An important component of the AASHTO Highway Safety Manual (HSM) is the ability to estimate the safety performance of highways and the effects of proposed countermeasures. The HSM utilizes safety performance functions (SPFs) to estimate the number of crashes over a specific roadway over a specific time period. The move toward scientific approaches in safety analysis has driven the need for further development of SPFs so that they provide more detailed estimates by crash type and crash severity. These aspects are a natural and important step in the progression of crash analyses that is currently focused primarily on frequency reduction. It is envisioned that the SPFs and crash distributions will be refined to determine the expected severity and crash types of various road facilities. Planners and designers can then better target and select countermeasures to address these particular aspects resulting in improved project selection. Having a wide range of safety evaluation tools that facilitate a comparative analysis of crash severity and crash type will lead to potential systematic and system wide improvement scenarios. Currently, the models in HSM Part C differ by chapter. In Chapter 10 (rural two-lane highways), predictive models provide estimates for total crashes and then values are allocated to crash types and severity levels based on tabulated proportions. In Chapter 11 (rural multilane highways), separate predictive models are provided for total crashes, fatal-and-injury (KABC) crashes, and fatal-and-injury (KAB) crashes, with crash frequencies for property-damage-only (PDO) crashes determined by subtracting the KABC crash frequency from the total crash frequency. Crashes for any given crash severity level are then allocated to crash types based on tabulated proportions. In Chapter 12 (urban and suburban arterials), separate predictive models are provided for combinations of three crash severity levels (total, fatal-and-injury, and PDO) and five crash types (multiple-vehicle non-driveway crashes, single-vehicle crashes, multiple-vehicle driveway-related crashes, vehicle-pedestrian crashes, and vehicle-bicycle crashes). Within two of the crash types (multiple-vehicle non-driveway crashes and single-vehicle crashes), predicted crash frequencies can be broken down into even more specific crash frequencies by tabulated proportions.To provide more consistency for users, it is desirable that a HSM second edition provide a consistent approach to predictive modeling by crash severity and crash type. This would require that models from past research be refit using the databases that were used to develop those models or for new models to be developed from new databases. Certainly, users should expect all future HSM chapters to use a consistent set of crash severity and crash type. Candidate crash severity levels from which final choices should be made include total crashes (all severity levels combined) and K, A, B, C, PDO, KA, KAB, KABC, AB, and ABC crashes. There is a need to model individual crash types. User needs and sample size issues should be addressed in this modeling effort. In addition, there is a need to account for the variations of specific crash types by facility type. In other words, it may not be desirable to require modeling of a common set of crash types across all facility types, because some crash types are common (and, thus, easy to model) on some facility types and relatively uncommon (and thus difficult to model) on other facility types. A consistent approach, with reasonable variations by facility type, is needed for the HSM second edition. Once a consistent and practical set of categories for crash severity and crash type have been defined, these prediction models should be developed and implemented throughout HSM for future editions.
The objectives of this research are to develop:
- Crash severity and crash type SPFs or distributions or both that can be used in the estimation of the crash type and crash severity likely on the facility types contained or intended for use in the HSM;
- Recommendations of how the research results can be incorporated into the HSM and associated tools, including the development of associated chapters or chapter content in AASHTO standard format for the HSM second edition and recommended procedures for consistent use of crash severity and crash type SPFs or distributions or both; and
- A description of the statistical and practical advantages and disadvantages of the methodology developed in the research and potential barriers to implementation.
This research should provide a consistent approach for developing and validating crash severity and crash type prediction models to improve the capabilities of the current HSM and associated tools to estimate the safety performance outcomes associated with modifications to the highway and road user environments.
Proposers are asked to develop and include a detailed research plan for accomplishing the project objectives. Proposers are expected to describe research plans that can realistically be accomplished within the constraints of available funds and contract time. Proposals must present the proposers' current thinking in sufficient detail to demonstrate their understanding of the issues and the soundness of their approach to meeting the research objectives. The work proposed must be divided into tasks and proposers must describe the work proposed in each task in detail.
A key aspect of the research plan is providing opportunities for the panel to review and oversee the work. The research plan should describe a suitable number of interim deliverables and include one face-to-face meeting (to be determined by the panel and NCHRP) and an appropriate number of web-enabled teleconferences (including one within 1 month of the contract’s execution date).
A phased research plan is suggested:
1. Phase 1: Background research and data assessment.
2. Phase 2: Model development and validation.
3. Phase 3: Development of content for implementation into the HSM and associated software tools.
4. Phase 4: Review and respond to comments from the TRB Highway Safety Performance Committee subsequent to AASHTO balloting. It is anticipated that Phase 4 will occur beyond the 30-month research phase. This phase must be accounted for in the budget and a no-cost time extension may be approved if necessary.
The research team will be expected to deliver presentations at a total of four AASHTO and/or TRB committee meetings during the course of the research project or upon completion.
Research deliverables should include a final report that documents the entire research effort including all final data sets, software codes, and supporting documentation used for the development and validation of crash severity and crash type SPFs or distributions or both.
STATUS: The panel met in December to select a contractor. Contract negotiations are underway.