Texas Department of Transportation

Interstate 37 in Texas 

Interstate Access 

Justification 

SAFETY DATA CASE STUDY 

FHWA-SA-21-076 

Federal Highway Administration Office of Safety 

Roadway Safety Data Program 

http://safety.fhwa.dot.gov/rsdp
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SAFETY DATA CASE STUDY FHWA-SA-21-076

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SAFETY DATA CASE STUDY FHWA-SA-21-076

1. Report No.

FHWA-SA-21-076

2. Government Accession 

No.

3. Recipient's Catalog No.

 

4. Title and Subtitle

Interstate 37 in Texas Interstate Access Justification

5. Report Date

February 2022

6. Performing Organization Code

7.Author(s) 

Kristin Kersavage and Ian Hamilton

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. Sarah Weissman Pascual provided editorial review and support for 

technical content. Funding for this effort provided in part by the Highway Safety Manual Implementation Pooled Fund, 

TPF-5(255).

16. Abstract

This case study presents a safety analysis performed by the Texas Department of Transportation (TxDOT) for an Interstate 

Access Justification Report. The Interstate 37 (I-37) corridor is an important freight route, critical hurricane evacuation 

linkage, and the only crossing of the Nueces River within 25 miles. This corridor is part of the National Primary Highway 

Freight System. US 77, which connects to I-37 at the end of the project area, is a part of the Texas Highway Freight 

Network. Several planning studies, including the I-37 Corridor Study Report, I-69 to I-410, identified the need for mainline 

widening and the replacement of the Nueces River Bridge. As a result, TxDOT planned a widening project along this 

stretch of interstate to accommodate greater traffic demand, necessitating additional travel lanes and several key design 

changes, including environmentally sensitive alterations to the Nueces River Bridge. TxDOT reviewed the study area’s 

historic crash frequency and rates to make targeted improvements to the project design. After this review, the TxDOT 

project team used the Highway Safety Software to predict crashes along I-37 for both a No Build scenario with no changes 

to the corridor and a Build scenario with proposed capacity, mobility, and safety alternatives. The results indicated that 

predicted crashes are expected to decrease with implementation of the proposed improvements compared to the existing 

conditions for both the base (2020) and design (2040) years. Total crashes are expected to decrease by 7.53 crashes (12.8 

percent) per year for the base year and by 13.44 crashes (14.3 percent) per year for the design year.

17. Key Words: 

Safety, Highway Safety Software, Highway Safety 

Manual, Interstate access, HSS

18. Distribution Statement

No restrictions.

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this 

page) Unclassified

21. No. of Pages

 15 

22. Price

Technical Documentation Page
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SAFETY DATA CASE STUDY FHWA-SA-21-076

Acronyms

Acronym Description

AADT annual average daily traffic

AASHTO American Association of State Highway and Transportation Officials 

CMF crash modification factor

CRIS Crash Records Information System

FHWA Federal Highway Administration

HCS Highway Capacity Software

HSM Highway Safety Manual 

HSS Highway Safety Software

IAJR Interstate Access Justification Report

SPF safety performance function

STARS Statewide Traffic Analysis and Report System

TX Texas

TxDOT Texas Department of Transportation
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SAFETY DATA CASE STUDY FHWA-SA-21-076

Table of Contents

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

Safety Performance Analysis ....................................................................................................... 3

Key Takeaways and Future Enhancements................................................................................ 7

Conclusions .................................................................................................................................... 8

References...................................................................................................................................... 9

List of Figures

Figure 1. Graphic. I-37 project location...................................................................................... 2

Figure 2. Graphic. Crash cluster locations with road narrowing (2010-2017). ...................... 5

 

List of Tables

Table 1. Historical crash data by year for the I-37 corridor..................................................... 4

Table 2. Study area crash rate compared with statewide average urban and....................... 6

Table 3. Predicted crashes for the existing conditions and proposed improvements .......... 7
 

SAFETY DATA CASE STUDY FHWA-SA-21-076

Executive Summary

This case study presents a safety analysis performed by the Texas Department of Transportation 

(TxDOT) for an Interstate Access Justification Report. The Interstate 37 (I-37) corridor is an 

important freight route, critical hurricane evacuation linkage, and the only crossing of the Nueces River 

within 25 miles. This corridor is part of the National Primary Highway Freight System. US 77, which 

connects to I-37 at the end of the project area, is a part of the Texas Highway Freight Network. 

Several planning studies, including the I-37 Corridor Study Report, I-69 to I-410, identified the need for 

mainline widening and the replacement of the Nueces River Bridge. As a result, TxDOT planned a 

widening project along this stretch of interstate to accommodate greater traffic demand, necessitating 

additional travel lanes and several key design changes, including environmentally sensitive alterations to 

the Nueces River Bridge. TxDOT reviewed the study area’s historic crash frequency and rates to make 

targeted improvements to the project design. After this review, the TxDOT project team used the 

Highway Safety Software to predict crashes along I-37 for both a No Build scenario with no changes to 

the corridor and a Build scenario with proposed capacity, mobility, and safety alternatives. The results 

indicated that predicted crashes are expected to decrease with implementation of the proposed 

improvements compared to the existing conditions for both the base (2020) and design (2040) years. 

Total crashes are expected to decrease by 7.53 crashes (12.8 percent) per year for the base year and 

by 13.44 crashes (14.3 percent) per year for the design year.
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SAFETY DATA CASE STUDY FHWA-SA-21-076

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 describes a safety analysis the Texas Department of Transportation (TxDOT) 

performed for an Interstate Access Justification Report (IAJR) to replace and upgrade an 

important and environmentally sensitive portion of interstate 15 miles west of Corpus Christi, 

TX. Interstate 37 (I-37) is a major connection between San Antonio and Corpus Christi, and 

the I-37 corridor in Nueces County and San Patricio County is a mix of urban and rural area 

types. The Nueces River bridge is a focal point for north/south traffic between these two urban 

areas, and the bridge required replacement. TxDOT used McTrans’ Highway Safety SoftwareTM

(HSS) module to predict crashes along a segment of I-37 from the Redbird Lane cross street 

overpass to the US 77 Interchange, shown in figure 1, and conduct a data-driven analysis of 

several proposed improvements in and around the Nueces River bridge. 
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SAFETY DATA CASE STUDY FHWA-SA-21-076

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

Note: The white location pins, white/black dashed line, north arrow, route markers, and county names and 

boundaries were added by the authors to delineate the project bounds.

Purpose and Need

The Corpus Christi Metropolitan Transportation Plan 2015-2040 projects population in Nueces and 

San Patricio counties to grow 18 percent between 2015 and 2050. The Corpus Christi 

Metropolitan Planning Organization also anticipates daily trips to grow more than 50 percent 

between 2006 and 2040. As a key regional facility, the I-37 corridor is an important freight 

route, critical hurricane evacuation linkage, and the only crossing of the Nueces River within 25 

miles. This corridor is also part of the National Primary Highway Freight System. US 77, which 

connects to I-37 at the end of the project area, is a part of the Texas Highway Freight 

Network. Several planning studies, including the I-37 Corridor Study Report, I-69 to I-410, 

identified the need for mainline widening and the replacement of the Nueces River Bridge. As a 

Figure 1. Graphic. I-37 project location.
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SAFETY DATA CASE STUDY FHWA-SA-21-076

result, TxDOT planned a widening project along this stretch of interstate to accommodate 

greater traffic demand, necessitating additional travel lanes and several key design changes, 

including environmentally sensitive alterations to the Nueces River Bridge.

To support the proposed design changes and better understand the safety impacts of this vital 

mobility project, the project team performed a safety analysis based on the planned series of 

capacity, mobility, and safety improvements along the corridor, including: 

• Replacing the existing Nueces River Bridge. 

• Widening I-37 from six lanes to eight lanes, providing one additional lane in each 

direction. 

• Installing shoulder rumble strips.

• Reconstructing exit and entrance ramps for local access. 

• Providing a vertical clearance of 18 ft 6 inches for underpasses along I-37. 

• Making minor profile adjustments to the frontage roads.

These improvements require an IAJR, which includes a safety analysis of the existing conditions 

and proposed improvements. 

Project Description

• Sponsoring agency: TxDOT. 

• Project location: Nueces County and San Patricio County, TX. 

• Project bounds and length of project: I-37 from Redbird Lane to US 77 Interchange

(2.6 miles). 

• Facility type(s): Multilane interstate freeway.

• Area type: Rural and urban.

• Project status (as of October 2021): Under construction. 

Safety Performance Analysis

This section provides an overview of the safety analysis methods for the existing conditions and 

proposed improvements and the final results.

Analysis Overview 

For the analysis, TxDOT obtained 8 years of crash data (2010 through 2017) from TxDOT’s 

Crash Records Information System (CRIS) and used the Statewide Traffic Analysis and Report 

System (STARS II) to obtain annual average daily traffic (AADT) information. Table 1 displays 

the historical crash data for the I-37 corridor by year used in the analysis. TxDOT initially 

mapped historic crashes and calculated crash rates to screen for high safety priority locations.
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SAFETY DATA CASE STUDY FHWA-SA-21-076

Table 1. Historical crash data by year for the I-37 corridor.

Year Total Crashes Crash Rate (per 100 million 

vehicle miles traveled)

2010 40 75.09 

2011 38 76.85 

2012 50 103.51 

2013 71 141.99 

2014 63 114.12 

2015 47 81.46 

2016 45 81.81 

2017 64 111.24 

Average 52 102.87

Analysis and Results

TxDOT’s crash analysis noted crash clusters along the study corridor with the most significant 

cluster occurring south of the I-37/US 77 interchange (38 percent of all crashes that occurred 

during the study period). This location involves a narrowing from four to three southbound 

lanes as I-37 approaches the Nueces River Bridge. Furthermore, the three northbound lanes in 

this location involve a shared center lane where I-37 and US 77 split (figure 2). This crash 

analysis supported the design decision to widen I-37 from three to four lanes in both directions

to avoid a lane drop in the southbound direction and segregate traffic into two lanes for both 

diverging routes. 
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SAFETY DATA CASE STUDY FHWA-SA-21-076

 © 2021 Google® © 2021 Maxar Technologies.

Note: The white circles, black numbers, and route markers were added by the authors 

to display the number of crashes at each location. 

Additional analysis of contributing crash factors also noted:

• Two-thirds of all crashes involved a single vehicle traveling straight as the first harmful 

event and manner of collision.

• Rain and wet road conditions comprised over 50 percent of all crashes.

• Nearly 40 percent of all crashes were speed related. 

• The majority of crashes (approximately 66 percent) resulted in no injuries, followed by 

crashes that resulted in a possible injury (approximately 17 percent). Four percent of 

crashes involved a fatality or serious injury.

The study team compared the crash rate along the I-37 corridor with the statewide average 

crash rates for urban and rural area types. The study corridor was originally built as a rural 

freeway, but it is increasingly accommodating urban traffic. Given the transitional nature of this 

shorter corridor (from the more urban development south of the river to the more rural land 

use north of the river), TxDOT used both metrics to assess the corridor’s performance

relative to peers. Table 2 indicates the I-37 crash rates were higher than the statewide average 

rural rates for all years. The I-37 crash rates were higher than the statewide average urban

crash rates for 2010 through 2014, but lower than the statewide average urban crash rates for 

2015 through 2017. This comparison to both contexts helped illustrate the critical safety need 

as part of this primarily widening project.

Figure 2. Graphic. Crash cluster locations with road narrowing (2010-

2017). 
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SAFETY DATA CASE STUDY FHWA-SA-21-076

Table 2. Study area crash rate compared with statewide average urban and rural 

crash rates.

Year

I-37 Crash Rate 

(per 100 million 

vehicle miles 

traveled)

Urban Statewide 

Average Crash Rate 

(per 100 million 

vehicle miles 

traveled) 

Rural Statewide 

Average Crash Rate 

(per 100 million 

vehicle miles traveled) 

2010 75.09 74.34 40.65 

2011 76.85 67.71 39.95 

2012 103.51 90.90 46.81 

2013 141.99 95.23 53.53 

2014 114.12 113.17 55.35 

2015 81.46 148.09 58.98 

2016 81.81 150.96 52.77 

2017 111.24 146.40 53.90 

Average 102.87 110.85 50.24

Before performing the crash prediction analysis, TxDOT applied crash modification factors 

(CMFs) to the existing crash numbers to assess potential crash reductions for implemented 

improvements. The two most important treatments included CMFs for increasing the number 

of lanes (0.74; CMF ID #8336) and installing shoulder rumble strips (0.89; Torbic et al., 2009). 

As noted in the existing conditions analysis, the transition from four to three lanes was a 

suspected contributor to crashes south of the US 77 interchange. Based on observed crash 

numbers, the addition of a travel lane in each direction is expected to result in a reduction of 

2.8 crashes per year. Furthermore, as most crashes along the I-37 corridor involved a single 

vehicle, TxDOT anticipated shoulder rumble strips to produce substantial safety benefits. Based 

on the number of roadway departure crashes on the corridor, TxDOT expected the rumble 

strips to reduce crashes by 2.4 per year. Combined, both countermeasures should reduce 

crashes along the study corridor by 5.2 per year.

To conduct its predictive analysis, the project team used crash and traffic data for analysis in 

HSS, which is a module in McTrans’ Highway Capacity SoftwareTM (HCS) package version 7.7. 

This downloadable, locally installed software package is widely used for capacity analysis 

projects, but the HSS package can effectively apply the HSM Part C Predictive Method. HCS 

provides a broad suite of traffic simulation tools, and the data inputs and results of HSS are 

consistent with the HSM. Agencies that wish to implement the HSS would need the same inputs 

as other HSM-related tools and software. TxDOT used the default safety performance 

functions (SPFs) in the HSS to predict crashes based on the traffic volume for the base year 

(2020) and the projected traffic volume for the design year (2040). TxDOT predicted base and 

design year crashes with the existing conditions, a No Build option where no improvements are 

implemented, as well as for a Build scenario that included the proposed improvements (table 3).
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SAFETY DATA CASE STUDY FHWA-SA-21-076

Table 3. Predicted crashes for the existing conditions and proposed improvements 

by severity.

Scenarios

Number of 

Predicted 

Total Crashes

per Year

Number of 

Predicted Fatal and 

Injury Crashes per 

Year

Number of 

Predicted Property 

Damage Only 

Crashes per Year

Base Year (2020) 

Existing Conditions 58.65 19.36 39.29 

Base Year (2020) 

Proposed 

Improvements

51.13 17.35 33.78 

Design Year (2040) 

Existing Conditions 94.28 30.29 63.98 

Design Year (2040) 

Proposed 

Improvements

80.84 26.24 54.60 

For both the base (2020) and design (2040) years, predicted crashes are expected to decrease 

with implementation of the proposed improvements compared to the existing conditions. 

Predicted total crashes are expected to decrease by 7.52 crashes (12.8 percent) per year for 

the base year and by 13.44 crashes (14.3 percent) per year for the design year.

Documentation and Use of Analysis Results

The project team completed the analysis as part of an IAJR. Based on the historic crash review, 

TxDOT identified target countermeasures that addressed the corridors specific issues 

(e.g., lane drops, traffic weaving, and single vehicle crashes). Additionally, TxDOT’s review of 

crash rates further reinforced that the character of the corridor was shifting from rural to 

increasingly urban, and that the widening and capacity improvements were justified. Finally, the 

predictive methods applied in HSS also showed a safety benefit to the proposed improvements 

and provided TxDOT with the confidence that the mobility improvements would also produce 

safety benefits.

Key Takeaways and Future Enhancements

Although TxDOT is in the process of developing Texas-specific SPFs, it relied on default, 

uncalibrated HSM models for the purposes of this analysis. The HSM Part C predictive methods 

were developed using data from States other than Texas, which could impact the accuracy of 

the predictions. Because of this, the HSM recommends applying calibration factors to the crash 

predictions to adjust the predicted crashes to local conditions. The project team did not have a 

calibration factor at the time of project development. As such, the project team focused on the 

relative comparison of crash predictions rather than the absolute value.
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SAFETY DATA CASE STUDY FHWA-SA-21-076

Conclusions

The I-37 corridor is an important freight route, critical hurricane evacuation linkage, and the 

only crossing of the Nueces River within 25 miles. The project team performed a safety analysis 

for implementing a series of capacity, mobility, and safety improvements along the corridor, 

which included replacing the existing Nueces River Bridge, widening I-37 from six to eight lanes, 

reconstructing entrance and exit ramps, providing an 18-ft 6-inch vertical clearance for 

underpasses, and making minor profile adjustments to frontage roads. Based on a review of 

existing crashes, TxDOT was able to make targeted improvements that addressed the 

corridor’s most critical safety needs. To assess anticipated future crashes based on the 

projected growth in traffic, the project team used HSS to implement the HSM Part C Predictive 

Method and compare predicted crashes for the existing geometric conditions to the predicted 

crashes for the proposed design conditions with improvements. The safety analysis indicated 

that the proposed corridor design is expected to reduce crashes when the improvements are 

implemented. While TxDOT would have preferred to use calibrated or State-specific SPFs in its 

analysis (as opposed to uncalibrated models from the HSM), ongoing research in this area will 

produce Texas-specific SPFs for TxDOT to use in future safety analyses across the State. 
9 

References

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

Safety Manual: 1st Edition, Washington, D.C. Resources available online: 

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

 Corpus Christi Metropolitan Planning Organization. (2014). Corpus Christi Metropolitan 

Transportation Plan: 2015-2040. Available online: https://www.corpuschristimpo.org/01_mtp/2015-2040/2014117_2015-40_MTP_final.pdf. 

 Federal Highway Administration. (2021). “CMF Clearinghouse” (website). Available 

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

Michael Baker International. (2020). Interstate Access Justification Report: Interstate 37. 

TxDOT: Corpus Christi District.

 Phone Interview on July 13, 2021, with Khalid Jamil.

 Texas Department of Transportation. (2018). I-37 Corridor Study Report: I-69 to I-410. 

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

 Torbic, D. J., Hutton, J., Bokenkroger, C. D., Bauer, K. M., Harwood, D., Gilmore, D.K.,

Dunn, J. M., Ronchetto, J. J., Donnell, E.T., Sommer, H., Garvey, P., Persaud, B., Lyon, 

C. (2009). Guidance for the Design and Application of Shoulder and Centerline Rumble 

Strips, NCHRP Report 641, Washington, D.C. Available online:

http://www.cmfclearinghouse.org/studydocs/nchrp_rpt_641-GuidanceRumbleStrips.pdf.

Contact

Texas Department of Transportation

Khalid Jamil, Section Director, Traffic Simulation & Safety Analysis, Design Division

Khalid.Jamil@txdot.gov