Understanding the speed-crash relationship has long been of interest in roadway safety analysis. But the relationship cannot be adequately established without taking into account the many factors that influence both speed and crashes: roadway geometry and context, weather conditions, human factors, vehicle type distribution, and the dynamics of the vehicle and tire.
While a significant amount of research has been conducted to identify relationships between roadway design elements and crashes, research that has also considered the contribution of operating speeds or posted speed is limited. A general subjective understanding of the contribution of operating speed of a highway or freeway, through the dynamics of the vehicle, on the severity of a crash is known: higher speeds are associated with more severe crashes. What is desired is a quantitative understanding of how speed (operating and posted) in conjunction with roadway geometry relates to the likelihood of a crash and to crash severity.
This research would build on existing research to explore the relationships between roadway geometrics, speed (operating and posted), and crashes on high-speed rural highways. These relationships will help inform future design guidelines, posted speed practices, and potential safety countermeasures, which are related to desired outcomes for multiple American Association of State Highway and Transportation Officials (AASHTO) committees, such as design, traffic engineering, and safety.
The objective of this research is to explore the relationships between roadway geometrics, operating speed (as defined in FHWA-SA-10-001), posted speed, and crashes on high-speed rural highways.
At a minimum, the research team shall:
- Identify the relationships between speed (operating and posted), geometric characteristics, and crashes on high-speed rural highways, considering other design elements and road conditions.
- Determine the relative contribution of speed (operating and posted) with various roadway geometric characteristics to crashes (in terms of frequency and severity) for high-speed rural highways.
- Identify and measure the effectiveness of existing mitigation strategies for crashes and speed (operating and posted) on high-speed rural highways.
- Determine if existing crash modification factors (CMFs) for geometric elements can be modified with a speed component, and if speed-related CMFs can be developed that could ultimately be incorporated into the AASHTO Highway Safety Manual (HSM) Part C.
Accomplishment of the project objective will require completion of the following tasks, at a minimum.
PHASE I – Planning
Task 1. Conduct a literature review on previous research about the relationships between roadway geometrics, operating and posted speed, and crashes on high-speed rural highways. The review shall include current practice at state and local agencies; AASHTO; the Institute of Transportation Engineers (ITE) guidelines and standards; safety data from the National Highway Traffic Safety Administration (NHTSA); and published and unpublished research conducted through the NCHRP and the Federal Highway Administration (FHWA) and other national, international, state, and pooled-fund sponsored research.
Task 2a. Propose a stakeholder engagement plan to gather related information, such as undocumented or informal implementation strategies, major concerns, and potential data sources. The stakeholders should include up to 20 representatives from state departments of transportation (DOTs), local agencies, AASHTO groups, TRB committees, etc.
Task 2b. Gather stakeholder inputs based on approved plan from Task 2a.
Task 3. Identify potential data sources, especially with respect to operating speed data and how these data could be analyzed in conjunction with roadway geometric characteristics. Evaluate the quality of the data sources and whether they can support the desired evaluation.
Task 4. Prepare a detailed work plan based on a comprehensive literature review, stakeholder inputs, and analysis of available resource data. The plan should cover the following aspects at a minimum:
- Data collection;
- Method for statistical modeling and data analysis to identify the relationships between roadway geometric characteristics, speed (operating and posted), and crashes (frequency and severity) on high-speed rural roadways, considering other design elements and road conditions;
- Strategies for assessing the effects of study factors (operating and posted speed, roadway geometric characteristics) on crashes and perform sensitivity analysis with respect to inclusion of covariates, model forms, etc; and
- Method to develop improved CMFs or crash modification functions (CMFunctions) for geometric elements, incorporating operating or posted speed.
Task 5. Prepare Interim Report No. 1, documenting the results of Tasks 1 through 4, and provide an updated plan for the remainder of the research no later than 8 months after contract award. The updated plan must describe the method and rationale for the work proposed for Phases II and III.
PHASE II – Study
Task 6. Execute the work plan based on approved Interim Report No. 1.
Task 7. Prepare an annotated outline of the draft research report.
Task 8. Prepare Interim Report No. 2 that documents Tasks 6 and 7 and provide an updated work plan for the remainder of the research no later than 15 months after approval of Phase I. The updated plan must describe the process and rationale for the work proposed for Phase III.
Meet with the NCHRP to review the report and obtain approval for subsequent tasks.
PHASE III – Reporting
Task 9. Prepare and submit the draft research report based on approved Interim Report No. 2 for the NCHRP panel’s review.
Task 10. Refine and update the draft research report based on comments from the NCHRP project panel.
Task 11. Present the research findings to appropriate AASHTO technical committees for comment and propose any revisions to the NCHRP. The research team should anticipate making at least two presentations during the research to appropriate technical committees at annual meetings of the AASHTO Committee on Design, Committee on Traffic Engineering, or Committee on Safety. Revise the draft research report after consideration of review comments.
Task 12. Prepare the final deliverables, including:
- A conduct of research that documents the entire research effort and any lessons learned;
- 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.” 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.