NCHRP 15-59 [Final]
Horizontal Sightline Offset Design Criteria, Exceptions, and Mitigation Strategies
| Project Data
||Published as NCHRP Research Report 910.|
When the sight lines along horizontal curves are impaired, designers are challenged to find acceptable solutions. Designers compensate for the limitations on driver sight distance in various ways, including: accepting deficient sightlines, lowering design speed, increasing shoulder width, or providing additional signage. There are advantages and disadvantages to the trade-offs being made; therefore, many agencies have used the design exception process to address the trade-offs for sight distance in such situations. Research is needed to evaluate these situations and determine what criteria or mitigation will provide acceptable solutions when impaired horizontal sightline offsets are encountered.
The objectives of this research are to: (1) evaluate the safety and operational performance as well as the trade-offs and risks associated with state-of-the-practice mitigation treatments when horizontal sightline offset criteria and guidance are not met; and (2) recommend updates to AASHTO’s Policy on Geometric Design of Highways and Streets (Green Book) for horizontal sightline offset design criteria and guidance on curved roadway alignments adjacent to barriers or other types of impediments that may impact the driver’s line of sight. The research should address a broad range of issues related to horizontal sightline offset design criteria and guidance for curved roadway alignments and may include, but not be limited to, the following:
- Review of research, practice, and innovative approaches in the United States and other countries that complement the research objective, including substantive approaches to assessing safety for different types of roadways.
- Identification of factors (e.g., human factors, vehicle characteristics, vertical alignment, construction costs, and other built or natural environmental conditions) that influence horizontal sightline offset design and performance.
- In situations where horizontal sightline offset design criteria could not be met, what trade-offs and risks were considered in choosing the final design elements (e.g., shoulder width, barrier height, design speed)? What elements are most commonly considered to best resolve deficiencies? What mitigation strategies were implemented and how were they chosen? How did the facilities perform after mitigation?
- In future situations when horizontal sightline offset criteria cannot be met, develop guidance on what trade-offs and risks should be considered, what elements are most likely to best resolve deficiencies, and what mitigation strategies should be evaluated based on risks and trade-offs.
- Proposed new text for the next edition of AASHTO’s Policy on Geometric Design of Highways and Streets (Green Book).
A kick-off teleconference of the research team and NCHRP shall be scheduled within 1 month of the contract’s execution. The work plan proposed must be divided into tasks, with each task described in detail. The tasks must be divided into two phases. Phase I will consist of information gathering and planning tasks, culminating in the submittal of an interim report. The interim report will describe the work completed in the Phase I tasks and provide an updated work plan for the Phase II tasks and an outline of the recommended updates to the AASHTO Green Book. The updated Phase II work plan should address the manner in which the proposer intends to use the information obtained in Phase I to satisfy the project objective. A face-to-face interim meeting with NCHRP will be scheduled to discuss the interim report. Work on Phase II tasks shall not begin until the updated work plan is approved by NCHRP. The project schedule shall include 1 month for NCHRP review and approval of the interim report. The final deliverables will include the text for the updated AASHTO Green Book (suitable for review and balloting by the AASHTO Technical Committee on Geometric Design), plus a final report that documents the entire research effort and other deliverables as described in the research plan. Deliverables should also include an executive summary and a Microsoft PowerPoint presentation describing the background, objectives, research method, findings, and conclusions. The research team will be expected to present the results at a minimum of two meetings of AASHTO committees or other relevant national organizations to be determined by the NCHRP.