The AASHTO Highway Safety Manual provides tools for predicting the safety performance of a roadway facility. These tools include safety performance functions (SPFs) which incorporate geometric and other conditions to predict the crashes expected on a facility. SPFs are developed using crash numbers, geometric conditions, traffic conditions, and other data from one or more states, resulting in less accurate safety performance measures when the SPFs are used for analyses in other states. SPFs can be customized for a specific condition or region using a calibration factor. The calibration factor is then multiplied by the HSM model results to yield crash predictions that better represent the observed crash number in that state. Jurisdictions can develop jurisdiction-specific SPFs using their own data, allowing analyses that more closely represent their individual experiences. Although the development of customized SPFs is generally considered more accurate for crash predictions, it involves a higher level of data needs, expertise, and cost.
As the state of the practice in data-driven safety analysis advances, states are challenged to calibrate or develop models that meet their needs. Specific challenges include the availability of sufficient data or funding to collect data. In addition, states increasingly have questions about whether and how to apply particular factors or models to facility types that are not exactly the same as the ones used to develop the models. Practitioners have questions about whether calibration factors or SPFs are transferable and could be used by other jurisdictions. An initial step to addressing these questions is to synthesize the work states have already done to calibrate the Highway Safety Manual SPFs or develop their own SPFs.
The objective of this synthesis is to document state DOT practice on calibration factors and development of jurisdiction-specific SPFs.
Information to be gathered includes (but is not limited to):
1. Calibration factors and SPFs that states have developed;
2. Have cities, counties, and MPOs within a state developed their own SPFs or calibrated existing SPFs?;
3. Are CFs developed in-house or by consultants, and what are the respective general levels of effort?;
4. What type of analysis tools, e.g., FHWA Calibrator, are used for development of calibration factors?;
5. Factors, including barriers and challenges, and the process used to make a decision to calibrate existing SPFs or develop jurisdiction-specific SPFs;
6. What calibration factors are used and how often they are updated?;
7. Development of region-specific calibration factors or SPFs (i.e., mountain, piedmont, coastal) and how they are used;
8. Metrics and methods of validation on calibrated factors;
9. Adoption of calibration factors from other states and metrics and techniques used to assess transferability; and
Relevant factors, such as crash reporting threshold, that would impact the applicability of calibration factors or SPFs to other jurisdictions.
Information will be gathered through a literature review, a survey of state DOTs, a virtual meeting (if possible) to delve into questions whose answers are not clear from the survey, and follow-up interviews with selected agencies for the development of case examples. Information gaps and suggestions for research to address those gaps will be identified.
Since Highway Safety Manual work may be led by different departments among the state DOTs or there maybe be users in multiple departments, distribution of the survey to safety engineering, traffic engineering, and roadway design staff is recommended. Follow up interviews are recommended with many of the State DOT staff to obtain additional information and clarify survey responses.
Initial information may be available in case studies or other documents developed by FHWA (https://safety.fhwa.dot.gov/rsdp/hsm.aspx, http://www.cmfclearinghouse.org/resources_spf.cfm, and other pages), NCHRP, or AASHTO (http://www.highwaysafetymanual.org/Pages/Implementation.aspx), but it is expected that the most complete and current information will be obtained through the survey of state DOTs.
First Panel: February 15, 2023 Virtual Meeting
Teleconference with Consultant: March 13, 2023
Second Panel: December 15, 2023
Brenton Bogard, Ohio Department of Transportation
Daniel Carter, North Carolina Department of Transportation
Kohinoor Kar, Arizona Department of Transportation
Johnathan Markt, HDR
Bonnie Polin, Massachusetts Department of Transportation
Kishor Shrestha, Washington State University
Matt Hinshaw, Federal Highway Administration
Kelly Hardy, AASHTO
Bernardo Kleiner, Transportation Research Board