The National Academies

NCHRP 15-25 [Completed]

Alternatives to Design Speed for Selection of Roadway Design Criteria

  Project Data
Funds: $325,000
Research Agency: Midwest Research Institute
Principal Investigator: Douglas W. Harwood
Effective Date: 5/13/2004
Completion Date: 7/31/2008


The objective of this research is to recommend comprehensive improvements or augmentations to the design-speed approach for setting geometric design criteria. This research is intended to apply to all types of roads.


The Task 6 interim report was discussed with the project oversight panel in February 2005. The research team developed a broad range of alternative approaches and the panel selected a few for futher development.  Due to contractural difficulties, the work will be completed under NCHRP 15-47.


Design speed has long been a prime factor in the design of roadway geometric elements, such as vertical and horizontal alignment and cross section. The current design process does not always result in the desired consistency in roadway alignment or driver behavior along these alignments. The desired product of good geometric design is a roadway alignment and cross section that will encourage the driver to operate safely and consistently with the function of the facility. Further, an ideal geometric design is both consistent with the context of the setting and cost-effective.

Many factors must be considered when designing a roadway; the dominance of design speed in setting geometric design criteria (i.e., ranges of acceptable design values) raises several issues:
  • Contrary to expectations, operating speed is often higher than design speed. The factors that affect operating speed are not well understood, but the worst case assumptions used in design (e.g., older, ill-maintained vehicles; inattentive and impaired drivers; poor weather and light conditions) contribute to the lack of correlation. The vehicle aspect is particularly difficult because the characteristics of the fleet are continually changing.
  • The public often desires that operating speeds be low. They often focus on a low design speed as the best way to ensure low operating speeds.
  • Designers must be creative in reaching the most effective design; focusing on design speed can mask some innovative solutions. Creativity and innovation are particularly important when the environment, land use, and right-of-way constrain the design or when a roadway is on the boundary between functional classifications. Designing a safe and cost-effective low-volume road can also be challenging.
  • Even though the AASHTO definition of design speed has changed, plaintiff arguments in tort liability suits often misapply the design-speed concept, and redressing this in a courtroom can be difficult.


Task 1. Analyze the strengths and limitations of using design speeds to set geometric design criteria for the individual design elements of a project.

Task 2. Identify factors in addition to design speed that should be considered in setting geometric design criteria, and estimate the importance of each factor.

Task 3. Summarize knowledge on the effects of design elements on operating speeds, including a review of speed-prediction models.

Task 4. Characterize approaches to selecting a design speed.

Task 5. For the overall process and individual design elements, describe potential improvements and augmentations to the criteria-selection process and prioritize them. These should include both modest improvements and more strategic ones that could have a major effect on the design process. Prepare detailed, alternative work plans and budgets to develop the individual improvements and augmentations.

Task 6. Within 6 months of the contract start, submit an interim report summarizing the results of Tasks 1 through 5.

Task 7. Carry out the approved work plans.

Task 8. Recommend changes to the AASHTO Green Book and Roadside Design Guide. Units of measure must be consistent with the current publications. Identify any conflicts with other AASHTO documents.

Task 9. Prepare a final report that documents the entire research effort. The product of Task 8 shall be included as a stand-alone appendix, suitable for submission for review by the AASHTO Technical Committees on Geometric Design and Roadside Safety.

To create a link to this page, use this URL: http://apps.trb.org/cmsfeed/TRBNetProjectDisplay.asp?ProjectID=405