Minnesota Department of Transportation

Minnesota’s I-35 

Planning Study 

SAFETY DATA CASE STUDY 

FHWA-SA-21-077 

Federal Highway Administration Office of Safety 

Roadway Safety Data Program 

http://safety.fhwa.dot.gov/rsdp
i SAFETY DATA CASE STUDY FHWA-SA-21-077

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ii SAFETY DATA CASE STUDY FHWA-SA-21-077

1. Report No.

FHWA-SA-21-077

2. Government Accession 

No.

3. Recipient's Catalog No.

 

4. Title and Subtitle

Minnesota’s I-35 Planning Study

5. Report Date

February 2022

6. Performing Organization Code

7.Author(s) 

Kristin Kersavage

8. Performing Organization Report No.

9. Performing Organization Name and Address

Vanasse Hangen Brustlin, Inc (VHB)

940 Main Campus Drive 

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-June 2022 

14. Sponsoring Agency Code

FHWA

15. Supplementary Notes

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

Manual Implementation Pooled Fund, TPF-5(255). Sarah Weissman Pascual provided editorial review and support for 

technical content.

16. Abstract

This case study describes the Minnesota Department of Transportation’s (MnDOT’s) use of the Enhanced Interchange 

Safety Analysis Tool (ISATe) as a part of a planning study for an approximately 7-mile segment of Interstate 35 (I-35) in 

Carlton County. I-35 carries commuter and recreational traffic, and it is a main freight corridor for the area. Most of the 

bridges in the study area were 45 to 55 years old, and many of the bridges and pavement sections along the corridor were 

reaching the end of their service life and would require rehabilitation or replacement. This study identified safety, 

operational, and mobility needs along the corridor and developed alternatives which were then coordinated with upcoming 

infrastructure replacement projects. The I-35 planning study is an example of incorporating quantitative safety analysis as 

part of a comprehensive review process. MnDOT’s work to obtain stakeholder feedback and analyze existing safety 

conditions allowed the project team to establish safety as a project need. Additional evaluation of specific needs led 

MnDOT to develop a suite of alternatives that balanced environmental sensitivity with access and safety. ISATe provided 

the technical capacity to assess the study area’s baseline conditions based on existing geometrics and traffic, and estimate 

predicted crashes based on forecast conditions. The benefit-cost analysis provided further confidence that MnDOT’s 

alternatives will be cost effective and will provide a high level of benefit to users of the I-35 corridor.

17. Key Words: 

Safety, ISATe, Highway Safety Manual, Freeway, 

Interchange

18. Distribution Statement

No restrictions.

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this 

page) Unclassified

21. No. of Pages

 22 

22. Price

Technical Documentation Page
iii SAFETY DATA CASE STUDY FHWA-SA-21-077

Acronyms 

Acronym Description

A suspected serious injury 

AADT annual average daily traffic

AASHTO American Association of State Highway and Transportation Officials 

B suspected minor injury 

C possible injury

CSAH County State-Aid Highway

FA fatal and incapacitating injury 

FHWA Federal Highway Administration

HSM Highway Safety Manual 

ISATe Enhanced Interchange Safety Analysis Tool

ITS intelligent transportation system

K fatality

MnCMAT Minnesota Crash Mapping Analysis Tool

MnDOT Minnesota Department of Transportation

mph miles per hour

O property damage only

SEE Social, Environmental, and Economic

TH Trunk Highway

VMT vehicle miles traveled 
iv SAFETY DATA CASE STUDY FHWA-SA-21-077

Table of Contents 

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

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

Challenges ........................................................................................................................14

Conclusions.......................................................................................................................15

References ........................................................................................................................16

List of Figures

Figure 1. Graphic. I-35 project location. ......................................................................... 2

Figure 2. Graphic. I-35 study process. ............................................................................. 3

Figure 3. Graphic. Concept A Roundabout. ................................................................... 9

Figure 4. Graphic. Concept B Split Diamond with Collector Distributor. ................. 9

Figure 5. Graphic. Concept D Split Diamond with Eastbound Collector .................10

Figure 6. Graphic. Concept A Wider Bridges. .............................................................11

Figure 7. Graphic. Concept F Wider Bridges with Modified Ramps and Loops.......11

List of Tables

Table 1. Historical total crash data by segment for the I-35 corridor (2011-2015). .. 5

Table 2. Historical KA crash data by segment for the I-35 corridor (2011-2015)...... 5

Table 3. Historical crash data by interchange for the I-35 corridor (2011-2015)....... 6

Table 4. Comparison of ISATe predicted crashes and observed crashes by crash.... 7

Table 5. Comparison of ISATe predicted crashes for the existing conditions and.... 7

Table 6. Predicted crashes per year for the No Build scenario and alternatives.....12

Table 7. Benefit-cost analysis results for each concept...............................................13
v SAFETY DATA CASE STUDY FHWA-SA-21-077

Executive Summary

This case study describes the Minnesota Department of Transportation’s (MnDOT’s) use of the 

Enhanced Interchange Safety Analysis Tool (ISATe) as a part of a planning study for an approximately 7-

mile segment of Interstate 35 (I-35) in Carlton County. I-35 carries commuter and recreational traffic, 

and it is a main freight corridor for the area. Most of the bridges in the study area were 45 to 55 years 

old, and many of the bridges and pavement sections along the corridor were reaching the end of their 

service life and would require rehabilitation or replacement. This study identified safety, operational, 

and mobility needs along the corridor and developed alternatives which were then coordinated with 

upcoming infrastructure replacement projects. The I-35 planning study is an example of incorporating 

quantitative safety analysis as part of a comprehensive review process. MnDOT’s work to obtain 

stakeholder feedback and analyze existing safety conditions allowed the project team to establish safety 

as a project need. Additional evaluation of specific needs led MnDOT to develop a suite of alternatives 

that balanced environmental sensitivity with access and safety. ISATe provided the technical capacity to 

assess the study area’s baseline conditions based on existing geometrics and traffic, and estimate 

predicted crashes based on forecast conditions. The benefit-cost analysis provided further confidence 

that MnDOT’s alternatives will be cost effective and will provide a high level of benefit to users of the 

I-35 corridor.
1 SAFETY DATA CASE STUDY FHWA-SA-21-077

Introduction 

The Transportation Research Board’s Safety Performance and 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”). The Federal Highway Administration (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 this and other 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 case study presents a planning-level safety analysis published by the Minnesota Department 

of Transportation (MnDOT) in 2017. MnDOT used the Enhanced Interchange Safety Analysis 

Tool (ISATe) as a part of a planning study for an approximately 7-mile segment of Interstate 35 

(I-35) in Carlton County, MN (figure 1). There are 4 interchanges and 17 bridges in the study 

area, which carries commuter and recreational traffic and serves as a freight corridor. Many of 

the bridges and segments of pavement along the corridor are reaching the end of their service 

life. This study identified safety, operational, and mobility needs along the corridor which can be 

implemented in coordination with upcoming infrastructure replacement projects. The planning 

study included several components: 

• Traffic operations analysis. 

• Safety analysis. 

• Traffic forecast. 

• Social, environmental, and economic (SEE) assessment. 

• Existing infrastructure evaluation. 

• Corridor design review. 

Although the I-35 Planning Study used these metrics to inform the future design and function of 

the corridor, this case study specifically focuses on the relevant components of MnDOT’s safety 

analysis.
2 SAFETY DATA CASE STUDY FHWA-SA-21-077

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

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

Purpose and Need

I-35 carries commuter and recreational traffic, and it is a primary freight corridor for the area.

Most of the bridges in the study area were 45 to 55 years old, and many of the bridges and

pavement sections along the corridor were reaching the end of their service life. These would

require rehabilitation or replacement. This study identified safety, operational, and mobility

needs along the corridor and developed alternatives which were then coordinated with

upcoming infrastructure replacement projects. The I-35 Planning Study was divided into three

phases (figure 2):

• Phase 1 – Purpose and Need/Issue Development.

• Phase II – Development and Alternative Evaluation.

• Phase III – Recommendations and Implementation Plan.

Figure 1. Graphic. I-35 project location.
3 SAFETY DATA CASE STUDY FHWA-SA-21-077

MnDOT evaluated existing safety conditions as part of a broad Phase I, Purpose and Need 

Identification process. For Phase II, Alternatives Development, the project team used ISATe to 

compare the safety performance of the existing conditions and alternatives and determine how 

each would impact safety performance. The project team also used ISATe results to perform a 

benefit-cost analysis to determine if the alternatives are economically justified. The project team 

then identified recommended alternatives in Phase III, Recommendations and Implementation 

Plan, and prioritized the projects.

Project Description

• Sponsoring agency: MnDOT.

• Project location: Carlton County, MN.

• Project bounds and length of project: 7 mi along I-35 from 2 mi south of the I-35/

Trunk Highway (TH) 210 interchange to the I-35/County State-Aid Highway (CSAH) 61

interchange.

• Facility type(s): 4-lane, divided Interstate highway.

• Area type: Rural.

• Project status (as of October 2021): Construction of the roundabout at I-35 and

TH 33 completed in 2018.

Figure 2. Graphic. I-35 study process. 

© MnDOT
4 SAFETY DATA CASE STUDY FHWA-SA-21-077

Safety Performance Analysis

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

final results.

Existing Conditions Safety Analysis

The I-35 Planning Study obtained information on existing safety concerns and future needs from 

several sources, including public engagement meetings and historical crash analysis. The project 

team obtained the most recent five years of crash data available at the time the analysis (2011 

through 2015) from the Minnesota Crash Mapping Analysis Tool (MnCMAT).

1 The I-35 study 

corridor began two miles south of the I-35/TH 210 interchange and extended to the I-35/CSAH 

61 interchange. The I-35 analysis investigated freeway segments and interchanges separately.

The project team segmented mainline crash data into the following sections for analysis:

• Approximately 2 mi south of TH 210 to the TH 210 interchange.

• TH 210 interchange to the TH 33 interchange.

• TH 33 interchange to the TH 45 interchange.

• TH 45 interchange to the CSAH 1 interchange.

• CSAH 1 interchange to the CSAH 61 interchange.

For the safety analysis, the project team compared the historical crash data with the statewide 

average crash rate and calculated the critical crash rate (i.e., comparison of actual crash rate to 

expected crash rate) and crash rate index (i.e., the calculated crash rate divided by the critical 

crash rate at each location) to identify initial safety concerns. A crash rate index greater than 

one indicates that a location has a crash rate higher than the critical crash rate and represents a 

potential safety need. Tables 1 and 2 display the historical crash data for the segments along the

I-35 corridor by total crashes and fatal (K) and suspected serious injury (A)2

 crashes.

1 MnCMAT has since been replaced by the MnCMAT2 system. 2 MnDOT defined A injury crashes as “incapacitating injury” at the time of the study.
5 SAFETY DATA CASE STUDY FHWA-SA-21-077

Table 1. Historical total crash data by segment for the I-35 corridor (2011-2015).

Start of 

Segment

End of 

Segment

Segment 

Length 

(mi)

Annual 

Average 

Daily Traffic 

(AADT)

Total 

Crash 

Frequency

Calculated

Crash 

Rate1 

Critical 

Crash 

Rate

Crash 

Rate 

Index2

2 miles 

south of 

TH 210

TH 210 2.00 16,400 31 0.52 0.68 0.76

TH 210 TH 33 1.47 26,000 45 0.65 0.66 0.97

TH 33 TH 45 1.07 17,600 21 0.61 0.76 0.80

TH 45 CSAH 1 2.26 27,000 64 0.57 0.62 0.93

CSAH 1 CSAH 61 2.5 27,000 61 0.49 0.61 0.81

Total N/A N/A N/A 222 N/A N/A N/A 

1 Crashes per million vehicle miles traveled (VMT). 2 Crash rate divided by the critical crash rate.

Table 2. Historical KA crash data by segment for the I-35 corridor (2011-2015).

Start of 

Segment

End of 

Segment

Segment 

Length 

(mi)

Annual 

Average 

Daily Traffic 

(AADT)

KA Crash 

Frequency

Calculated

Crash 

Rate1 

Critical 

Crash 

Rate

Crash 

Rate 

Index2

2 miles 

south of 

TH 210

TH 210 2.00 16,400 2 3.34 0.79 4.22

TH 210 TH 33 1.47 26,000 1 1.44 0.78 1.84

TH 33 TH 45 1.07 17,600 1 2.90 0.84 3.45

TH 45 CSAH 1 2.26 27,000 2 1.79 0.75 2.38

CSAH 1 CSAH 61 2.5 27,000 0 0.00 0.75 0.00

Total N/A N/A N/A 6 N/A N/A N/A 

1 Crashes per 100 million VMT.

2 Crash rate divided by the critical crash rate.

The project team also assigned crashes to the following interchanges:

• I-35 and TH 210

• I-35 and TH 33

• I-35 and TH 45

• I-35 and CSAH 1
6 SAFETY DATA CASE STUDY FHWA-SA-21-077

Since there were no standard crash rates for interchanges, the project team developed average 

crash rates using interchanges in out-state Minnesota (out-state referring to MnDOT Districts 

1 through 4 and 6 through 8; i.e., those outside of the Minneapolis/St. Paul area Metro District). 

Metro District interchanges are more urban with higher traffic volumes, and these locations did 

not suit the I-35 context. Table 3 displays the historical crash data for the interchanges along 

the I-35 corridor by total crashes and fatal and suspected serious injury crashes. Four out of the 

five segments and three out of four interchanges had a KA injury crash rate greater than the 

statewide average.

Table 3. Historical crash data by interchange for the I-35 corridor (2011-2015).

Location 

Description

Interchange 

Type

Total 

Crash 

Frequency

Total 

Crash 

Rate1 

Critical 

Crash 

Rate2

Total 

Crash 

Rate 

Index3

KA Crash 

Frequency

KA Crash 

Rate4 

Critical 

Crash 

Rate2

KA 

Crash 

Rate 

Index3

TH 210/W 

of Carlton 

County

Diamond 34 0.64 1.03 0.63 1 1.90 1.21 1.57

TH 

33/Cloquet

Full 

Directional 72 1.09 1.00 1.09 5 7.55 1.19 6.36

TH 

45/Scanlon

Folded 

Diamond 54 1.01 1.03 0.98 0 0.00 1.21 0.00

CSAH 1 

Near Esko Diamond 22 0.41 1.03 0.40 2 3.73 1.21 3.09

Total N/A 182 N/A N/A N/A 8 N/A N/A N/A 

1 Crashes per million entering vehicles. 2 Average for the specified interchange types in out-state Minnesota. 3 Crash rate divided by the critical crash rate.

4 Crashes per 100 million entering vehicles.

Crash Prediction Analysis and Results

ISATe is a Microsoft ExcelTM-based spreadsheet crash prediction tool that automates the 

methods in HSM Chapters 18 and 19. These pertain to freeways and ramps and quantify the 

safety implications of different freeway design elements based on geometric design and the 

average crash frequency. The project team used ISATe to analyze baseline conditions, as well as 

future alternatives, including a future year (2040) “No Build” alternative and alternatives with 

safety improvements.

Baseline Conditions

The ISATe crash analysis for the baseline conditions predicted crashes on all mainline segments 

and ramps between TH 210 and TH 45 based on existing road geometry, traffic control, and 

traffic volumes. Table 4 compares the historical average observed crashes and the uncalibrated

predicted crashes from ISATe. Results indicate that the total predicted crashes (46.9) are 

similar to the observed crashes (48.6). This indicated that ISATe was closely matching observed 

conditions, with a difference of less than 5 percent between predicted and observed crashes.
7 SAFETY DATA CASE STUDY FHWA-SA-21-077

Table 4. Comparison of ISATe predicted crashes and observed crashes by crash 

severity – existing conditions.

Crash Severity

Average Observed 

Crashes 

(crashes/year)

ISATe Predicted 

Crashes 

(crashes/year)

Fatal 0.2 0.6 

Suspected Serious Injury 1.6 1.6 

Suspected Minor Injury (B) 5.0 6.9 

Possible Injury (C) 9.0 9.2 

Property Damage Only (O) 32.8 28.6 

Total 48.6 46.9

Year 2040 No Build Safety Analysis

Using forecasted traffic volumes for the year 2040, the project team performed an ISATe safety 

analysis for a No Build scenario with no other changes to the existing geometric and traffic 

control conditions. The traffic forecasts assumed a 1.5-percent growth in annual volumes. Table 

5 displays the predicted crashes for the No Build alternative and the existing conditions. The 

results indicate that crashes are expected to increase in the future due to the traffic volume 

growth.

Table 5. Comparison of ISATe predicted crashes for the existing conditions and No 

Build alternative.

Crash Severity

Predicted Crashes for 

Existing Conditions 

(crashes/year)

Predicted Crashes for 

No Build Alternative 

(2040; crashes/year)

Fatal 0.6 0.8 

Suspected Serious Injury 1.6 2.0 

Suspected Minor Injury 6.9 9.0 

Possible Injury 9.2 12.5 

Property Damage Only 28.6 39.0 

Total 46.9 63.2

Alternatives Safety Analysis

The I-35 planning study conducted a comprehensive review of existing safety, mobility, and 

environmental concerns along the corridor. MnDOT developed planning-level concepts for the 

corridor to improve safety and mobility based on: 
8 SAFETY DATA CASE STUDY FHWA-SA-21-077

• An existing traffic operations analysis.

• An existing safety analysis and SEE assessment.

• An existing infrastructure evaluation.

• A corridor design review to identify corridor issues and deficiencies.

Environmental review and stakeholder input revealed that the interchanges at TH 33 and TH 45 

needed additional design evaluation to address access and safety concerns for the corridor as a 

whole. 

The concepts for the I-35 and TH 33 interchange included: 

• Concept A: Roundabout.

• Concept B: Split Diamond.

• Concept C: Split Diamond North Frontage Only.

• Concept D: Split Diamond South Frontage Only.

• Concept E: Tight Diamond.

• Concept F: Diverging Diamond.

The planning level concepts for the I-35 and TH 45 interchange included: 

• Concept A: Wider TH 45 Bridges.

• Concept B: Four Loops Only.

• Concept C: Folded Diamond West.

• Concept D: Bridge Braid Ramps.

• Concept E: Tight Diamond.

• Concept F: Wider TH 45 Bridges with Modified Ramps and Loops.

MnDOT reviewed each alternative for disqualifying flaws and considered right of way and 

environmental impacts in addition to potential safety and access benefits. The project team and 

project stakeholders narrowed the preferred list of alternatives to five planning level concepts

for detailed traffic and safety analyses. These concepts included: 

TH 33 Interchange

• Concept A: Single Roundabout (figure 3).

• Concept B: Split Diamond with Collector Distributor (figure 4).

• Concept D: Split Diamond with Eastbound Collector Distributor (figure 5).
9 SAFETY DATA CASE STUDY FHWA-SA-21-077

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

Note: The black lines and street name text were added by the authors to delineate the project bounds.

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

Note: The black lines and street name text were added by the authors to delineate the project bounds.

Figure 3. Graphic. Concept A Roundabout. 

Figure 4. Graphic. Concept B Split Diamond with Collector Distributor. 
10 SAFETY DATA CASE STUDY FHWA-SA-21-077

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

Note: The black lines and street name text were added by the authors to delineate the project bounds.

TH 45 Interchange

• Concept A: Wider Bridges (figure 6).

• Concept F: Wider Bridges with Modified Ramps and Loops (figure 7).

Figure 5. Graphic. Concept D Split Diamond with Eastbound Collector Distributor. 
11 SAFETY DATA CASE STUDY FHWA-SA-21-077

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

Note: The black lines and street name text were added by the authors to delineate the project bounds.

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

Note: The black lines and street name text were added by the authors to delineate the project bounds.

Figure 6. Graphic. Concept A Wider Bridges. 

Figure 7. Graphic. Concept F Wider Bridges with Modified Ramps and Loops. 
12 SAFETY DATA CASE STUDY FHWA-SA-21-077

The project team used ISATe to perform a safety analysis for each of the five concepts using 

the proposed geometric changes for TH 33 and TH 45. TH 33 Concepts B and D are expected 

to divert traffic onto I-35 due to the additional access to the freeway. As a result, the project 

team forecasted traffic on local access roads to account for the route diversion. The project 

team analyzed both the change in crashes on the local system as well as the freeway to account 

for the additional traffic accessing I-35. 

Table 6 displays the predicted crashes from ISATe for the No Build scenario and four of the 

concepts. TH 33 Concept A was not included in the analysis since ISATe cannot analyze 

roundabouts, and a roundabout at that terminal is included in all alternatives. MnDOT noted 

that since all alternatives included this design, excluding it from ISATe would not bias the 

comparison.

Table 6. Predicted crashes per year for the No Build scenario and alternatives. 

Scenario System K A B C O Total

No Build

Freeway 0.8 2.0 9.0 12.5 39.0 63.2 

Change on 

Local System - - - - - - 

Total 0.8 2.0 9.0 12.5 39.0 63.2

TH 33 Concept B 

(Split Diamond 

with

Collector 

Distributor) 

Freeway 0.8 2.1 9.1 12.7 40.3 65.0 

Change on 

Local System -0.06 -0.11 -0.61 -1.22 -3.60 -5.61

Total 0.7 2.0 8.5 11.5 36.7 59.4

TH 33 Concept D

(Split Diamond 

with

Eastbound 

Collector

Distributor) 

Freeway 0.8 2.1 9.1 12.6 39.8 64.3 

Change on 

Local System -0.05 -0.09 -0.50 -1.05 -3.12 -4.82

Total 0.7 2.0 8.6 11.6 36.7 59.5

TH 45 Concept A

(Widen TH 45 

Bridges) 

Freeway 0.7 2.0 8.7 12.2 38.2 61.8 

Change on 

Local System - - - - - - 

Total 0.7 2.0 8.7 12.2 38.2 61.8

TH 45 Concept F

(Widen TH 45 

Bridges with 

Modified Ramps 

and Loops) 

Freeway 0.7 1.9 8.3 11.3 37.2 59.4 

Change on 

Local System - - - - - - 

Total 0.7 1.9 8.3 11.3 37.2 59.4
13 SAFETY DATA CASE STUDY FHWA-SA-21-077

The results indicate that total crashes are expected to decrease for all concepts (TH 33 

Concepts B and D and TH 45 Concepts A and F) when compared to the No Build alternative, 

when considering the change in crashes on both the freeway and local system. For TH 33 

Concepts B and D, total crashes are expected to increase on the freeway, while crashes are 

expected to decrease on the local system due to traffic diverting from the local system to the 

freeway. However, when considering the change in crashes on both the freeway and local 

system, TH 33 Concepts B and D experience a total expected crash reduction compared to the 

No Build scenario.

Documentation and Use of Analysis Results

Using the aforementioned safety analysis, the project team performed a benefit-cost analysis 

using cost estimates for each concept (table 7). Crash costs reflect MnDOT’s 2016 statewide 

estimates for each crash severity level (i.e., what was available at the time of the study). Benefits 

included travel time and delay (e.g., vehicle hours traveled), operations costs (e.g., VMT), and 

crashes reduced, while the costs include initial capital costs (separated by component 

categories and service life), operation and maintenance costs, and remaining capital value (i.e., 

the value of the improvement beyond the 20-year study period). Examples of remaining capital 

value included right of way acquisition, grading, drainage, and utility relocation. The benefit-cost 

analysis determined that all design concepts result in a benefit-cost ratio greater than 1.0, 

indicating the improvements are economically justified. TH 33 Concepts B and D result in the 

highest benefit-cost ratios, as well as the largest gross benefit.

Table 7. Benefit-cost analysis results for each concept.

TH 33 Concept 

B (Split 

Diamond with

Collector

Distributor) 

TH 33 Concept D

(Split Diamond 

with Eastbound

Collector

Distributor) 

TH 45 

Concept A 

(Widen 

TH 45 

Bridges) 

TH 45 Concept F 

(Widen TH 45

Bridges with

Modified Ramps

and Loops) 

Benefits $46.9 million $35.1 million $5.0 million $14.6 million

Costs $8.0 million $4.6 million $3.6 million $9.2 million

Benefitcost ratio 5.9 7.6 1.4 1.6 

In addition to quantitative safety analysis, the I-35 planning study documented additional

recommended improvements which were developed in coordination with stakeholders and the 

public. These forums included stakeholder workshops, a public open house, a booth at a day 

event on the Fond du Lac Reservation, and online engagement (social media and study 

webpage). Based on the study process, evaluation, and coordination with stakeholders and the 

public, MnDOT developed a recommended list of improvements. MnDOT prioritized these

into three groups to be programed and completed as funding becomes available. 
14 SAFETY DATA CASE STUDY FHWA-SA-21-077

The first group of projects include:

• Clearing and grubbing.

• Truck rollover warning sign at TH 45.

• CSAH 1 stop sign and stop bar.

The second group of projects include:

• TH 33 Concept B – Split Diamond with Collector Distributor or TH 33 Concept D –

Split Diamond with Eastbound Collector Distributor.

• TH 45 Concept A – Widen TH 45 Bridges or TH 45 Concept F – Widen TH 45 Bridges

with Modified Ramps and Loops.

• Moorhead Overpass – Widen bridge to accommodate a trail or build a separate trail

bridge.

The third group of projects include:

• Improve I-35 horizontal alignment at TH 33 to meet 70-mph curve.

• TH 210 Entrance Ramps – Reconstruct ramps to meet 50-mph vertical curve or

lengthen acceleration lanes.

Challenges

MnDOT noted a few challenges as part of its analysis and planning processes. Although the 

ISATe can be used to perform a safety analysis and predict crashes, ISATe cannot predict 

crashes for all intersection geometries. The terminal intersection at the TH 33 interchange, 

which is a roundabout, could not be included in the safety analysis. However, MnDOT noted 

that excluding it would not bias the planning-level comparison of the final design concept 

alternatives, as all alternatives considered a roundabout at that interchange terminal.

More broadly, planning studies of this size and complexity must often contend with rapidly 

changing conditions and external decisions. Cloquet, MN, which contains part of the study area, 

is growing and constantly changing. The I-35 study took a comprehensive look at the current 

and expected conditions at the time, but the community and road conditions often change 

before a project can advance to the next stage. For example, a reduced conflict intersection 

project was subsequently funded on TH 33 just outside of the planning limits of the I-35 study. 

Although the intersection project should improve safety, it was not included or modeled in this 

study. 

Additionally, the project team noted challenges related to public engagement. First, members of 

the public who gave input into the project may not be the same people who work or live in the 

area. Second, it can be difficult for members of the public who are not familiar with safety 
15 SAFETY DATA CASE STUDY FHWA-SA-21-077

research to understand modeled crashes, as they may not be intuitive for everyone. This may 

cause people to not accept aspects of the project or analysis and reinforces the need to convey 

safety benefits in relatable terms depending on the audience.

Conclusions

The I-35 planning study is an example of incorporating quantitative safety analysis as part of a 

comprehensive review process that includes other dimensions of the transportation system

(i.e., mobility, accessibility, and environmental stewardship). MnDOT’s work to obtain 

stakeholder feedback and analyze existing safety conditions allowed the project team to 

establish safety as a project need. Further evaluation of specific needs led MnDOT to develop a 

suite of alternatives that balanced environmental sensitivity with access and safety. ISATe 

provided the technical capacity to assess the study area’s baseline conditions based on existing 

geometrics and traffic and estimate predicted crashes based on forecast conditions. 

Furthermore, the benefit-cost analysis provided further confidence that MnDOT’s alternatives 

will be cost effective, as well as provided a high level of benefit to users of the I-35 corridor.
16 

References

 AASHTO. (2011). “Part C – Predictive Method” (website). Available online: 

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

 SRF Consulting Group, Inc. (2017). I-35 Planning Study Final Report. 

Contact

Minnesota Department of Transportation

Derek Leuer, State Traffic Safety Engineer

derek.leuer@state.mn.us