INSIDE THE DOT
South Dakota Department of Transportation
Calibration of HSM Predictive Methods for Rural State and Local Highways
Xiao Qin, SDSU
The new surface transportation act, Moving Ahead for Progress in the 21st Century (MAP-21), has made traffic safety a top priority. MAP-21 will require state DOTs to set performance goals for reducing serious injuries and fatalities. If a state DOT does not meet performance goals within two years, the Federal Highway Administration (FHWA) could place more limits on how safety funding may be spent. In addition, MAP-21 will require improvements in the areas of data collection and safety analysis, a regularly updated Strategic Highway Safety Plan (SHSP), and an established evaluation process. It is important that the South Dakota Department of Transportation SDDOT be prepared with the most accurate tools to make safety project decisions and meet safety performance goals. SDDOT has made recent efforts to improve the safety program, such as the application of Highway Safety Manual (HSM) predictive methods. SDDOT uses HSM Safety Performance Functions (SPF) and Crash Modification Factors (CMFs) to compare predicted crashes to observed crashes at locations statewide. Safety Performance Functions predict the number of crashes at a location given the traffic volume and site characteristics. Crash Modification Factors predict the number of crashes at a site if a safety improvement is implemented. Locations where the number of actual crashes is higher than the number of predicted crashes may have safety issues and are further investigated. In addition, SDDOT uses SPFs and CMFs from the Highway Safety Manual to determine if there are safety needs at locations where resurfacing projects are planned. Additional safety features are incorporated into programmed projects if there is a strong potential to reduce crashes. The SPFs and CMFs in the Highway Safety Manual have been developed using data from states, excluding South Dakota. The SPFs may not be accurate as they are not calibrated to South Dakota’s conditions, such as the local driver demographics, animal populations, weather conditions, or unique geographic characteristics. In addition, each SPF and CMF assumes a set of base site conditions, such as a minimum roadway width of 12 feet and a minimum shoulder width of 6 feet. These base conditions may not be realistic for some South Dakota’s roadways, and it may be necessary to determine a set of base conditions applicable to South Dakota highways. SDDOT has provided funding for a pooled-fund study intended to develop quality CMFs for low traffic volume roads and the CMFs developed through the study may be appropriate for SDDOT. Research is needed to calibrate the HSM Safety Performance Functions to South Dakota’s conditions and provide future guidance on calibration and application of HSM predictive methods. There are 10 intersection types and eight segment types for which HSM SPFs are available and need to be calibrated.
1 Calibrate HSM predictive models to South Dakota’s data for all rural facility types where SPFs are available and provide guidance on the selection of CMFs.
2 Develop guidelines for future calibration of HSM predictive methods.
1 Meet with the project’s technical panel in Pierre, SD to review the project scope and work plan.
2 Review and summarize literature pertinent to the calibration of Highway Safety Manual predictive methods, including information about other state DOT’s efforts to develop or calibrate HSM Safety Performance Functions.
3 Apply the HSM Part C calibration technique to sample data provided by SDDOT.
4 Review base conditions for SPFs defined in the HSM and define a set of base conditions appropriate for SPF models used on SD highways.
5 Identify the facility types for which SPFs will be calibrated based on data availability and needs of SDDOT and local agencies.
6 Recommend any necessary modifications to HSM procedures and calculate an appropriate sample size for calibration.
7 Determine which CMFs are appropriate for use in calibration and provide guidance on future application of CMFs.
8 Submit a technical memorandum and make a presentation to the technical panel summarizing the literature search and propose a SPF calibration methodology for approval.
9 9) Upon approval of the technical panel, apply a statistical analysis to determine whether SPFs need to be developed for different regions, such as Black Hills versus non-Black Hills roads, local and state roads, or other categories deemed important by the technical panel or through the literature review.
10 Calibrate the Highway Safety Manual SPFs to South Dakota’s data using the proposed methodology.
11 Develop guidelines for future calibration of SPFs.
12 Prepare a final report summarizing research methodology, findings, conclusions, and recommendations.
13 Make an executive presentation to the SDDOT Research Review Board at the conclusion of the project.
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