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                Interactive Highway Safety Design Model (IHSDM) FAQs: IHSDM and Highway Safety Manual (HSM)
    

    
    
    
      There is an important and direct relationship between IHSDM and AASHTO’s Highway Safety Manual. This section explains that relationship and the advantages of using IHSDM to apply HSM Part C predictive methods.

**Acronyms/Abbreviations**
The following acronyms/abbreviations are used in the FAQs below.

Crash Prediction Module (CPM)
	Design Consistency Module (DCM)
	Driver/Vehicle Module (DVM)
	Economic Analyses tool (EA tool)
	Federal Highway Administration (FHWA)
	Highway Safety Manual (HSM)
	Interactive Highway Safety Design Model (IHSDM)
	Policy Review Module (PRM)
	Research and Development (R&D)
	Traffic Analysis Module (TAM)

            Expand All
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        **What is the relationship between the IHSDM–HSM Predictive Method 2021 Release and AASHTO's HSM?**
        The HSM was developed by the Transportation Research Board and AASHTO, with AASHTO publishing the 1st Edition HSM in June 2010. The HSM contains four parts (A, B, C and D). Part C (Predictive Method) documents Crash Prediction Methodologies for three types of highways: rural two-lane highways, multilane rural highways, and urban/suburban arterials. In addition, the HSM 2014 Supplement contains Predictive Methods for Freeways and Ramps (HSM chapters 18 and 19, respectively). The IHSDM CPM includes — to the extent possible — faithful software implementation of the crash prediction methods documented in Part C of the HSM.

The 2016 Release expanded the IHSDM CPM by implementing draft HSM crash predictive methods for, six-or-more lane and one-way urban/suburban arterials developed under NCHRP Project 17-58 (Safety Prediction Models for Six-Lane and One-Way Urban and Suburban Arterials). The 2019 Release expanded the CPM further by implementing crash prediction methods for roundabouts, based on methods developed under NCHRP Project 17-70 (Development of Roundabout Crash Prediction Models and Methods) and documented inNCHRP Research Report 888 (Development of Roundabout Crash Prediction Models and Methods). The 2021 Release expanded the CPM further yet by implementing crash prediction methods for some intersection types not covered in the HSM, based on methods developed under NCHRP Project 17-68 (Intersection Crash Prediction Methods for the Highway Safety Manual) and documented in NCHRP Web-Only Document 297 (Intersection Crash Prediction Methods for the Highway Safety Manual). These new predictive methods are expected to be incorporated into the future 2nd Edition HSM.

For more information on the HSM, visit the HSM public website.
  


      **Why use IHSDM to apply HSM Part C Predictive Methods?**
        IHSDM offers many advantages for users wishing to implement HSM Part C methods, including:

IHSDM can evaluate many highway segments, intersections, and interchanges over a number of years.
	IHSDM supports Location-Based Data Input that automatically segments the highway into homogeneous segments as per HSM Part C. Data can be entered in stations or mileposts.
	IHSDM supports Site-Based Data Input, which is especially useful for projects where detailed, location-based geometry is not available.
	The IHSDM CPM includes a faithful implementation of the entire HSM Part C. For Freeway/Interchange projects that impact non-freeway highways, users can apply HSM Part C methods to other network components (e.g., cross-roads, connectors, local roads, and intersections).
	IHSDM can handle complex and simple alignments.
	IHSDM seamlessly evaluates a highway that changes facility type (e.g., rural two-lane to rural multilane).
	IHSDM implements the Empirical-Bayes procedure.
	The IHSDM Navigation Tree helps users to organize projects, highways, evaluations, and so on.
	IHSDM Evaluation Reports and Graphs provide extensive documentation of results/output.
	The IHSDM Highway Viewer provides graphical representation of the design.
	Extensive documentation is available via the IHSDM Help Browser
	A tutorial provides step-by-step comprehensive guidance.
	The IHSDM System Administration Tool (AdminTool) includes a Calibration Utility to help agencies implement HSM Calibration Procedures.In addition, IHSDM includes the EA Tool, which allows users to conduct economic analyses within IHSDM, using CPM evaluation results (crash frequencies and severities). IHSDM Economic Analyses Tool Help and an IHSDM Economic Analyses Tool Tutorial Lesson are available in the software bundle to help you get started.
  


      **My agency wants to calibrate HSM Part C crash prediction models. Can IHSDM assist in this effort?**
        Yes! A Calibration Utility is in the IHSDM AdminTool, to assist agencies in implementing the calibration procedures described in the Appendix to Part C of AASHTO’s HSM.

The Crash Prediction panel of the AdminTool includes these sections:

Calibration Data Sets.
	Crash Distribution Data Sets.
	Model Data Sets.The Calibration Data Sets interface provides a mechanism for users to enter, edit and organize the site data to be used to calculate the calibration factors for the various crash prediction models available in the IHSDM CPM (and, thus, in HSM Part C). In addition to containing the user-entered site data, each Calibration Data Set is also linked to a Crash Distribution Data Set and a Model Data Set.
Within the Calibration Data Set interface, the user can choose to either Calibrate Using Site Data or Manually Specify a Calibration Factor for each of the crash prediction models covering rural two-lane highways, rural multilane highways, urban/suburban arterials, roundabouts, freeway segments, and freeway ramps/interchanges (ramps, C-D roads and ramp terminals).

When running a CPM evaluation, the user indicates which Calibration Data Set to use in that particular evaluation. The CPM then applies the appropriate calibration factors from the user-selected Calibration Data Set.
Tutorial Lesson 10 (CPM Calibration) provides step-by-step instructions and hands-on exercises related to the calibration process.

    
          Last updated: Wednesday, January 4, 2023 U.S. DEPARTMENT OF TRANSPORTATION

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