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, traffic conditions and other data from one or more states resulting in less accurate safety performance measures when 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 predict crashes that better represents the observed crash number in that state. States can develop jurisdiction-specific SPFs using their own state data, allowing analyses that more closely represent the individual states’ experiences. Although the development of customized SPF 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 similar to the ones used to develop the models. State practitioners have questions about whether calibration factors or SPFs are transferable and could be used by other states. An initial step to addressing this is synthesizing the work states have already done to calibrate the HS M SPFs or develop their SPFs.
The objective of this synthesis is to document state DOT current practices on calibration factors and development of jurisdiction-specific SPFs.
Information to be gathered includes (but is not limited to):
• Calibration factors and SPFs that states have developed;
• Decision factors, including barriers and challenges, related to the decision to calibrate existing SPFs or develop jurisdiction specific SPFs;
• What calibration factors are used and how often they are updated;
• Analysis of calibration factor and/or model sensitivity;
• Development of region-specific calibration factors or SPFs (i.e., mountain, piedmont, coastal) and how they are used;
• Adoption of calibration factors from other states;
• Metrics and methods of validation on calibrated factors;
• Metrics and techniques used to assess transferability; and
• Relevant factors, such as crash reporting thresholds, which 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, 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.
First Panel: TBD
Teleconference with Consultant: TBD
Second Panel: TBD