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An Assessment of Geometric Design Policies and Processes

Accomplishing 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 resulting from policy implementation has remained essentially unchanged since that time. The current design process can be briefly outlined as follows:
·         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 that will influence subsequent design decisions from within established ranges. These include design speed and design traffic, which includes not only volume (e.g., design hour volume) but also type of vehicle (i.e., design vehicle).
·         Based on selection of a design speed, and in some cases design traffic, 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. Acceptable safety is presumed through application of the process and technical guidance, but is nonetheless an indirect outcome of the process.
·         Relies on relatively simple mathematical ‘models’ as the basis for derivation of dimensional values (e.g., point-mass model for selection of curve radius and superelevation).
During the past 60 years much has changed in the vehicle fleet, knowledge about driver characteristics (i.e., human factors), and safety and operations. AASHTO has committed to continually update their policies. Yet, for the most part, such updates have not altered the fundamental process or even, in most cases, the basic design models. Mathematical simplifications driven by lack of information and/or ease of computing may no longer be appropriate, and may result in suboptimal outcomes in the aggregate. Therefore, an assessment of the current design process is needed to ensure that recent advances and emerging issues are appropriately leveraged and/or reflected in the policies.
Of note: A “Strategic Geometric Design Research Needs Workshop” was held in Williamsburg, Virginia, in July 2004,as a joint effort by three committees— the AASHTO Technical Committee on Geometric Design and the Transportation Research Board’s (TRB’s) Geometric Design Committee (AFB10) and Operational Effects of Geometrics Committee (AHB65).At the workshop, a Research Problem Statement (“Investigation of Alternative Geometric Design Highway Design Processes: Strategic Research”; documented in Transportation Research Circular E-C110 [1]) similar in concept was voted as one of the highest priorities, but was not funded.
There has been much research in the area of capacity, speed, and to some degree, relating crashes to highway features. Recent and relevant research efforts include but are not limited to:
·         NCHRP Synthesis 299:Recent Geometric Design Research for Improved Safety and Operations [2], NCHRP Report 374 (Safety Effects of Highway Geometric Elements) [3], and the body of research that has led to FHWA’s Interactive Highway Safety Design Model [4]. NCHRP Synthesis 20-5 / Topic 42-04 [5] was funded in 2010 and will serve to update NCHRP Synthesis 299.
·         AASHTO sponsored research has led to revisions in design policy in the areas of stopping sight distance (NCHRP Report 400) [6] and Intersection Sight Distance (NCHRP Report 383).[7]
·          NCHRP Report 439 [8] addressed some aspects of horizontal curve design, including recommended changes to policy.
·         A new look at passing sight distance with suggested policy revisions was also recently completed (NCHRP Report 605).[9]
·         Research is completed or ongoing in key areas dealing with freeways and interchanges, urban and suburban arterial segment and intersection design and safety, and rural highway segment and intersection design and safety. AASHTO’s new 1st Edition Highway Safety Manual [10] has incorporated results of several of those studies.
Other notable efforts have focused on the design process. These include:
·         A completed research synthesis dealing with design exceptions, published as NCHRP Synthesis 316 [11]
·         Ongoing synthesis 20-05 studies: 1) topic 39-09, Design Flexibility Considerations for Established Cities [12] and 2), topic 40-07, Trade-Off Considerations in Highway Geometric Design.[13]
·         The results of NCHRP 15-27, Safety Impacts of Design Element Trade-offs have been published in NCHRP Report 633 [14].
·         NCHRP Project 15-25, Alternatives to Design Speed for Selection of Roadway Design Criteria [15], should provide background and focus for some of the effort. However, the issue goes beyond design speed alone.
NCHRP Project 15-34, Performance-Based Analysis of Geometric Design of Highways and Streets [16], has an objective to develop a guide for performance-based analysis of geometric design throughout the development of a project. [17] The guide is to identify existing tools for estimating performance and illustrate their use. Further, the guide will describe additional tools or enhancements to existing tools needed for estimating performance and a plan for developing them. The final report is to identify recommendations for text changes in the AASHTO Policy on Geometric Design of Highways and Streets. [17]
As documented above, much good and notable related-research has been conducted. However, the vast majority of studies have accepted the underlying theory and structure of the existing design models/process and built upon that “foundation.” For example, research to update the stopping and passing sight distance models did not change the underlying models, and did not seem focused on assessment of the safety impacts. The research proposed here aims to critically assess the underlying models (i.e., to evaluate the “foundation”), and to tie the underlying theory to performance.
The research objective is to identify needed updates to current design policy, and/or necessary changes in the fundamental design process to incorporate the latest traffic operations and safety knowledge. Two basic questions will be addressed:
1.      Does the structure of the current AASHTO Policy formulation meet the needs of all stakeholders?
2.      What gaps exist in the current Policies and how should those gaps be filled?
Tasks in this project would include:
·         Perform a critical review of the format, structure and basic assumptions included in the AASHTO Policies governing geometric design of highways and streets.
·         Review and assess the structure of current AASHTO Policy formulation to assure it meets the needs of all stakeholders.
·         Assess whether, and to what extent the current design process reflects the explicit consideration of performance (e.g., level of service, safety) and promotes efficient, if not optimal, combination of design elements to yield designs that are cost-effective when considering life-cycle benefits and costs.
·         Assess and address the building of our knowledge base on the fundamental technical inputs to current geometric design features. For example, at present, the primary technical input for most features is design speed; are there other inputs that should be considered, e.g., safety-related factors?
·         Identify the current applicability of the source data that was used as a basis for the past research that formulated the AASHTO Policies. Such policies primarily include the A Policy on Geometric Design of Highways and Streets (Green Book) [18] and Roadside Design Guide [19], as well as the Guide for Achieving Flexibility in Highway Design [20].
·         A critique of AASHTO design models as to their adequacy and applicability across the range of locations, traffic conditions and functional classifications.
·         Recommend needed refinements to the following design models:
o   horizontal curvature
o   stopping sight distance
o   passing sight distance
o   maximum/minimum grade and length of grade
o   minimum vertical curve length
o   cross sectional guidance
o   roadside encroachment
·         Develop a work plan to make the needed design model refinements. The work plan methodology may include applications of knowledge from published literature for some model refinements. New data collection and analysis may be needed to support other model refinements.
·         Execute the work plan following prioritization and approval from the panel.
This research effort should directly involve the AASHTO Geometric Design Task Force and Subcommittee on Design.
Recommended Funding:    $700,000
Research Period:    27 months
Improving the current design process and/or establishing new design frameworks will help to accomplish AASHTO and state agency goals. For example, AASHTO’s Strategic Plan 2009-2013 includes objectives to “establish safety as a national priority” and to “increase mobility.” Policies and procedures that explicitly consider these performance goals at all organizational levels (including the project decision level) will maximize the likelihood that those goals are achieved.
1.      “Geometric Design Strategic Research,” Transportation Research Circular Number E-C110. Transportation Research Board - Geometric Design Committee and Operational Effects of Geometrics Committee, Washington, D.C., January 2007.
2.      Fitzpatrick, K. and M. Wooldridge. NCHRP Synthesis 299: Recent Geometric Design Research for Improved Safety and Operations, Washington, D.C., 2001
3.      National Cooperative Highway Research Program. NCHRP Report 374: Effect of Highway Standards on Safety. Transportation Research Board, National Research Council, Washington, D.C., 1995.
4.      Interactive Highway Safety Design Model (IHSDM) web site: www.ihsdm.org
5.      NCHRP Synthesis 20-5 / Topic 42-04: Recent Geometric Design Research for Improved Safety, Operations and Maintenance. TRB website:
  1. Fambro, D., and K. Fitzpatrick. NCHRP Report 400: Determination of Stopping Sight Distances, 1997.
  2. Harwood, D.W., J.M. Mason, and R. Brydia. NCHRP Report 383: Intersection Sight Distance, 1996.
  3. Bonneson, J. A., NCHRP Report 439: Superelevation Distribution Methods and Transition Designs, 2000.
  4. Harwood, D.W. et al. NCHRP Report 605: Passing Sight Distance Criteria, 2008.
  5. Highway Safety Manual web site: www.highwaysafetymanual.org
  6. Mason, J.M., and K.M. Mahoney. NCHRP Synthesis 316: Design Exception Practices - A Synthesis of Highway Practice, Washington, D.C., 2003.
  7. NCHRP Synthesis 20-5 / Topic 39-09: Design Flexibility Considerations for Established Cities. TRB website:
  8. NCHRP Synthesis 20-5 / Topic 40-07: Trade-off Considerations in Highway Geometric Design. TRB website:
  9. Stamatiadis, N. et al. NCHRP Report 633: Impact of Shoulder Width and Median Width on Safety, 2009.
  10. NCHRP Project 15-25, Alternatives to Design Speed for Selection of Roadway Design Criteria. TRB web site: http://www4.trb.org/trb/crp.nsf/All+Projects/NCHRP+15-25
  11. NCHRP Project 15-34, Performance-Based Analysis of Geometric Design of Highways and Streets. TRB web site: http://www.trb.org/TRBNet/ProjectDisplay.asp?ProjectID=414
  12. Mahoney, K.M. Performance Based Design: Integrating Safety Research into Geometric Design. Presentation at the 82nd Annual Meeting of the Transportation Research Board, Washington, D.C., January 15, 2003.
18. “A Policy on Geometric Design of Highways and Streets.” AASHTO, Washington, D.C., 2004.
  1. “Roadside Design Guide, 3rd Edition.” AASHTO, Washington, D.C., 2002.
  2. “A Guide for Achieving Flexibility in Highway Design, 1st Edition.” AASHTO, Washington, D.C., 2004.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Source Info:Mike Dimaiuta, Eric Donnell, R.J. Porter and Gerald Sudimick
Date Posted:09/15/2010
Date Modified:09/15/2010
Index Terms:Highway design, Geometric design, Policy making, Vehicle fleets, Mobility, Highway operations, Evaluation and assessment,
Cosponsoring Committees: 

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