State highway agencies, and others responsible for road systems, do not currently have useful tools for reflecting safety in their decisions concerning freeway and interchange projects. This diminishes the weight placed on safety considerations in these decisions. When difficult choices must be made, greater confidence is often placed on predictions of such factors as cost, operational impacts, and environmental impacts, which are expressed in quantitative terms. Therefore, an effective tool to quantify the safety impacts of proposed projects for consideration in planning, design, and operations decisions is needed.
Safety prediction procedures have been developed for rural two-lane highways, rural multilane highways, and urban and suburban arterials, and these will be included in the First Edition of the Highway Safety Manual (HSM) to be published in 2009/10. Base safety models were developed by the Federal Highway Administration for freeways and interchanges, and these are included in the network screening module of the SafetyAnalyst software and in the Interchange Safety Analysis Tool (ISAT). ISAT was developed as an interim tool to meet immediate needs. The ISAT tool provides crash estimates of three typical interchange configurations (diamond, partial cloverleaf, and full cloverleaf), but has limited capabilities and flexibility. This spreadsheet-based tool can be significantly enhanced to improve the quality of the crash estimates and the tool’s overall usefulness.
Research is needed to develop an enhanced prediction methodology and safety analysis tool (ISAT or a successor) for corridor and site-specific analysis. These results will lead to the development of a new chapter on safety predictive models for freeways and interchanges for the HSM and the documentation for the expansion of the Interactive Highway Safety Design Model (IHSDM).
The objectives of this research are to develop (1) an overall framework for the enhancement of safety prediction methodologies for freeways and interchanges to support decision making for planning, network, corridor analysis, and individual site analysis; (2) safety analytical models and procedures within that framework; (3) models and procedures for a corridor and individual site application tool (e.g., enhanced ISAT or successor); (4) a chapter for the future edition of the HSM; and (5) documentation for inclusion of the models in the IHSDM.
Accomplishment of the project objectives will require at least the following tasks.
(1). Evaluate existing knowledge for predicting safety performance of freeways and interchanges to identify strengths and gaps in existing models. (2). Based on the findings of Task 1, develop an overall framework and plan to enhance the methodology for safety predictions for freeways and interchanges, and to integrate the enhancements into ISAT (or successor), HSM, and IHSDM. The framework and plan will define the analytical tools available and needed to support decision making for planning; design; and network, corridor and individual site analyses regarding freeways and interchanges. The framework will identify the steps to improve each of these areas from planning through operations. These steps will include, as a minimum: (1) defining the interchange influence area for the predictive models; (2) identifying additional predictive parameters to include in the models and the data required; (3) investigating additional Accident Modification Factors (AMFs) for use with the predictive models and for potential site improvements, and the data required; (4) prioritizing recommended improvements to base models (SPFs) and AMFs for freeway segments, ramp auxiliary lanes, ramp terminals, and crossroads for incorporation into the application tools; (5) implementing the enhanced ISAT or successor tool; (6) developing chapter content for the future edition of the HSM; and (7) developing documentation for inclusion of the models in the IHSDM. (3). Submit an interim report that provides the results of Tasks 1 and 2. The report shall include, as a minimum:
- Research framework and priorities.
- Phase II work plan with task descriptions, timelines, staff, and effort by task, including but not limited to:
- Research protocols.
- Additional information necessary to justify the recommended research approach.
- Data sources and collection procedures.
- List of models and parameters to be developed.
- Description of how the models will be validated and calibrated to at least one state.
- Description of how the new models can be implemented into existing safety procedures, such as SafetyAnalyst, IHSDM, HSM, etc.
- Description of the enhanced ISAT tool or successor including how the contractor will develop the draft application tool and user manual, and how it will be demonstrated to the project panel.
- Description of the methodology and user group for testing the enhanced ISAT or successor.
- List of other deliverables and their uses.
(4). Meet with the NCHRP panel to review the Task 3 interim report approximately 1 month after its submittal. Submit a revised interim report addressing the panel’s review comments. The contractor shall not proceed with Phase II without the approval of the NCHRP.
(5). Execute the approved Phase II work plan. (6). Submit the draft enhanced ISAT or successor tool including the knowledge base and user manual. Propose a methodology and recommend a user group for testing the tool. (7). Conduct a demonstration of the application tool via a web conference (or similar arrangement) with the project panel. (8). Conduct a user evaluation of the draft enhanced ISAT or successor, collate and address comments and concerns, and revise the draft tool accordingly. (9). Submit a final report documenting the entire research effort, including as appendices the prediction methodology, the enhanced ISAT or successor tool and user manual, chapter for HSM, and documentation for inclusion of models in the IHSDM.
The final report is pending publication. Note: Content from the final report was included in the HSM First Edition Supplement (2014).