Statistics from the Fatality Analysis Reporting System (FARS) indicate that more than 25 percent of fatal crashes occur at horizontal curves, with most of these crashes being roadway departures. Although researchers and practitioners agree that curvature plays a role in crash frequency and severity, safety performance functions (SPFs) and severity distribution functions for horizontal curves have not been thoroughly investigated or widely implemented.
The American Association of State Highway and Transportation Officials (AASHTO) Highway Safety Manual (HSM) provides SPFs of various facility types for segments and intersections, but not for curve segments. Rather, curves are evaluated by applying an adjustment factor (AF) to estimate the predicted crash frequency of a curve segment. Unfortunately, not all the SPF models within the HSM have AFs for horizontal curves. Recent studies have been implemented to develop AFs for curve segments of certain facility types to begin filling this gap. However, applying a horizontal curve AF to an existing segment SPF assumes that the underlying prediction model of a tangent segment only needs to be adjusted to appropriately estimate a horizontal curve’s influence on the segment’s safety performance. This method may not be the best way to assess the safety performance of horizontal curves. A more thorough investigation may reveal that common geometric attributes used to estimate the safety performance of tangent segments have a different degree of influence on the safety performance of horizontal curves. Taking this possibility further, the attributes most important for predicting the safety performance of a horizontal curve may differ due to context (rural versus urban), facility type, or other factors such as geometric and operational characteristics, and roadside elements. Therefore, additional research is needed to better understand the attributes that most influence the safety performance of horizontal curves.
The objective of this research is to develop a guide and a tool to quantify the safety performance of horizontal curves using geometric and operational characteristics for applications across a range of highway activities including planning, design, operations, and safety management
At the minimum, the research shall:
- Advance the predictive safety performance methodologies for horizontal curves on rural and urban freeways, rural and urban multilane divided roadways, rural and urban multilane undivided roadways, and rural and urban two-lane undivided roadways.
- Develop and validate a statistically valid predictive methodology to quantify the safety performance of locations with curves for use by state and local transportation agencies to help evaluate the likely safety performance of different horizontal curves at a given location. The study should consider geometric and operational characteristics of contiguous highway segments and various facility types. Consideration should be given to different horizontal curve types such as simple curves, compound curves, reverse curves, and broken-back curves.
Accomplishment of the project objective will require completion of the following tasks, at a minimum.
Task descriptions are intended to provide a framework for conducting the research. The NCHRP is seeking the insights of proposers on how best to achieve the research objective. 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 objective.
Task 1. Conduct a critical review of the road safety literature to identify factors associated with the safety performance of horizontal curves. The review should summarize the factors considered, discuss related modeling efforts, and identify gaps in the current state of knowledge and practice on the research topic.
Task 2. Identify data currently used as well as other public data that could help develop exposure and safety prediction models for use in locations with horizontal curves. Develop a comprehensive data collection, management, and validation plan. The plan should include metadata for raw and processed data, data ownership information, data restrictions (if applicable), and recommendations for data archiving (see Special Note A).
Task 3. Prepare a detailed Phase II work plan that describes a methodological framework for developing safety prediction models. Examine key factors for evaluating the safety performance of horizontal curves, using geometric and operational characteristics.
For the modeling task, the proposed Phase II work plan should include, at a minimum:
- Methods for multi-source 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 and tool; and
- Development of a spreadsheet tool.
The methods for prediction of crash frequency and severity for various facility types should be repeatable, be capable of evaluating existing and future conditions, and be practical and readily implementable by state and local transportation agencies of all sizes to help evaluate potential safety performance at a given location.
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.
Note: Following a 1-month review of Interim Report No. 1 by the NCHRP, the research team will be required to meet virtually with the NCHRP project panel to discuss the interim report. Work on Phases II and III of the project will not begin until authorized by the NCHRP. Phase I budget shall not exceed $50,000.
Task 5. Execute the work plan based on the approved Interim Report No. 1.
Discuss in detail the impact of the significant variables, including sign of impact (+/-), magnitude, and elasticities (or marginal effects), with the purpose of improving the scientific understanding of the safety performance of locations with horizontal curves. Include in the documentation for model development:
- Detailed methodology;
- Data summary and descriptive statistics by facility type and horizontal curve geometric characteristics;
- Validated models and results, including variable importance ranking and the elasticities (or marginal effects) of analyses by facility type;
- Metrics used for assessing the quality of statistical models and the results of quantitative analyses; and
- Tables and graphs showing relationships between crash frequency/severity and key independent variables.
Provide all assumptions, data limitations, and other constraints (e.g., the range for valid input fields, conditions that could not be assessed with the methodologies because the data sources used to develop the method did not include these conditions).
NOTE. Model development and validation require the selection of independent variables, functional forms, model methods, etc. Consistent with reproducibility best practices, the researchers should provide documentation and justification for the selection of variables and functional forms in each model beyond the simple use of p-values. This includes documentation of preliminary and intermediate models testing variables not included in the final models. Examples of selection criteria and methods that should be considered and documented include, at a minimum, improved predictive power in model validation for predictive models such as accuracy and variance; elasticities, marginal effects, or other interpretive measures for sign and magnitude of estimated coefficients; considerations of both omitted and included variables bias; and costs and difficulties in collecting and maintaining data elements for transportation agencies and practicing engineers. All codes and documentation shall be provided to the panel. For model validation, the purpose of the models being developed, sample size, and other parameters should be considered and the most appropriate method selected.
Task 6. Develop a user-friendly, updatable, and easy-to-maintain spreadsheet tool and user manual that can be used and maintained by state and local transportation agencies for practitioners to quantify the safety performance of horizontal curves using geometric and operational characteristics for applications across a range of highway activities including planning, design, operations, and safety management. The spreadsheet tool should not be developed with third-party software, but with commonly available software.
Task 7. Prepare an annotated outline of the draft guide.
Task 8. Prepare Interim Report No. 2 that documents Tasks 5 through 7 and provides an updated work plan for the remainder of the research no later than 12 months after approval of Phase I. The updated plan must describe the process and rationale for the work proposed for Phase III.
Note: Following a 1-month review of Interim Report No. 2 by the NCHRP, the research team will be required to meet in person with the NCHRP project panel to discuss the interim report. Work on Phase III of the project will not begin until authorized by the NCHRP.
Task 9. Prepare and submit the draft guide based on the approved Interim Report No. 2 for the NCHRP panel’s review.
Task 10. Refine and update the draft guide and the spreadsheet tool based on comments from the NCHRP project panel.
Task 11. Prepare draft language for consideration by AASHTO to incorporate the research results in the next update of the AASHTO HSM (herein called AASHTO Deliverable). Include sample problems, assumptions, effect sizes, data limitations, and other constraints (e.g., the range for valid input fields, conditions that cannot be assessed with the methodologies because the data sources used to develop the method did not include these conditions).
Task 12. Present the research findings to appropriate AASHTO technical committees for comments and propose any revisions to 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 Committee on Design. Revise the draft research report after consideration of review comments.
Task 13. Prepare the final deliverables including the following:
- A conduct of research that documents the entire research effort and any lessons learned;
- The final guide;
- The AASHTO Deliverable;
- All raw and cleaned data collected and used in this research (data should be provided in as close to its raw form as possible, based on contractual or legal restrictions): 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 any annotated data management codes used for fusing, integrating, and cleaning data;
- Annotated model development and validation code;
- The spreadsheet tool (including any annotated spreadsheet macro code, if used), user manual, and any other spreadsheet tool documentation;
- Media and communication material (e.g., presentations, 2-page executive level flyer, graphics, graphic interchange format, press releases); and
- A stand-alone technical memorandum titled “Implementation of Research Findings and Products” (see Special Note J for additional information). Additional funding may be available for a follow-up contract on the implementation of the results.
Note: Following receipt of the draft final deliverables, the remaining 3 months shall be for NCHRP review and comment and for research agency preparation of the final deliverables.
A. To ensure that data produced under this project are accessible, a data archiving and sharing plan shall be prepared by the agency performing the research. The plan must be submitted with Task 3 and shall include a description of expected activities, schedules, limitations, milestones, and required resources. Researchers should describe briefly the expected schedule for data production and archiving in the CRP permanent repository, the format of the final dataset, the documentation to be provided, and whether or not any analytic tools and/or results also will be provided. The plan must include a description of actions planned to ensure quality. Moreover, the plan must specify the resources required to archive and distribute the data expected to be obtained in the course of performing the research. In general, public release of data will not occur before full review and approval by the CRP.
The data archiving and sharing plan must include the following sections:
• Background and significance,
• Expected data formats,
• Description of data archiving and quality assurance plan,
• Description of data sharing plan,
• Schedule for data archiving and public release of data,
• Milestones for the implementation of the plan, and
• Resources and budget.
B. The Information and Instructions for Preparing Proposals for the Transportation Research Board’s Cooperative Research Programs were revised in May 2022. Please take note of the new and revised text which is highlighted in yellow.
C. Proposals must be submitted as a single PDF file with a maximum file size of 10 MB. The PDF must be formatted for standard 8½” X 11” paper, and the entire proposal must not exceed 60 pages (according to the page count displayed in the PDF). Proposals that do not meet these requirements will be rejected. For other requirements, refer to chapter V of the instructions.
D. The Information and Instructions for Preparing Proposals for the Transportation Research Board’s Cooperative Research Programs have been modified to include a revised policy and instructions for disclosing Investigator Conflict of Interest. For more information, refer to chapter IV of the instructions. A detailed definition and examples can be found in the CRP Conflict of Interest Policy for Contractors. The proposer recommended by the project panel will be required to submit an Investigator Conflict of Interest and Disclosure Form as a prerequisite for contract negotiations.
E. Proposals will be rejected if any of the proposed research team members’ work for organizations represented on the project panel. The panel roster for this project can be found at https://www.mytrb.org/OnlineDirectory/Committee/Details/6731. Proposers may not contact panel members directly; this roster is provided solely for the purpose of avoiding potential conflicts of interest.
F. Proprietary Products - If any proprietary products are to be used or tested in the project, please refer to Item 6 in the Information and Instructions for Preparing Proposals.
G. Proposals are evaluated by the NCHRP staff and project panels consisting of individuals collectively knowledgeable in the problem area. The project panel will recommend their first choice proposal considering the following factors: (1) the proposer's demonstrated understanding of the problem; (2) the merit of the proposed research approach and experiment design; (3) the experience, qualifications, and objectivity of the research team in the same or closely related problem area; (4) the plan for ensuring application of results; (5) how the proposer approaches inclusion and diversity in the composition of their team and research approach, including participation by certified Disadvantaged Business Enterprises; and, if relevant, (6) the adequacy of the facilities. A recommendation by the project panel is not a guarantee of a contract. The National Academy of Sciences (NAS - the contracting authority for the National Academies of Sciences, Engineering, and Medicine) will conduct an internal due diligence review and risk assessment of the panel’s recommended proposal before contract negotiations continue.
Note: The proposer's approach to inclusion and diversity as well as participation by Disadvantaged Business Enterprises should be incorporated in Item 11 of the proposal.
H. Copyrights - All data, written materials, computer software, graphic and photographic images, and other information prepared under the contract and the copyrights therein shall be owned by the National Academy of Sciences. The contractor and subcontractors will be able to publish this material for non-commercial purposes, for internal use, or to further academic research or studies with permission from TRB Cooperative Research Programs. The contractor and subcontractors will not be allowed to sell the project material without prior approval by the National Academy of Sciences. By signing a contract with the National Academy of Sciences, contractors accept legal responsibility for any copyright infringement that may exist in work done for TRB. Contractors are therefore responsible for obtaining all necessary permissions for use of copyrighted material in TRB's Cooperative Research Programs publications. For guidance on TRB's policies on using copyrighted material please consult Section 5.4, "Use of Copyrighted Material," in the Procedural Manual for Contractors.
I. Proposals should include a task-by-task breakdown of labor hours for each staff member as shown in Figure 4 in the Information and Instructions for Preparing Proposals. Proposals also should include a breakdown of all costs (e.g., wages, indirect costs, travel, materials, and total) for each task using Figures 5 and 6 in the brochure. Please note that TRB Cooperative Research Program subawards (selected proposers are considered subawards to the National Academy of Sciences, the parent organization of TRB) must comply with 2 CFR 200 – Uniform Administrative Requirements, Cost Principles, and Audit Requirements for Federal Awards. These requirements include a provision that proposers without a "federally" Negotiated Indirect Costs Rate Agreement (NICRA) shall be subject to a maximum allowable indirect rate of 10% of Modified Total Direct Costs. Modified Total Direct Costs include all salaries and wages, applicable fringe benefits, materials and supplies, services, travel, and up to the first $25,000 of each lower tier subaward and subcontract. Modified Total Direct Costs exclude equipment, capital expenditures, charges for patient care, rental costs, tuition remission, scholarships and fellowships, participant support costs and the portion of each lower tier subaward and subcontract in excess of $25,000.
J. The required technical memorandum titled “Implementation of Research Findings and Products” should (a) provide recommendations on how to best put the research findings/products into practice; (b) identify possible institutions that might take leadership in applying the research findings/products; (c) identify issues affecting potential implementation of the findings/products and recommend possible actions to address these issues; and (d) recommend methods of identifying and measuring the impacts associated with implementation of the findings/products. Implementation of these recommendations is not part of the research project and, if warranted, details of these actions will be developed and implemented in future efforts.
The research team will be expected to provide input to an implementation team consisting of panel members, AASHTO committee members, the NCHRP Implementation Coordinator, and others in order to meet the goals of NCHRP Active Implementation: Moving Research into Practice, available at http://onlinepubs.trb.org/onlinepubs/nchrp/docs/NCHRP_ActiveImplementation.pdf
K. If the team proposes a Principal Investigator who is not an employee of the Prime Contractor, or if the Prime Contractor is proposed to conduct less than 50% of the total effort (by time or budget), then section five of the proposal should include: (1) a justification of why this approach is appropriate, and (2) a description of how the Prime Contractor will ensure adequate communication and coordination with their Subcontractors throughout the project.
L. All budget information should be suitable for printing on 8½″ x 11″ paper. If a budget page cannot fit on a single 8½″ x 11″ page, it should be split over multiple pages. Proposers must use the Excel templates provided in the Information and Instructions for Preparing Proposals for the Transportation Research Board’s Cooperative Research Programs.