. 1

Purpose and Structure of the Guide ......................................................................................... 1

CHAPTER 2. APPLICABILITY .................................................................................................. 3

Interchange Function ................................................................................................................. 3

Interchange Configuration ........................................................................................................ 3

Single Roundabout Interchanges .............................................................................................. 3

Parclo Configurations ............................................................................................................... 3

Not Valid for Spreadsheet Analysis ......................................................................................... 4

Interchange Influence and Interchange Areas ........................................................................ 5

Crash Severities Analyzed ......................................................................................................... 6

CHAPTER 3. SPREADSHEET INSTRUCTIONS..................................................................... 7

Workflow .................................................................................................................................... 7

Step 1: Read Welcome and Instructions Worksheets ............................................................. 7

Step 2: Complete Input Tables .................................................................................................. 8

Interchange Configurations ...................................................................................................... 8

Area Type ................................................................................................................................. 9

Traffic Volumes ....................................................................................................................... 9

Freeway Characteristics ......................................................................................................... 10

Crossroad Characteristics ....................................................................................................... 11

Step 3: Interpret the Outputs .................................................................................................. 14

Calibration .............................................................................................................................. 16

Additional Guidance on Predictive Method ........................................................................... 16

REFERENCES ............................................................................................................................. 17
iv

LIST OF FIGURES

Figure 1. Graphic. Interchange configurations included in the spreadsheet tool. ............................ 4

Figure 2. Graphic. Study interchange area definition.(3) .................................................................. 5

Figure 3. Screenshot. Spreadsheet workflow.(2) ............................................................................... 7

Figure 4. Screenshot. Spreadsheet field coding.(2) ........................................................................... 8

Figure 5. Illustration. Nearest interchange gore distance. .............................................................. 10

Figure 6. Illustration. Intersection skew calculation. ..................................................................... 11

Figure 7. Illustration. Example interchange spreadsheet inputs. .................................................... 12

Figure 8. Screenshot. User input from PDO worksheet.(2) ............................................................. 14

Figure 9. Screenshot. Applicability section.(2) ............................................................................... 14

Figure 10. Screenshot. Model output from PDO worksheet.(2) ...................................................... 15

Figure 11. Graph. Total crash frequency from Charts worksheet.(2) .............................................. 15

Figure 12. Screenshot. SDF model output.(2) ................................................................................. 16

LIST OF TABLES

Table 1. Parclo spreadsheet input summary. .................................................................................... 9

Table 2. Required area type inputs. .................................................................................................. 9

Table 3. Summary of required traffic volume inputs. ...................................................................... 9

Table 4. Summary of freeway lane information inputs. ................................................................. 11

Table 5. Summary of crossroad characteristic inputs. ................................................................... 13
v 

LIST OF ABBREVIATIONS

A incapacitating injury

AADT annual average daily traffic

B non-incapacitating injury

C possible injury

CI confidence interval 

COV coefficient of variation

DDI diverging diamond interchange 

FHWA Federal Highway Administration

HOV high-occupancy vehicle 

HSM Highway Safety Manual

K fatal

KA fatal and serious injury

KABC fatal and injury crash categories

KABCO injury classification scale

LT left turn

Parclo partial cloverleaf

PDO property damage only 

SDF severity distribution function 

SPDI single-point diamond interchange

TDI tight diamond interchange 

VPD vehicles per day

XR crossroad

1 

CHAPTER 1. INTRODUCTION

OBJECTIVE

This user guide is intended as a companion to the Federal Highway Administration (FHWA) 

report Safety Comparisons Between Interchange Types (herein referred to as “report”) and the 

spreadsheet tool FHWA Interchange Configuration Safety Comparison Tool (herein referred to as 

“spreadsheet tool”).(1,2)

 The report contains all of the background research, modeling details, and 

supplementary information. All calculations can be performed within the spreadsheet tool. 

BACKGROUND

The research project aimed to develop a planning-level safety assessment tool and interchange 

safety comparison process for FHWA and State departments of transportation to use when

reviewing Interchange Justification Reports. The safety assessment allows agencies to quantify 

the safety performance of proposed designs against a base (or reference) condition for service 

interchanges.

The project team developed a spreadsheet tool to implement the methods, geometric and 

operational characteristics, and parameters described in the corresponding report. The 

spreadsheet tool provides annual crash frequency predictions for individual crash severity levels 

using the KABCO injury classification scale, where K represents fatal, A incapacitating injury, B 

non-incapacitating injury, C possible injury crashes, and O property damage only (no apparent 

injury). The spreadsheet tool includes an estimate of the variability of the prediction so users can

assess the significance of differences among predictions for multiple interchange configuration 

types under consideration. The report and spreadsheet tool together provide a planning-level 

safety assessment tool analysts can use to compare potential safety performance effects of 

freeway access and interchange design decisions.

PURPOSE AND STRUCTURE OF THE GUIDE

This user guide provides detailed instructions for using the spreadsheet implementation tool and 

interpreting the results it provides. The user guide is organized into the following three chapters:

1. Introduction: Provides context and identifies the accompanying spreadsheet tool and 

report.

2. Applicability: Describes interchange configurations the spreadsheet tool can evaluate and 

the corresponding interchange influence areas on which the predictive model is based. 

3. Spreadsheet Instructions: Describes how to use the spreadsheet tool. This chapter also 

explains the required inputs for the spreadsheet tool and provides references to the report 

for users to interpret the spreadsheet inputs and outputs. 

3 

CHAPTER 2. APPLICABILITY 

INTERCHANGE FUNCTION

The spreadsheet tool is only valid for service interchanges, which connect freeways to local 

surface streets or arterials. System interchanges, which connect freeways to other freeways, were 

not included in the predictive model and thus are not considered in the spreadsheet tool. 

INTERCHANGE CONFIGURATION

The spreadsheet tool applies to the following interchange configurations: 

• Diamond interchange. 

• Compressed diamond interchange. 

• Roundabout diamond interchange. 

• Diverging diamond interchange (DDI). 

• Partial cloverleaf (parclo) type A (parclo A).

• Parclo type B (parclo B). 

• Parclo type AB (parclo AB). 

• Single-point diamond interchange (SPDI). 

• Tight diamond interchange (TDI). 

Single Roundabout Interchanges

Single roundabout interchanges are commonly considered and installed as service interchanges; 

however, this study found that jurisdictions that mention single roundabout interchanges are often 

referring to diamond interchanges with roundabouts at the crossroad ramp terminals. Single 

roundabout interchanges are not included in the model or spreadsheet; analysts should determine 

whether the diamond interchange definition fits their proposed configuration if they intend to use 

the spreadsheet tool. 

Parclo Configurations 

For the study design, data collection, and analysis, the project team focused on parclo 

configurations most commonly considered and installed (A2, A4, B2, B4, AB2, and AB4) and 

developed models with the intent of capturing any safety performance differences between these 

parclo types. Due to the sheer number of possible combinations, the project team did not include 

other parclo configurations in this research. The report includes descriptions of the parclo 

configurations. 

The spreadsheet combines diamond interchanges and compressed diamond interchanges into one 

category. Parclo AB and parclo B are also combined into one category. These groupings indicate 

that the predictive method found no safety performance differences between the two interchange 

configurations. Figure 1 shows all nine interchange configurations viable for comparative 

analysis using the spreadsheet tool. 
4 

Source: FHWA. 

Figure 1. Graphic. Interchange configurations included in the spreadsheet tool.

Not Valid for Spreadsheet Analysis 

The following interchange configurations and characteristics were excluded from the predictive 

model and thus not suitable for analysis using the spreadsheet tool: 

• Full cloverleaf interchange. 

• Displaced left-turn interchange. 

• Single roundabout interchange. 

• Ramps to frontage road interchange. 

• Braided ramp interchange. 

• “Dogbone” or double roundabout interchange. 

• System interchange. 

• Interchange with more than six ramps. 

• Interchange with direct-connection ramps. 

Planners often search for unique solutions to operational or safety concerns to fit local design 

constraints; therefore, they are encouraged to seek alternative methods for evaluating interchange 

configurations not included in this study. 
5 

INTERCHANGE INFLUENCE AND INTERCHANGE AREAS 

The general study area, or area of influence, necessary for conducting safety and operational 

analysis for interchange access improvements can vary substantially in size and scope. The 

spreadsheet tool focuses on the interchange area rather than the interchange influence area. To 

eliminate variability across all nine studied interchange configurations, the interchange area 

applicable to the predictive model developed for this project was defined as follows: 

• Freeway mainline: Includes 1,500 ft upstream and downstream of the painted gores 

farthest from the crossroad on either side.

• Crossroad: Includes 100 ft upstream and downstream of the gore or curb return of the 

outermost ramp connection for each terminal (crossroad). 

• Ramps (maximum of six). 

• Ramp terminals. 

Figure 2 represents the interchange study area defined in this project. 

Source: FHWA. 

Figure 2. Graphic. Study interchange area definition.(3)
6 

When applying the predictive models for safety comparisons, users should determine the 

maximum possible interchange area dimensions among the alternatives considered and apply 

those dimensions to all alternatives. For alternative interchanges with a smaller footprint, users 

would apply the Highway Safety Manual (HSM) predictive models to account for differences in 

interchange area dimensions among the alternatives.(4)

CRASH SEVERITIES ANALYZED

The spreadsheet tool provides separate predictions for property damage only (PDO) crashes and 

fatal and injury crashes (K, A, B, and C on the KABCO scale). Fatal and injury crashes are also 

known as KABC crashes. Additionally, the spreadsheet tool employs severity distribution 

functions (SDFs) to predict the probability of individual KABC crash outcomes. The resulting 

probability from the SDF is multiplied by the prediction for KABC crashes to determine the 

predicted number of crashes by severity outcome.
7 

CHAPTER 3. SPREADSHEET INSTRUCTIONS

WORKFLOW

The spreadsheet tool is divided into six worksheets. Refer to the report for methodology and 

supporting details for the model used in the spreadsheet tool. 

The Welcome and Instructions worksheets provide background information for users. The 

Abbreviations Used section in the Instructions worksheet defines abbreviations used in the 

spreadsheet and this user guide. 

Users enter input in the PDO and SDF worksheets. The PDO worksheet inputs are carried 

through to the KABC worksheet because the predictive methodology uses the same inputs for 

PDO and KABC crash frequency. Users enter separate inputs in the SDF to determine the 

probability of individual crash severity outcomes. Each worksheet provides predicted crash 

outcomes, which the Charts worksheet tab summarizes. The PDO and KABC worksheets

calculate the predicted crash frequency at the interchange level. The SDF worksheet calculates

the impact of various interchange features on crash severity. Users can see the crash frequency 

results and the 95-percent confidence interval (CI) ranges on the Charts worksheet. Figure 3 is a 

screenshot of the various worksheets within the spreadsheet with steps 1–3 of the intended 

workflow listed for reference.

Source: FHWA. 

Figure 3. Screenshot. Spreadsheet workflow.(2)

STEP 1: READ WELCOME AND INSTRUCTIONS WORKSHEETS

The Instructions worksheet provides descriptions of each worksheet and detailed descriptions for 

all inputs required for using the spreadsheet.

The spreadsheet identifies inputs and outputs using colors and patterns, which are described in 

figure 4. 
8 

Source: FHWA. 

Figure 4. Screenshot. Spreadsheet field coding.

(2)

Users modify cells highlighted as “Required input information” on both the PDO and SDF 

worksheets. These cells are the only cells in the spreadsheet intended for user modification.

This worksheet also indicates the key crash frequency model results and the results from applying

the SDFs. Additionally, this worksheet alerts users when information is linked to another 

worksheet. 

Required input cells will alert the user if an incorrect value has been entered, and users should 

refer to the Instructions worksheet for additional information. 

Cells are formatted to produce errors when nonconforming input is present. Users should enter 

values into each cell individually and not copy and paste cells to duplicate information across 

tables, or the conditional error formatting will be lost.

STEP 2: COMPLETE INPUT TABLES

The two input tables are located at the top of the PDO worksheet and the SDF worksheet. Users 

should complete both of the input tables before interpreting the results. The following sections 

provide context and additional information about the required inputs. 

Interchange Configurations

Nine interchange configurations are grouped into seven outputs in the PDO worksheet and six 

outputs in the SDF worksheet. Users may input the same geometric and operational 

characteristics for all types under consideration, or they may vary input parameters when 

conducting a simultaneous test. The spreadsheet and predictive model use the 

diamond/compressed diamond combined category as the base condition, meaning the user will 

need to provide inputs to this category regardless of whether these interchange types are 

considered. 

For parclo interchanges, the spreadsheet requires additional inputs on the SDF worksheet to 

differentiate between the various parclo configurations included in the study. Table 1 summarizes

the inputs for parclo interchange configurations in the implementation spreadsheet.
9 

Table 1. Parclo spreadsheet input summary.

N/A = not applicable.

Area Type

The predictive model differentiates between rural and urban area types. Urban areas are defined 

as having a population greater than 5,000. Table 2 provides an example input for area type in the 

implementation spreadsheet.

Table 2. Required area type inputs.

Worksheet Relevant Input Parameters User Input in Spreadsheet

PDO Urban area type 1 if in an urban area; 0 otherwise

SDF N/A N/A

Traffic Volumes

Freeway annual average daily traffic (AADT) volume, the crossroad AADT volume, and the 

ramp AADT volumes are all required inputs for the spreadsheet. For freeway mainline and cross

streets, the AADT is the bidirectional traffic volume. The coefficient of variation (COV) of ramp 

volumes is also a required input for the model. The implementation spreadsheet automatically 

calculates this value based on user inputs of individual ramp volumes. Note that up to six ramp 

volumes may be entered into the spreadsheet. If the subject interchange has fewer than six ramps, 

then the unnecessary rows can be left blank. Table 3 provides a summary of required traffic 

volume inputs for crash frequency and SDF worksheets. 

Table 3. Summary of required traffic volume inputs.

Worksheet Relevant Input Parameters User Input in Spreadsheet

PDO

Freeway AADT Volume in vpd

XR AADT Volume in vpd

Total ramp AADT Volume in vpd 

COV of ramp volumes Calculated in spreadsheet 

(requires individual ramp AADTs)

SDF

Freeway AADT ≥200,000 vpd 1 if freeway through lanes carry more than 

200,000 vpd; 0 otherwise

XR AADT greater than or equal to 

30,000 vpd 

1 if crossroad through lanes carry more 

than 30,000 vpd; 0 otherwise 

vpd = vehicles per day; XR = crossroad. 

Worksheet Relevant Input Parameters User Input in Spreadsheet

PDO N/A N/A

SDF

Type A or B parclo interchange

(includes A2/A4/B2/B4) 

1 if user is considering one of these types; 

0 otherwise

Type AB parclo interchange 

(includes AB2/AB4) 

1 if user is considering one of these types; 

0 otherwise
10 

Freeway Characteristics 

The distance between adjacent gore areas along the freeway is accounted for in the predictive 

model as a factor that increases crash frequency. This distance is also accounted for in the SDF as 

a factor that increases the probability of fatal and serious injury (KA) severity crashes within the 

crash distribution. The nearest interchange gore distance is measured from the closest gore areas 

between adjacent interchanges in the same direction of travel. Figure 5 describes this parameter.

Source: FHWA. 

Figure 5. Illustration. Nearest interchange gore distance.

The predictive model accounts for managed lanes on the freeway as a factor associated with 

increased crash frequency when at least one managed lane is present within the defined 

interchange area. In this case, managed lanes refer to high-occupancy vehicle (HOV) or 

high-occupancy toll lanes. This methodology development did not include other managed lane 

strategies, such as part-time shoulder use, reversible, contraflow, or bus-only lanes. The 

spreadsheet input is a yes or no response from the user regarding whether one or more managed 

lanes are within the interchange area. 

The number of through lanes on the freeway is accounted for in the predictive model to evaluate 

AADT per lane as well as a factor specifically accounting for the effect of the number of lanes.

The base condition is four or fewer through lanes on the freeway (in both directions combined), 

meaning spreadsheet output is affected when five or more lanes are input. Users input the number 

of freeway through lanes (bidirectional total) as a whole number value. The SDF worksheet uses 

a yes or no input to account for the number of through lanes, depending on whether the total is 

greater than or equal to eight lanes. Table 4 provides a summary of freeway lane information 

inputs. 
11 

Table 4. Summary of freeway lane information inputs.

Worksheet Relevant Input Parameters User Input in Spreadsheet

PDO

Nearest interchange gore distance 

within 0.5 mi 

1 if distance ≤0.5 mi; 0 otherwise 

(figure 5) 

Managed lanes on freeway 1 if there are any managed lanes within the 

defined interchange area; 0 otherwise

Freeway number of through lanes 

(bidirectional total)

Total number of through lanes on the 

freeway within defined interchange area 

(do not include auxiliary or managed 

lanes)

SDF

Freeway posted speed limit Enter speed limit in mph 

Nearest interchange gore distance 

within 0.25 mi

1 if distance ≤0.25 mi; 0 otherwise

(see figure 5) 

Freeway has eight or more lanes 

(bidirectional total)

1 if freeway has ≥8 through lanes;

0 otherwise (do not include auxiliary or 

managed lanes)

Crossroad Characteristics 

The intersection skew angle is measured as the angle between the crossroad centerline and the 

freeway centerline at the interchange. The skew angle between crossroad and freeway is 

accounted for in the predictive model as a factor associated with an increase in crash frequency

when the skew angle is greater than 30 degrees. Figure 6 describes this parameter. 

Source: FHWA. 

typ = typical.

Figure 6. Illustration. Intersection skew calculation.
12 

The predictive model accounts for the interchange’s total number of through lanes on the 

crossroad to evaluate AADT per lane and a factor specifically accounting for the effect of the

number of lanes. The base condition for the SDF is four or fewer through lanes, meaning 

spreadsheet output is affected when five or more lanes are input. Users input the total number of 

crossroad through lanes (bidirectional total) as a whole number value. The number of through 

lanes on the crossroad is included in the SDF as a factor that moves the severity distribution 

toward more severe crash outcomes when more lanes are present. Users input yes or no as to 

whether the bidirectional total is four or more through lanes.

Left-turn lanes on the crossroad are accounted for in the predictive model as a factor associated 

with a decrease in crash frequency. The PDO worksheet accepts values ranging from zero to 

seven total left-turn lanes on the crossroad. This value includes all left-turn lanes from the ramp 

terminals onto the crossroad and from the crossroad onto the ramps. Figure 7 provides more 

details.

Source: FHWA. 

LT = left turn.

Figure 7. Illustration. Example interchange spreadsheet inputs.

The SDF worksheet accounts for the distance from crossroad ramp terminals to adjacent 

intersections along the crossroad. The distance is measured from the centerline of the ramp 

terminal to the nearest intersection outside the defined interchange area. Figure 6 illustrates the 

usual location of a typical crossroad ramp terminal.
13 

The number of pedestrian crossings conflicting with right-turning vehicles across the interchange 

is accounted for in the SDF as a factor that moves the severity distribution toward a higher 

probability of more severe outcomes. Users input a value from 0 to 7, representing the total 

number of conflicts present. A conflict is defined as a right-turn movement intersecting with a 

pedestrian movement. If there are multiple adjacent right-turn movements from a single leg of an 

intersection within the interchange area, users should count it as one conflict point if the 

right-turn movements cross paths with a pedestrian movement. 

Figure 7 provides an example of various crossroad characteristics at an interchange required for 

spreadsheet input. Table 5 summarizes input requirements for crossroad characteristics and 

features.

Table 5. Summary of crossroad characteristic inputs.

Worksheet Relevant Input Parameters User Input in the Spreadsheet

PDO

Intersection skew angle >30 

degrees

1 if angle between XR and freeway is >30 

degrees; 0 otherwise

XR number of through lanes 

(bidirectional total)

Total number of through lanes on the XR 

within the defined interchange area

Number of LT lanes on the XR at 

intersections

Total number of LT lanes on the XR 

within the defined interchange area 

(0 minimum, 7 maximum)

SDF

XR posted speed limit Enter speed limit in mph

Nearest XR adjacent intersection 

(signal or roundabout) within 0.10 

mi of ramp terminal

1 if the nearest intersection along the XR is 

≤0.10 mi from any of the XR ramp 

terminals; 0 otherwise

Number of pedestrian crossings 

conflicting with right-turning 

vehicles across the interchange

Total number of pedestrian crossings that 

right-turning vehicles conflict with within 

the defined interchange area; 0 otherwise

XR has four or more lanes 

(bidirectional total)

1 if XR has ≥4 through lanes;

0 otherwise

Figure 8 provides a snapshot of the User Input section of the PDO worksheet. In this sheet, the 

user can enter data for any interchange configurations for comparison. However, as noted in the 

Interchange Configurations section, data must be entered for the diamond/compressed option 

since the calculations work from this configuration as the baseline. Also, as noted, changes made 

in the PDO worksheet will automatically update the KABC worksheet. 
14 

Source: FHWA. 

Std = standard.

Figure 8. Screenshot. User input from PDO worksheet.(2)

Additionally, users are directed to review the Applicability section of the PDO worksheet. As 

shown in figure 9, this section includes the AADT ranges for each interchange configuration 

available in the tool. For each configuration, the recommended minimum and maximum AADT 

values are included for the freeway, crossroad, entrance ramps, and exit ramps. Caution should be 

used when interpreting results outside these bounds, and further consideration of the operational 

merits should be taken. For example, roundabout diamond interchanges generally had lower 

volumes and may not operate well at volumes higher than those used to develop the model. 

Source: FHWA. 

Figure 9. Screenshot. Applicability section.(2)

STEP 3: INTERPRET THE OUTPUTS

As shown in figure 10, the primary output of the spreadsheet is in the Model Output section on 

the PDO worksheet. This section provides the predicted crash frequency for each interchange 

configuration in units of crashes per year. Results are split into PDO severity-level crashes, 

combined KABC severity-level crashes, and the total (KABC plus PDO) interchange crashes. 

These values are based on the predictive model developed for this project and are applicable over 

the entire interchange area; the model does not determine the location of the predicted crashes 
15 

within the interchange. Chapter 5 in the report provides background information on the predictive 

model. 

Source: FHWA. 

LB = lower bounds; UB = upper bounds.

Figure 10. Screenshot. Model output from PDO worksheet.(2)

The PDO worksheet also provides the 95-percent CIs as a range of values between lower and 

upper bounds for the crash frequency results. CIs are a measure of statistical certainty: the wider 

the range, the more caution should be used in applying the model results. Additionally, the 

predictive model results suggest that while there may be a difference based on the expected 

value, the differences may not be statistically different, which should be considered when 

evaluating the safety effects of each interchange configuration. 

The Charts worksheet displays the model output in a visual format. Charts show the predicted 

crash frequency and 95-percent CI upper and lower bounds for PDO, KABC, and combined 

KABC plus PDO results. Figure 11 provides an example chart from the worksheet highlighting 

predicted total (KABC plus PDO) crashes for each configuration and applicable 95-percent CIs. 

Source: FHWA. 

Figure 11. Graph. Total crash frequency from Charts worksheet.(2)

As shown in figure 12, the SDF worksheet provides a breakdown of the SDF results by 

interchange configuration. The worksheet predicts the proportion of KABC crashes, given that a 

crash has occurred. Results are combined for K and A severity levels. These proportions are 

multiplied by KABC crash frequency on the KABC tab to provide a prediction for crash 

frequency by severity level. 
16 

Source: FHWA. 

Figure 12. Screenshot. SDF model output.(2)

Calibration

The predictive model provided in this worksheet is based on data from five States. As with other 

predictive models, local conditions may dictate that safety performance will differ by jurisdiction, 

and calibration is necessary for using the results to compare differences in terms of the number of 

crashes. If the tool is used without calibrations, users are encouraged to consider the relative (or

percent) difference between alternatives in terms of safety benefits. Users can refer to appendix A 

of the HSM Part C predictive method for the calibration procedure.(4)

Additional Guidance on Predictive Method

Users are encouraged to conduct evaluations within the parameters presented for interchange 

configurations discussed in this guide. However, the predictive method may not be applicable in 

some situations, or unique circumstances may not fit within the criteria for using the predictive 

method. In these situations, if crash prediction is necessary, users should consider applying the 

predictive method from chapters 18 and 19 of the HSM.

(4) If data required for the chapter 18 and 

19 predictive method are unavailable, and users opt to employ this tool, they should document 

any assumptions made, noting specific situations where the models have been extended for 

analysis. Further documentation should include additional factors that may contribute to a 

potential bias in safety estimation.

Practitioners should exercise caution when making decisions based purely on safety prediction 

models. Refer to the Conclusions and Recommendations section (chapter 7) of the report for 

additional insight into the applicability of model results to planning-level decisionmaking. 
17 

REFERENCES

1. Himes, S., V. Gayah, J. Gooch, G. O’Connor, and T. Le. Forthcoming. Safety 

Comparisons Between Interchange Types. Washington, DC: Federal Highway 

Administration.

2. FHWA. 2023. FHWA Interchange Configuration Safety Comparison Tool (software). 

3. Himes, S., V. Gayah, J. Gooch, G. O’Connor, and T. Le. 2023. Safety Comparison of 

Interchange Configurations. TechBrief No. FHWA-HRT-23-029. Washington, DC: 

Federal Highway Administration.

4. American Association of State Highway and Transportation Officials. 2010. Highway 

Safety Manual, 1st ed. Washington, DC: American Association of State Highway and 

Transportation Officials.


HRSO-10/03-23(WEB)E Recycled

Recyclable

Recommended citation: Federal Highway Administration,

Interchange Comparison Safety Tool User Guide 

(Washington, DC: 2023) https://doi.org/10.21949/1521985