The National Academies

NCHRP 15-47 [Final]

Developing an Improved Highway Geometric Design Process

  Project Data
Funds: $524,967
Research Agency: CH2M Hill
Principal Investigator: Tim Neuman
Effective Date: 3/15/2013
Completion Date: 6/14/2016

The draft update of the Very Low Volume Roads guide has been submitted to AASHTO and is proceeding towards publication by them.

The objectives of this research were to (1) develop a comprehensive, flexible design process to meet the needs of geometric designers in the future and (2) update AASHTO's Guidelines for Geometric Design of Very Low-Volume Roads. The design process (Objective 1) must consider:
  • Specification of the project purpose and need, including the modes that will be using the facility.
  • Context setting of the facility.
  • Desired performance outcomes for the facility for the various modes; including safety, mobility, and access management.
  • Methods for evaluating tradeoffs associated with different design alternatives.
  • Optimization of the design given the project’s financial and other constraints.
  • Flexibility to address issues that arise from stakeholder involvement or environmental reviews.
  • Documentation of decisions to address tort liability concerns.
The design of a highway—its three-dimensional features (horizontal alignment, vertical alignment, and cross-section) and appurtenances to provide for drainage, traffic control, and safety—requires a well-defined process. AASHTO and its predecessor, AASHO, have published highway design policy since the 1940s; the underlying highway design process has remained essentially unchanged since that time. That process has the following characteristics:
  • Dimensionally based, with design values for physical dimensions directly derived from tables, charts, and equations.
  • Requires establishment of fundamental design controls including location, terrain, and functional classification.
  • Requires designers to make choices for other major factors (e.g., design speed, design hour volume, design vehicle) that will influence subsequent design decisions from within established ranges.
  • Based on selection of a design speed, and in some cases design vehicular traffic volume, other design criteria are directly derived or obtained for minimum dimensions (e.g., lane width, curve radius) and/or maximum dimensions (e.g., grade) as appropriate for the design controls and assumptions.
  • Direct performance measures in terms of vehicle mobility, including speed and level of service, are explicitly considered in some design decisions (e.g., number of lanes).
  • Costs versus benefits are also an integral part of the design process, but are implicitly considered through recommended dimensional ranges for different area and terrain types.
  • Nominal safety is presumed through the application of the process and technical guidance but safety performance may not be explicitly considered.
  • Relies on mathematical models as the basis for derivation of dimensional values (e.g., point-mass model for selection of curve radius and superelevation).
NCHRP Project 15-25, "Alternatives to Design Speed for Selection of Roadway Design Criteria," examined a critical step in the traditional design process. The January 2005 Interim Report for that project points out that “While design speed has always been central to the geometric design process, actual road designs are strongly influenced by design speed in some cases, but not in all cases.” That report also notes that “on portions of the highway removed from sight obstructions and horizontal curves, highways designed for a broad range of design speeds may look nearly identical.” That project explored alternatives to that portion of the traditional process but lacked the necessary resources and scope to develop a comprehensive solution.

During the past 60 years, transportation needs have changed and much has been learned about the relationships among geometric design, vehicle fleet, human factors, safety, and operations. AASHTO has continually updated its policies to respond to these changes, but such updates have provided limited changes to the fundamental process or basic design approaches. Some agencies have begun using an expanded array of roadway functional classifications as a basis for selecting certain design criteria. An assessment of the current design process is needed to ensure that recent advances in knowledge (e.g., the AASHTO Highway Safety Manual) and emerging issues (e.g., complete streets, flexible design) are appropriately addressed.

PHASE I (Objective 1)


Task 1. Review Literature, Data Sources, and Current Practice. The objective of this task is to summarize the current state-of-the-knowledge and state-of-the-practice on traditional design processes and opportunities for improvement. This will be accomplished by conducting a literature review, surveying state and local highway agencies, interviewing leading highway agencies and practitioners, and identifying and evaluating available databases.


Task 2. Coordination with AASHTO and TRB Committees. There will be extensive coordination, throughout the research effort, with the AASHTO Technical Committee on Geometric Design, the TRB Geometric Design Committee, the TRB Operational Effects of Geometrics Committee, the TRB Highway Capacity and Quality of Service Committee, and the TRB Highway Safety Performance Committee. A key element in this coordination will be the conduct of a half-day workshop at their joint meeting tentatively scheduled for July 2013 in Irvine, California.


Task 3. Prepare White Papers. The objective of this task is to prepare white papers on key topics including:

  • Review and critique of the current design process and opportunities for improvement
  • Experiences of transportation agencies that have developed alternative design processes, including practical design, complete streets, context-sensitive solutions, livability, and similar initiatives
  • Potential alternative process concepts
  • Challenges and issues associated with adopting design process changes


Task 4. Prepare Phase II Work Plans and Interim Report. In Task 4, the research team will prepare specific work plans for Phase II of the project. These plans may include:

  1. Finalize Conceptual Plan for a Revised Design Process
  2. Identify Knowledge Gaps That Could Limit Implementation of the Revised Design Process
  3. Conduct Research to Fill in as Many Knowledge Gaps as Possible
  4. Prepare a Design Guide for Implementing the Revised Design Process
  5. Recommend Changes to the AASHTO Green Book

The interim report will include all results of Tasks 1 through 4, specifically including the Task 3 white papers and the Phase II work plans. Following submission of the interim report, the research team will meet with the NCHRP project panel to present the research results and to discuss the work plans.


PHASE II (Objective 1)


Task 5. Execute Approved Phase II Work Plan. In Task 5, the research team will execute specific work plans developed in Task 4.


Task 6. Final Report and Presentation. The research team will prepare a preliminary draft final report documenting the entire research effort and including the design guide developed in Task 5 as an appendix. The draft report will be revised in response to comments received from the NCHRP and the project panel. The final report will be submitted on or before the project completion date. As part of Task 6, the research team will present a final briefing to the AASHTO Technical Committee on Geometric Design at their meeting in the Summer of 2014.

PHASE III (Objective 2)

Task 7. Assess Highway Agency Experience with the Current Guidelines. Activities will include a survey of highway agencies on their experience with the current guidelines, telephone discussions with selected highway agencies to follow up on the survey responses, and a briefing and discussion with the AASHTO Technical Committee on Geometric Design.

Task 8. Prepare Technical Memorandum with Recommendations for Updating the Guidelines. The memorandum will identify the guideline sections for which updates will be considered, the nature of the updates that will be considered, as well as any expansion of the scope of the guidelines that will be considered within the project budget.

Task 9. Conduct Cost-Effectiveness Analyses. The analyses will focus on the appropriate level of investment in geometric design improvements for projects in specific traffic volume ranges.

Task 10. Prepare First Draft of Updated Guidelines.

Task 11. Prepare Final Draft of Updated Guidelines for Balloting by AASHTO.

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