The AASHTO Highway Safety Manual (HSM) 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 requires more data and expertise, and it has a higher 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 orfunding to collect data. In addition, states increasingly question 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 was to document state DOT practice on calibration factors and development of jurisdiction-specific SPFs. Information for this study was gathered through a literature review, a survey of state DOTs, and follow-up interviews with selected DOTs. Case examples of five state DOTs provide additional information on calibration factors and development of jurisdiction-specific SPFs.

Vikash V. Gayah and his colleagues at the Pennsylvania State University, University Park, Pennsylvania, collected and synthesized the information and wrote the report. The members of the topic panel are acknowledged on page iv. This synthesis is an immediately useful document that records state DOT practices on calibration factors and development of jurisdiction-specific SPFs that were acceptable within the limitations of the knowledge available at the time of its preparation. As progress in research and practice continues, new knowledge will be added to that now at hand. It published as NCHRP Synthesis 634.