As the number and prevalence of intersection types and configurations increase, it has become more challenging for practitioners to quantify the safety effects of constructing these designs. Currently the American Association of State Highway and Transportation Officials (AASHTO) Highway Safety Manual (HSM) provides safety performance functions for a few conventional intersections based on empirical research, but any deviation from the basic assumptions in the HSM requires the application of one or more crash modification factors (CMFs). Although there are a high number of CMFs from various sources, some apply to alternative intersection forms; fewer apply to complex traditional intersections that diverge from the basic intersections in the HSM. Intersection control evaluation policies and efforts typically promote the consideration of alternative intersections not covered in the HSM, which has limited the ability of practitioners to compare the safety performance of these alternative intersections to conventional designs.
One element that is common to all at-grade intersections is the conflict point, the location where two or more paths may cross. Previous research has demonstrated relationships between the number of conflict points at an intersection and the number of resulting crashes over a given time period. Similarly, crash severity can be roughly correlated to conflict type, angle, and speed. Alternative intersections have been touted as an improvement in safety over conventional forms due to a reduction in total conflict points, conflict point proximity, and speed. There is a need for a new methodology to assess a variety of intersections' safety performance from the perspective of conflict point type.
The objectives of this research are to
- Develop a quantifiable method to uniformly identify and describe at-grade intersection conflict point types under different contexts; and
- Develop and validate a predictive methodology, using intersection conflict point types to predict multimodal crash frequency and severity, to supplement those in the HSM Part C models.
Accomplishment of the project objectives will require completion of the following tasks, at a minimum.
PHASE I: Planning
Task 1. Conduct a critical review of the roadway literature on the safety performance of at-grade intersection conflict point types. Submit a technical memorandum that summarizes findings related to conflict point analysis modeling efforts and identifies gaps in the current state of knowledge and practice on the research topic.
Task 2. Based on the findings of Task 1, prepare an assessment of the adequacy of existing data and suggest other data that could help develop traffic exposure and safety prediction methodology based on intersection conflict point types. Develop a comprehensive data collection, management, and validation plan. The plan, which will be conducted in Phase II, should include a data dictionary, metadata for raw and processed data, data ownership information, and recommendations for data archiving. A proposal should avoid data that are not available for public release due to licensing or other restrictions.
Task 3. Prepare a detailed Phase II work plan that gives a quantifiable method to uniformly describe at-grade intersection conflict point types in different contexts (e.g., speed, multimodal traffic volumes, traffic control, land use, intersection geometry, etc.) and a framework for developing a safety performance prediction methodology using intersection conflict point types based on the findings from Tasks 1 and 2.
For the safety performance prediction methodology development task, the proposed Phase II work plan should include, at a minimum
- A list of intersection conflict point types and characteristics for each (figures to help illustrate the text are encouraged);
- Methods for multisource data integration/fusion;
- Data collection, management, and distribution plan;
- Identified tools or software to be used for data management, processing, and modeling;
- Metrics for assessing statistical models and analysis results;
- Validation approach for the methodology;
- A plan to compare modeling results, using case examples of methodology implementation in different contexts, including at least two types of intersection currently described in the HSM Part C and at least two other types of intersection not included in the HSM Part C; and
- A tool for implementing the methodology.
The methodology for prediction of crash frequency and severity for various facility types should be reliable, capable of evaluating existing and future conditions, practical, and readily implementable by state and local transportation agencies of all sizes to help evaluate potential safety performance at a given location. Using the methodology, practitioners should be able to quantify the safety performance of at-grade intersections based on intersection conflict point types for applications across the range of highway activities, including project level planning, design, operations, and safety management.
Task 4. Prepare Interim Report No. 1 documenting the results of Tasks 1 through 3 and provide an updated plan for the remainder of the research no later than 6 months after contract award. The updated plan must describe the methodology and rationale for the work proposed for Phases II and III.
PHASE II: Study
Task 5. Execute the work plan based on approved Interim Report No. 1.
Task 6. Develop a user-friendly, updatable, and easy-to-maintain tool to implement the developed methodology and user manual that can be used and maintained by state and local transportation agencies. The tool development should avoid the use of third-party software components that would not be available for updates and maintenance and should use commonly available or open-source software.
Task 7. Prepare an annotated outline of the draft project report. Include in the documentation for methodology development:
- Data summary and descriptive statistics;
- Detailed description of the methodology, including inputs and outputs;
- Validation method and results, including variable importance ranking and elasticities (or marginal effects) of analyses by intersection conflict point type;
- Performance metrics used for assessing the quality and results of the quantitative analyses; and
- Tables and graphs showing relationships between crash frequency, crash severity, and key independent variables.
Provide all assumptions, data limitations, and other constraints (e.g., range for valid input fields). Explain conditions that cannot be assessed using the methodology.
Task 8. Prepare Interim Report No. 2 that documents Tasks 5 through 7 and provide an updated work plan for the remainder of the research no later than 18 months after approval of Phase I. The updated plan shall describe the process and rationale for the work proposed for Phase III.
PHASE III: Reporting
Task 9. Prepare and submit the initial draft project report, tool and user manual based on approved Interim Report No. 2 for the NCHRP panel's review.
Task 10a. Prepare for workshop(s) with at least 10 representatives from state and local agencies from various geographic regions and of various sizes to review and collect feedback on the draft reports and presentations. Submit workshop materials and proposed attendee list for the NCHRP panel's review and approval.
Task 10b. Conduct one or more workshops as approved in Task 10a. Revise the draft project report, tool, user manual, and other workshop materials taking into account feedback gathered during the workshop and submit for the NCHRP project panel's review.
Task 11. Update the draft project report, tool, and user manual based on comments from the NCHRP project panel.
Task 12. Prepare draft language for consideration by AASHTO to incorporate the research results in the next update of the AASHTO HSM (hereafter called the AASHTO Deliverable): include sample problems, assumptions, effect sizes, data limitations, and other constraints (e.g., range for valid input fields, conditions that cannot be assessed with the research methodologies).
Task 13. Present the research findings to appropriate AASHTO technical committees for comments and propose any revisions to the NCHRP. The research team should anticipate making two presentations during the research to appropriate technical committees at annual meetings of the AASHTO Committee on Safety or the AASHTO Committee on Design. Revise the draft project report after consideration of review comments received during the meetings.
Task 14. Prepare the final deliverables, including
- The final project report;
- The AASHTO Deliverable;
- All raw and cleaned data collected and used in this research (data should be provided in as close to raw form as possible), input data sets, fused and integrated research data sets, data dictionaries, data models, etc.;
- Documentation of the steps in the data fusion and integration process, including annotated data management code used (if any) to fuse, integrate, and clean the data;
- The tool (including annotated code if any), user manual, and any other tool documentation;
- Media and communication material (e.g., presentations, 2-page executive-level flyer, graphics, graphic interchange formats (GIFs), press releases); and
- A stand-alone technical memorandum titled "Implementation of Research Findings and Products." for additional information. Additional funding may be available for a follow-up contract on the implementation of the results.
STATUS: A research contractor has been selected for the project. The contracting process is underway.