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  • Location: Statewide, Ohio

The Highway Safety Manual (HSM) uses safety performance functions (SPFs) to calculate estimated crash frequency per year based on volume and traffic input. The SPFs within the HSM are the national baseline for state agencies to calculate the likelihood of crashes along their roadways. In order to account for local conditions, an agency has two options – 1) develop calibration factors that are applied to the baseline HSM SPF or 2) develop new SPFs based on local data inputs to serve as the new baseline for analysis.

B&N worked as a subconsultant for ODOT to create SPFs specific to Ohio conditions to increase the accuracy of safety performance calculations and improve planning and design decisions. The SPF development process involved collecting geometric information, such as the presence and location of speed change lanes (acceleration and deceleration lanes) and weaving segments. After collecting the existing geometric data and historical crashes, analysis was performed to develop Ohio-specific SPFs as well as adjustment factors to account for the difference between sites (i.e., 6-foot shoulder width vs. 10-foot shoulder width).

Once the SPFs and corresponding adjustment factors were developed, B&N tested the new SPFs at 26 sites in Ohio to compare the various models. Four models were analyzed – 1) HSM-developed SPFs and adjustment factors, 2) HSM-developed SPFs and adjustment factors with Ohio-specific calibration factors, 3) Ohio-specific SPFs and HSM-developed adjustment factors, and 4) Ohio-specific SPFs and Ohio-specific adjustment factors. 

The research determined that the most accurate models were those that used HSM-developed SPFs and adjustment factors with Ohio-specific calibration factors or Ohio-specific SPFs and Ohio-specific adjustment factors. However, given that the Ohio-specific SPFs are directional (rather than bi-directional like the HSM-developed SPFs) and require fewer data inputs, the analysis is simpler and easier to implement. For these reasons, the new Ohio-specific SPFs and adjustment factors will be used moving forward once the ODOT safety analysis tools are updated.

The final report can be found here -