Safety Comparison TECHBRIEF of Interchange Configurations

Research, Development, 

and Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

https://highways.dot.gov/research

INTRODUCTION

Interchange Access Requests (IARs) document potential safety 

performance impacts to justify new or modified interchanges. 

Typically, IARs are written early in the project planning and design 

process, with information generally consistent with conceptual 

design. The details required for using the Highway Safety Manual

(HSM) predictive methods for freeways, ramps, and ramp 

terminals are often unknown at that stage (American Association 

of State Highway and Transportation Officials (AASHTO) 2010). 

Without these details, accurately predicting crash frequency and 

severity for interchange components is difficult. Furthermore, 

aggregating site-by-site predictions may not fully capture the safety 

performance impacts when the project location is considered.

To further explore and address the safety-related components of an 

IAR application, the Federal Highway Administration (FHWA) 

developed planning-level models and tools to predict crash frequency 

and severity for an existing or proposed interchange. Model inputs 

were limited to the following:

• The details known at the planning and conceptual design stage.

• The variables expected to affect crash frequency and severity. 

The planning-level models allow analysts to compare the potential 

safety performance effects of freeway access and interchange design 

decisions at the planning level. 

STATE SURVEY

This project identified interchange configurations that account 

for at least 75 percent of those considered in IARs. The 

project team surveyed FHWA division offices to determine 

the most common interchange configurations considered and 

constructed and the planning-level factors considered in the 

safety analysis. Additional questions revealed that most IARs 

focus on urban applications and service interchanges. 

FHWA Publication No.: FHWA-HRT-23-029

FHWA Contact: Wei Zhang (ORCID: 0000-0002-5175-7527), HRSO-10, 

202-493-3317, wei.zhang@dot.gov

This document is a technical summary of the forthcoming 

Federal Highway Administration report Safety Comparisons Between 

Interchange Types.
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Figure 1. Graphic. Interchange configurations.

Source: FHWA.

APPLICABILITY

The predictive method applies to the interchange 

configurations shown in figure 1:

• Diamond interchange.

• Compressed diamond interchange.

• Tight diamond interchange.

• Roundabout diamond interchange.

• Single-point diamond interchange.

• Diverging diamond interchange.

• Partial cloverleaf type A.

• Partial cloverleaf type B.

• Partial cloverleaf type AB.

PREDICTIVE MODELS

The predictive models developed provide annual predicted 

crash frequency for the interchange area, as shown in 

figure 2. The interchange area includes the freeway 

mainline, crossroad, ramps, and ramp terminals within 

the bounds shown in figure 2. Supplemental analyses, 

such as the HSM, provide additional support for influence 

areas larger than the interchange area (AASHTO 2010). 

Predictive models provide crash frequency for fatal and 

injury crashes and for property damage only crashes based 

on freeway, ramp, and crossroad annual average daily 

traffic (AADT) volume and interchange configuration 

type. In addition to configuration type, the predictive 

models provide adjustments for the number of freeway 

through lanes, number of crossroad through lanes, 

area type, interchange skew angle, presence of nearby 

interchange gores on the freeway, presence of managed 

lanes on the freeway, number of crossroad left-turn lanes 

at terminals, and variation in ramp AADT.

Predictive models also provide adjustments for crash 

severity within fatal and injury crashes based on the 

following factors:

• Interchange configuration type.

• Presence of nearby interchange gores on the freeway.

• Presence of nearby intersections on the crossroad.

• Freeway AADT.

• Crossroad AADT.
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• Freeway posted speed limit.

• Crossroad posted speed limit.

• Number of right-turn conflicts with pedestrians.

• Number of freeway lanes.

• Number of crossroad lanes.

IMPLEMENTATION TOOL

The predictive method is implemented through a 

spreadsheet tool, which provides separate outputs for 

fatal and injury crashes and property damage only 

crashes. The spreadsheet tool also provides predictions 

for individual injury severity levels from the KABCO 

injury classification scale (K, A, B, and C levels) 

(National Highway Traffic Safety Administration n.d.). 

The implementation tool includes an estimate of the 

prediction’s variability so users can assess the predictions 

for interchange configuration types under consideration.

CONCLUSIONS

This project aimed to develop planning-level models and 

tools to predict crash frequency and severity for an existing 

or proposed interchange to support analysis for IARs. This 

project identified interchange configurations that account 

for at least 75 percent of those considered in IARs. The 

final interchange configurations included in the predictive 

models are shown in figure 1. The predictive models use 

planning-level features and traffic volume information 

to provide an estimated crash frequency for comparative 

analysis. The predictive method is implemented through a 

spreadsheet tool, allowing users to assess predictions for 

various interchange configuration types under consideration. 

Figure 2. Graphic. Interchange area for crash prediction.

Source: FHWA.
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Researchers—This study was conducted by VHB under contract DTFH6116D00040. Scott Himes (ORCID: 0000-

0003-2932-7691), Vikash Gayah (ORCID: 0000-0002-0648-3360), Jeff Gooch (ORCID: 0000-0003-1227-0967), 

Ginny O’Connor, and Thanh Le (ORCID: 0000-0001-7558-8978) were the main researchers for this study.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct distribution is being 

made to the FHWA divisions and Resource Center.

Availability—This TechBrief may be obtained at https://highways.dot.gov/research.

Key Words—Crash frequency, interchange configurations, predictive modeling, safety.

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 TechBrief 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.

REFERENCES

AASHTO. 2010. Highway Safety Manual, 1st ed. 

Washington, DC: American Association of State 

Highway and Transportation Officials. https://www.

highwaysafetymanual.org/Pages/About.aspx, last accessed 

December 19, 2022.

National Highway Traffic Safety Administration. n.d. 

Model Minimum Uniform Crash Criteria (MMUCC), 

4th ed. Definitions. Washington, DC: National Highway 

Traffic Safety Administration. https://safety.fhwa.dot.

gov/hsip/spm/conversion_tbl/pdfs/kabco_definitions.pdf, 

last accessed December 19, 2022. 

JANUARY 2023 FHWA-HRT-23-029

HRSO-10/01-23(WEB)E

Recommended citation: Federal Highway Administration, 

Safety Comparison of Interchange Configurations 

(Washington, DC: 2023) https://doi.org/10.21949/1521973