The Frequently Asked Questions are based on the knowledge and experience of the developers of the HSM as well as what we have learned so far about implementing the Program. If the information you need is not here, go to the User Discussion Forum to ask your question.

General Information

What Is the Highway Safety Manual, and Why Was It Developed?
Is the HSM Required?
How Is the Highway Safety Manual Related to the AASHTO Green Book and Roadside Design Guide?
Where Can I Find an Overview of the HSM?
Where Can I Find Information on Training?
Can I Download the FHWA Webinar Recordings?
Where Can I Find Other HSM Users?
Where can I get help with the manual?
Where do I find /send potential errata?
How do I suggest additions to the HSM?
When will the next edition of the manual be published?
Can a State Department of Transportation Use Federal Highway Safety Improvement Program Funds to Purchase The HSM?

 

HSM Related Tools

SafetyAnalyst

What is SafetyAnalyst?
How is SafetyAnalyst related to the HSM?
How can my agency obtain the SafetyAnalyst software?

Interactive Highway Safety Design Model (IHSDM)

What is IHSDM?
How is IHSDM related to the HSM?
My agency wants to calibrate HSM Part C crash prediction models. How can IHSDM assist this effort?
If an agency has developed their own Safety Performance Functions (SPFs), can they be entered into IHSDM?
Can default crash severity and crash type distribution values be updated with agency-specific values in IHSDM?

Crash Modification Factor (CMF) Clearinghouse

What is the CMF Clearinghouse?
What is the purpose of the CMF Clearinghouse?
What is a CMF?
The CMF Clearinghouse presents both Crash Modification Factors and Crash Reduction Factors. What's the difference?
I've seen the term "Accident Modification Factor" (AMF) before. Is that different than a Crash Modification Factor?
How can I apply multiple CMFs?
What does the star quality rating mean?
How is the star quality rating different from the notations (bold, italics, etc.) in the Highway Safety Manual (HSM)?
Are there available trainings related to the application of CMFs?
How does the CMF Clearinghouse relate to the Highway Safety Manual (HSM)?
How do you determine statistical significance?
Who uses CMFs and how are they used?
How are CMFs added to the Clearinghouse and what is the process for review?
How do I choose between CMFs in my search results that have the same star rating but different CMF values?
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?

PLANSAFE

Does PLANSAFE Implement the Highway Safety Manual (HSM)?

 

Roadway Safety Management Process – HSM Part B

Minimum Segment Length (for Part B Analyses)

Should minimum segment lengths be established for use in HSM Part B analyses? SafetyAnalyst?

Estimating Anual Average Daily Traffic (AADT) on Minor Cross Roads and Rural Low Volume Roads

When AADT values are estimated, results vary dramatically and data dispersion is extreme. Is using estimated AADT values (particularly on minor cross roads and rural, low volume roads) a good idea in SafetyAnalyst?

HSM Part B/SafetyAnalyst Methods v. "Traditional" Procedures

If the underlying techniques in HSM Part C and SafetyAnalyst are so great, why do they give the same results as the older, apparently flawed procedures?

 

Predictive Methods – HSM Part C

Minimum Segment Length (for Part C Analyses)

Should minimum segment lengths be established for use in HSM Part C analyses?

HSM Part C and IHSDM

How is IHSDM related to HSM Part C?

HSM Part C v. SafetyAnalyst

Why don't the HSM Part C and SafetyAnalyst require the same input information?
Why don't HSM Part C and SafetyAnalyst give similar results?
Why are the SPF equations in HSM Part C different from those in SafetyAnalyst?
Since the SafetyAnalyst and HSM Part C often give different answers, which one should be used?

Estimating Average Annual Daily Traffic (AADT) on Minor Cross Roads and Rural Low Volume Roads

When AADT values are estimated, results vary dramatically and data dispersion is extreme. Is using estimated AADT values (particularly on minor cross roads and rural, low volume roads) a good idea in HSM Part C and SafetyAnalyst?

The Empirical-Bayes (EB) Procedure

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?
If multiple alternative designs for a project are to be evaluated using HSM Part C methods and the EB procedure is applicable to one or more – but not all - alternative designs, should the EB procedure be used where applicable? Or, should EB only be used if it can be applied to all alternative designs in the project? In other words, is it appropriate to compare results (expected crashes) for alternative designs using EB to alternatives not using EB?

Calibration Factors – Use of Factors from SafetyAnalyst in HSM Part C (and vice versa)

Can calibration factors from SafetyAnalyst be used in HSM calculations?

HSM Part C Methods v. "Traditional" Procedures

If the underlying techniques in HSM Part C and SafetyAnalyst are so great, why do they give the same results as the older, apparently flawed procedures?

Use of CMFs from outside HSM Part C with Part C models

Is it appropriate to use CMFs from outside HSM Part C with the Part C models? For example, if a project includes all-way STOP-controlled intersections (which have no CMFs in HSM Part C), is it acceptable to apply CMFs for this type of intersection from HSM Part D, or other sources (e.g., the CMF Clearinghouse) to the Part C models?
Is there any guidance for applying multiple CMFs within a project (i.e., is there a limit to the number of CMFs that should be multiplied together?)

Unpaved Roads

Does Chapter 10 – Predictive Method for Rural Two-Lane, Two-Way Roads apply to unpaved roads?

Urban v. Rural Classification

The HSM states that "the definition of urban and rural areas is based on Federal Highway Administration (FHWA) guidelines which classify urban areas as places inside urban boundaries where the population is greater than 5,000 persons." However, agencies may have facilities that exhibit a "rural" cross section (i.e. paved shoulders and roadside ditches) within urban areas as defined above. They may also have a number of facilities that exhibit an "urban" typical section (i.e. curb & gutter, closed drainage system, etc.) outside of urban areas as defined above. In conducting HSM analyses, should the agency assign the facility type based on its geographic location as defined above, or based on its geometric features (presence of ditches vs. curb & gutter)?

 

Calibration

IHSDM Calibration Utility

My agency wants to calibrate HSM Part C Predictive Models. How can IHSDM assist this effort?

Calibration vs. Safety Performance Function (SPF) Development

How much is gained in accuracy by using an agency-developed safety performance function (SPF) rather than a calibrated SPF? Under what circumstances are calibrated SPFs satisfactory and under what circumstances is there a clear advantage for agencies that develop their own SPFs?
How should the usefulness of agency-developed SPFs be assessed to determine whether they are preferable to calibrated SPFs? (goodness-of-fit measures? overdispersion parameter values? direct application of both SPFs to test data sets other than the data used in their development?)

Calibration Factors – Use of Factors from SafetyAnalyst in HSM Part C (and vice versa)

Can calibration factors from SafetyAnalyst be used in HSM calculations?
Is it appropriate to develop separate calibration factors for each crash type and severity level? (For example, the Chapter 12 predictive method includes the prediction of multi-vehicle and single-vehicle crashes separately by severity. Should separate calibration factors be developed for each crash type and severity?)

Calibration when observed crashes per year for all sites combined and/or the number of sites are less than the recommended minimum

What should be done in the case of insufficient data for a certain facility type? For example, an agency wants to calibrate the model for 3-leg signalized intersections on rural 2-lane roadways, but all sites in the agency database combined have less than the recommended 100 crashes per year. Can a valid calibration factor still be determined?

 

Safety Performance Function (SPF) Development

Calibration vs. Safety Performance Function (SPF) Development

See "Calibration"

Data Quality for SPF Development

What data quality is needed for agency datasets used to develop SPFs?

Agency-Specific SPFs v. HSM/SafetyAnalyst 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 or SafetyAnalyst, is there something wrong? Which SPF should be used?

Shapes of SPFs – variations from one agency to another

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?

Minimum Segment Length (for SPF Development)

What should an agency do with all of the very short segments that do not meet minimum suggested threshold of 0.1-mi length?

IHSDM and Agency-Specific SPFs

If an agency has developed their own SPFs, can they be entered into IHSDM?

 

Crash Modification Factors (CMFs) – HSM Part D

What is a CMF?
What is meant by the "base condition?"
What is the numerical value of a CMF?
How do I apply a CMF?
In the HSM, what does the CMF standard error mean?
How do I select a Treatment/CMF?
Can a CMF have a different effect on different crash types?
How do I calculate the effect of multiple treatments at one site?
What is the difference between CMFs in the HSM and CMFs in the CMF Clearinghouse?
What is the difference between the CMFs in Part D and Part C of the HSM?
What does a CMF calculation look like?

Crash Modification Factor Clearinghouse

See "HSM-Related Tools"

CMFs in the Roadway Safety Management Process

How can CMFs be applied in the Roadway Safety Management Process?
How can CMFs be used/applied in the Countermeasure Selection process? (What is the role of CMFs in the Countermeasure Selection process?)
How can CMFs be used/applied in the Economic Appraisal process? (What is the role of CMFs in the Economic Appraisal process?)
What is the role of CMFs in Safety Effectiveness Evaluation?
What are potential benefits associated with applications of CMFs in the safety management process?
What are potential challenges to applying CMFs in the safety management process (and opportunities to overcome these challenges)?

 

Specific Technical Issues

How should default values in the HSM be handled?

Does the HSM Cover…

Guidelines for crash testing safety hardware?
One-way streets?
Pedestrians? Bikes?
Parking?
Animal crashes?
At-grade rail crossings?

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