907.7 Highway Safety Manual

907.7.1 What is the Highway Safety Manual?

AASHTO's Highway Safety Manual (HSM) is a document used to quantify the safety impacts from decisions made in planning, design, operations and maintenance. The HSM can be used to:

• Identify sites with the highest potential for severe or total crash reduction (network screening)
• Identify contributing factors to crashes and the countermeasures for these factors.
• Evaluate the economic impact of safety countermeasures (cost/benefit analysis).
• Evaluate design alternatives and prioritize by safety impact (design prioritization).
• Predict severe and total crashes on roadways (traffic safety analysis).

The HSM is divided into four primary sections (topics):

• Part A: Introduction, Human Factors, and Fundamentals
• Part B: Roadway Safety Management Process
• Part C: Predictive Method
• Part D: CMF Applications Guidance

While there are four primary sections (as mentioned above), these sections are divided into four books as follows:

• Volume 1: Part A (chapters 1 – 3) and Part B (chapters 4 – 9)
• Volume 2: Part C (chapters 10 – 12)
• Volume 3: Part D (chapters 13 – 17)
• Supplement: Predictive Method for Freeways and Predictive Method for Ramps (chapters 18 – 19)

MoDOT has prioritized the implementation of HSM Part C for Traffic Safety analysis. More information on when MoDOT uses the HSM may be found in EPG 907.7.2 When is the Highway Safety Manual Used?

907.7.2 When is the Highway Safety Manual Used?

MoDOT requires an HSM analysis be completed for all applicable safety-related Design Exceptions. Safety related changes could include, but are not limited to lane width, shoulder width, shoulder type, rumble strips, turn lanes, horizontal alignment, grade, median width, side slopes and lighting. An example of such a scenario could be if a contractor wants to use 4 ft. shoulders instead of the 6 ft. shoulders stated in the design. An HSM analysis would provide an expected crash frequency change due to this particular design exception and would aid the designer in evaluating such a request.

An HSM analysis should be completed for any non-systemic safety project which utilizes Highway Safety Improvement Program (HSIP) funds. Systemic safety projects (e.g. rumbles, shoulders, striping, etc.) are not required to have an HSM analysis due to the nature of their implementation. A cost/benefit analysis will still need to be completed for all non-systemic projects using safety funds.

While a HSM analysis is only required for a design exception, it is a valuable tool which MoDOT makes available for assistance in planning, design, operations and maintenance. Some examples of when MoDOT staff might use the HSM are:

• A Transportation Planner wishes to prioritize spending based on safety needs. An HSM analysis may be run on several locations to prioritize which locations receive funding.
• A Highway Designer wishes to compare two design alternatives. An HSM analysis may be done to compare the safety impacts of each alternative.
• A Traffic Studies Specialist wishes to analyze the safety of a roadway segment to determine if centerline rumble strips may be needed. An HSM analysis may be done to compare the expected crash frequency with the observed crash frequency.
• A Traffic Studies Specialist has a customer call asking about the safety of a certain roadway. An HSM analysis may be done to compare the predicted crash frequency with the observed (or expected, if available) crash frequency.
• A Traffic Operations Engineer wishes to prioritize a list of potential safety improvements throughout their district. The HSM can provide a B/C ratio to rank each individual improvement.

907.7.3 How is a Location Specific HSM Analysis Performed? (Analysis of Alternative Designs and Strategy Specific Evaluations)

One major advantage of utilizing the HSM Part C and D is that it provides a means of predicting the safety impact from a particular countermeasure. Using HSM Part C/D methodology, a user may find a predicted number of crashes with the new countermeasure and compare that with the expected number of crashes for this roadway. The following section provides a step by step process in performing a location specific HSM analysis.

Step 1: Determine the type of study area (Intersection or Segment) and select the segment or intersection type from the list below.

The following roadway segment types are supported by the HSM:

• Rural Two Lane, Two Way Roads
• Rural Multilane Highways
• Urban and Suburban Arterials
• Two Lane Undivided Arterials

• Three Lane Arterials including TWLTL

• Four Lane Undivided Arterials

• Four Lane Divided Arterials

• Five Lane Arterials including TWLTL

• Freeways

• Rural Freeway Segment with 4–8 lanes

• Urban Freeway Segment with 4–8 lanes

• Freeway Speed Change lanes associated with entrance ramps and exit ramps

• Interchanges (ramps)

The following intersection types are supported by the HSM:

• Three leg stop controlled on Rural Two Lane, Two Way Roads
• Four leg stop controlled on Rural Two Lane, Two Way Roads
• Three leg stop controlled on Rural Multilane Highways
• Four leg stop controlled on Rural Multilane Highways
• Three leg stop controlled on Urban and Suburban Arterials
• Four leg stop controlled on Urban and Suburban Arterials
• Four leg signalized on Rural Two Lane, Two Way Roads
• Four leg signalized on Rural Multilane Highways
• Three leg signalized on Urban and Suburban Arterials
• Four leg signalized on Urban and Suburban Arterials

There are specific equations for each segment type and intersection type. It is very important to select the proper category to obtain an accurate analysis.

Step 2: Download the appropriate spreadsheet based on the type selected in Step 1.

Go to the SharePoint HSM site and download the appropriate spreadsheet. The rural two lane, rural multilane and urban/suburban arterial spreadsheets are found in the "Existing Chapters – Spreadsheets" folder while the interchange and freeway spreadsheet is found in the "Freeways and Interchanges" folder.

Step 3: Split the study area into homogenous sections.

An important part of the HSM analysis is how the study area is split into sections. Extra care needs to be taken to be sure proper procedure is followed in breaking each roadway segment into smaller sections. The proper procedure in breaking a roadway segment can be found in Part C of the HSM.

Step 4: Gather the appropriate data for each section of the roadway segment.

Data needed could include AADT, lane width, number of lanes, shoulder type, shoulder width, rumbles present, barriers present, calibration factors, segment length, horizontal curve radius, grade and more.

Step 5: Input data into the appropriate HSM Spreadsheet.

Input the roadway geometry and traffic characteristics for each segment of the study area.

Step 6: Run the Spreadsheet analysis.

Once all data is input, go to the Site Total or Project Total to determine a predicted number of crashes for the study area. If you are evaluating a particular countermeasure, take the total number of predicted crashes obtained from the HSM analysis and multiply it by the Crash Modification Factor for that particular countermeasure. This will provide a predicted number of crashes with the proposed countermeasure and will allow for quick evaluation of safety treatments.

Notes:

A separate analysis may be needed to compare design alternatives or future geometry changes.

The predicted crash frequency may be compared to the observed (or expected, if available) crash frequency to objectively determine if a safety issue exists.

907.7.4 Where is Information Found in the HSM?

EPG 907.7.4 provides a quick reference for important topics in the HSM.

Rural Two Lane, Two Way Roads (page 10-1 of Vol. 2)

• General Notes:
• Definitions (page 10-2)

• Step by Step Procedure (page 10-4)

• Segments:

• General Definitions (page 10-11)

• Roadway Segmentation (page 10-11)

• Base Conditions and SPF (page 10-14)

• Crash Modification Factors (page 10-23)

• Intersections:

• General Definitions (page 10-11)

• Three Leg Stop Controlled Intersection SPF (page 10-18)

• Four leg Stop Controlled Intersection SPF (page 10-19)

• Four Leg Signalized Intersection SPF (page 10-20)

• Crash Modification Factors (page 10-31)

• Sample Problems (page 10-35)

Rural Multilane Highways (page 11-1 of Vol. 2)

• General Notes:

• Definitions (page 11-2)

• Step by Step Procedure (page 11-5)

• Segments:

• General Definitions (page 11-11)

• Roadway segmentation (page 11-11)

• Undivided Roadways

Base Conditions and SPF (page 11-14)

Crash Modification Factors (page 11-25)

• Divided Roadways

Base Conditions and SPF (page 11-17)

Crash Modification Factors (page 11-29)

• Intersections:

• Three Leg Stop Controlled Intersections (page 11-20)

• Four Leg Stop Controlled Intersections (page 11-20)

• Four Leg Signalized Intersections (page 11-20)

• Crash Modification Factors (page 11-32)

• Sample Problems (page 11-37)

Urban and Suburban Arterials (page 12-1 of Vol. 2)

• General Notes:

• Definitions (page 12-2)

• Step by Step Procedure (page 12-7)

• Segments:

• General Definitions (page 12-14)

• Roadway Segmentation (page 12-14)

• Safety Performance Functions (page 12-17)

• Crash Modification Factors (page 12-40)

• Intersections:

• Not Roundabouts:

Safety Performance Functions (page 12-28)

Crash Modification Factors (page 12-43)

• Roundabouts:

Interim Procedure (page 12-47)

• Sample Problems (page 12-49)

Freeways (page 18-1 of Supplement)

• General Notes

• Definitions (page 18-2)

• Step by Step Procedure (page 18-8)

• Freeway Segments

• General Definitions (page 18-21)

• Roadway segmentation (page 18-22)

• Base Condition and SPF (page 18-24)

• Crash Modification Factors (page 18-35)

• Speed Change Lanes

• General Definitions (page 18-21)

• Base Condition and SPF (page 18-29)

• Crash Modification Factors (page 18-45)

• Sample Problems (page 18-58)

Ramps (page 19-1 of Supplement)

• General Notes:

• Definitions (page 19-2)

• Step by Step Procedures (page 19-10)

• Ramp Segments

• General Definitions (page 19-25)

• Roadway Segmentation (page 19-26)

• Safety Performance Functions (page 19-28)

• Crash Modification Factors (page 19-46)

• Ramp Terminals

• General Definitions (page 19-25)

• Roadway Segmentation (page 19-26)

• Safety Performance Functions (page 19-35)

• Crash Modification Factors (page 19-54)

• Sample Problems (page 19-80)

907.7.5 What Additional Resources are Available to Aid in an HSM Analysis?

There are many additional resources available to aid in an HSM analysis.

Highway Safety Manual

• Each district has several copies of the Highway Safety Manual. See Design Division and/or Highway Safety and Traffic Division for a manual.
• Additional copies may be purchased from the AASHTO Bookstore.

Spreadsheets

• Freeways and Interchanges Safety Analysis Tool : Enhanced (iSATe). This automates HSM methodologies for freeway and interchange analysis. This spreadsheet may be found in the "Freeways and Interchanges" section of the HSM SharePoint site.
• Rural Two Lane, Rural Multi-Lane, Urban/Suburban Spreadsheets. There are multiple spreadsheets available which are each used for a specific type of roadway. These spreadsheets may be found in the "Existing Chapters – Spreadsheets" of the HSM SharePoint site.

HSM Help Documents

• HSM User Guide. Developed by NCHRP 17-50, this document is great for an overview of HSM methodologies. This would be a great document to help an individual acclimate to HSM procedures. This document may be found on the HSM SharePoint site.
• HSM Training Guide. Developed by FHWA, this document goes into more depth than the User Guide and can be another resource to reference when performing an HSM analysis.

HSM Online Resources

• AASHTO website: Provides online training courses, recorded webinar trainings and much more. This is a very valuable resource for HSM methodologies.
• FHWA website: Provides additional training material. Very similar to AASHTO Website.
• CMF Clearinghouse: Provides a database of Crash Modification Factors for Safety Strategies.

Central Office Highway Safety and Traffic

• Feel free to contact Central Office Highway Safety and Traffic for any questions or issues encountered during an HSM analysis.