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​These Frequently Asked Questions (FAQs) are based on the knowledge and experience of the developers and practitioners of the Highway Safety Manual, as well as various publications and websites, including the Highway Safety Manual discussion forum, the Federal Highway Administration’s Crash Modification Factor Clearinghouse, and more. FAQs are sourced from the Highway Safety Manual Frequently Asked Questions compendium, developed by the Transportation Research Board – Highway Safety Performance Committee’s (ANB25) User Liaison and Technology Facilitation Subcommittee. A PDF of the latest version of this document (June, 2018) can be found here.

Roadway Safety Management Process (Part B)

Network Screening (Chapter 4)

What is the difference between safety performance function (SPFs) for Part B network screening versus SPFs for Part C predictive methods?
Should minimum segment lengths be established for use in HSM Part B analyses?
How should crashes of different severities be considered in roadway network screening process?

Diagnosis (Chapter 5)

Should diagnostic analysis be performed only at locations showing elevated frequency or severity of crashes reflected by SPF?
How can CMFs be used / applied in the Countermeasure Selection process? (What is the role of CMFs in the Countermeasure Selection process?)

Economic Appraisal (Chapter 7)

How can CMFs be used and applied in the Economic Appraisal process? What is the role of CMFs in the Economic Appraisal process?
What economic measures indicate whether a project is economically justified?
How can economic appraisal methods be used for budget setting?
What are the impacts of using observed crash frequency in economic analyses?
Should economic or comprehensive crash costs be used in economic ANALYSES?
Can certain crash types or severities be ignored in economic analyses?
How should weighted crash costs be determined?
What are the impacts of using weighted crash costs in economic analyses?
Can different crash costs be used for different purposes?
What are the impacts of using conservative crash costs in economic analyses?

Prioritize Projects (Chapter 8)

What is the ideal method to prioritize projects or alternatives within a budget?
What is the ideal method to prioritize the projects or alternatives with the largest crash reduction regardless of cost?
When should the incremental benefit-cost analysis method be used?
What are independent and mutually exclusive projects, and how do they affect prioritization?

Safety Effectiveness Evaluation (Chapter 9)

What is the role of CMFs in Safety Effectiveness Evaluation?


Predictive Method (Part C)


What are the limitations of current HSM Part C crash predictive models?
Where can I find information on safety analysis for specific conditions such as animal crashes, Pedestrians and Bicycles, Parking, One-way Streets, and At-grade Rail Crossing?
What is the difference between safety performance functions (SPFs) for network screening versus SPFs for prediction?

Data Requirements and Preparation

Is there a minimum length criteria for segments to be used in HSM Part C analyses?
Are there advantages to having segments of a minimum length when using part c methods?
Is using estimated AADT values acceptable when applying HSM Part C Models?
For the Part C Predictive Methods, is there a minimum AADT that is needed to make HSM analysis valid?
Under what conditions is it appropriate to apply the EB procedure?
If the EB procedure is applicable for only some sites (homogeneous segments or intersections) within a project, is it appropriate to apply the EB to those sites and not others – or should EB only be used if it can be applied to all sites in a project?
Is it appropriate to compare results for alternative designs using EB to alternatives not using EB?
What is the difference between observed, predicted, and expected average crash frequency?
Is it appropriate to use CMFs from outside HSM Part C with the Part C models?
How should I best determine whether a roadway segment or intersection is rural or urban?
What if my site is not exactly like a site in the HSM?

Predictive Method for Rural Two-lane, Two-way Roads (Chapter 10)

Does Chapter 10 - Predictive Method for Rural Two-Lane, Two-Way Roads apply to unpaved roads?
Where can I get additional information about how to calculate the roadside hazard rating?

Specialized Procedures Common to All Part C Chapters (Appendix A)

How should default values in the HSM be handled?
Should separate calibration factors be developed for each crash type and severity level?
Should separate calibration factors be developed for different AADT ranges?
Should calibration factors be developed for portions of a highway network?
How often should calibration factors be updated?
What if I do not have a local calibration factor for the HSM Part C SPFs? What is the effect on the result?
Can calibration factors from SafetyAnalyst be used in HSM calculations?
How large should a site sample size be for calibration?
What should be done in the case of insufficient data for a certain facility type? can a valid calibration factor still be determined?
Is it important to replace the HSM default crash severity and collision type distribution tables by jurisdiction-specific data?
Can data assembled for the development of calibration factors be used for a recalibration effort? Can data assembled for the development of calibration factors be used for development of jurisdiction-specific SFPs?
How should an agency prioritize data collection efforts when undertaking the calibration of several Part C predictive models?
When should an agency develop jurisdiction-specific SPFs?
How much is gained in accuracy by using an agency-developed SPF rather than a calibrated SPF?
How should the usefulness of agency-developed SPFs be assessed to determine whether they are preferable to calibrated SPFs?
What data quality is needed for agency datasets used to develop SPFs?
If a highway agency develops agency-specific SPFs with its own data, and the agency-developed SPFs have a different shape from those found in HSM Part C, is there something wrong? Which SPF should be used?
Why do the shapes of SPF curves for a given facility type often vary markedly from one agency to another or from one region to another within a state? Are such variations real or are they artifacts of sample size or data quality issues?

Predictive Method for Freeways (Chapter 18)

How can I assess the safety tradeoffs between median barrier and flush median without sufficient clear zone?
How was ramp metering considered in the development of the freeway crash prediction models?
Does the freeway crash predictive method apply beyond the last interchange when a freeway transitions into a grade separated facility, and if so where does it end?
Why does lighting not have any CMF in freeway and ramp models? Is the effect insignificant or has there not been data to support model development for this?
Should the same criteria that are used to break the freeway segments into homogenous segments also be used to break speed‐change lanes to shorter speed‐change segments?
It appears from the examples that the CMF for weaving decreases with increasing traffic volumes. This seems counter intuitive.

Predictive Method for Ramps (Chapter 19)

Can the Predictive Method be applied to ramps providing two-way travel?
What is the difference between a connector ramp and entrance or exit ramp at service interchange?
For ramp terminals, the CMF for a channelization of a right turn lane is greater than 1.0. Why is this the case?


HSM-Related Tools

What HSM-related tools are available?

AASHTOWare Safety Analyst

What is Safety Analyst?
How is SafetyAnalyst related to the HSM?
How can my agency obtain the Safety Analyst software?
Why don’t the HSM Part C and Safety Analyst require the same input information?
Why don’t HSM Part C and Safety Analyst give similar results?
Can calibration factors from Safety Analyst be used in HSM calculations?

Interactive Highway Safety Design Model (IHSDM)

What is IHSDM?
How is IHSDM related to the HSM?
How can my agency obtain the IHSDM software?
Is IHSDM Technical Support available?
My agency wants to calibrate HSM Part C Predictive Models. How can IHSDM assist this effort?
If an agency has developed their own SPFs, can they be entered into IHSDM?
Can default crash severity and crash type distribution values be updated with agency-specific values in IHSDM?

HSM Spreadsheets

Which HSM Part C methods do the HSM spreadsheets (i.e., those produced under NCHRP Project 17-38 and State agency enhanced versions) implement?
What is ISATe and which HSM Part C methods do ISATe implement?

CMF Clearinghouse

What is the CMF Clearinghouse?
What is the purpose of the CMF Clearinghouse?
How does the CMF Clearinghouse relate to the HSM?
What is the difference between CMFs in the HSM and CMFs in the CMF Clearinghouse?
What does the star quality rating mean?
How is the star quality rating in the CMF Clearinghouse different from the notations (bold, italics, etc.) in the Highway Safety Manual?
How do I choose between CMFs in my search results that have the same star rating but different CMF values?
The CMF Clearinghouse presents both Crash Modification Factors and Crash Reduction Factors. What's the difference?
How do the Clearinghouse crash severity terms Fatal, Serious Injury, and Minor Injury relate to the KABCO injury scale?
How are states using the CMF Clearinghouse?


Does PLANSAFE Implement the Highway Safety Manual?