The National Academies

NCHRP 15-52 [Completed]

Developing a Context-Sensitive Functional Classification System for More Flexibility in Geometric Design

  Project Data
Funds: $300,000
Research Agency: University of Kentucky
Principal Investigator: Nick Stamatiadis
Effective Date: 1/15/2015
Completion Date: 1/31/2018


The objective of this research is to identify potential improvements to the traditional functional classification system to better incorporate the context, user needs, and functions of the roadway facility. The potential improvements should lead to a flexible framework that can be used by planners and designers in the development of optimal geometric design solutions.

The final report is available in prepublication form at http://www.trb.org/Publications/Blurbs/176004.aspx. The research team worked with two states to apply the guidance, validate it, and improve it based on the feedback received. The formal report is being prepared for publication.

Since 1984, the AASHTO “Green Book” (A Policy on the Geometric Design of Highways and Streets) and other roadway design criteria have been based on a functional classification system of a hierarchical roadway network composed of arterials, collector roads, and local roads. This classification is further broken out by an urban or rural designation. This system is described in Highway Functional Classification Concepts, Criteria and Procedures (FHWA-PL-13-026). This system of highway classification has been under increasing scrutiny and discussion due to some incompatibilities with context-sensitive design, practical design, and other innovative approaches. The following are some concerns:
  • Designation as urban or rural is insufficient to adequately account for the range of contexts for a roadway.
  • The current system is focused on the needs of vehicle drivers and does not help in serving the needs of other types of users (e.g., transit riders, pedestrians, bicyclists). In particular, it does not help with design decisions that must balance benefits for one mode against disbenefits for another (e.g., narrower lanes that benefit pedestrians but make it harder for trucks to use the road).
  • Classification leads to recommended or limited design choices that may not be optimal for the particular roadway. These restrictions promote “designing to standards” rather than a careful consideration of the safety, operational, and other impacts of design decisions.
  • The public often questions the use of these classifications as the basis for design decisions.
The current functional classification system has implications on many aspects of transportation agency functions (e.g., http://goo.gl/ah3fER). Research is needed that is not constrained by the traditional functional classification system and explores potential consequences to uses other than geometric design.

Task 1. Review the traditional functional classification system and identify strengths and weaknesses as related to geometric design decisions.

Task 2. Identify and describe existing alternative classification schemes and methods for assigning those classes to roadways. Include alternatives used by state DOTs and other agencies as well as the “new urbanism” approaches.

Task 3. Outline promising new or updated approaches for classifying roadway facilities. For each approach, the expected benefits to the geometric design process should be described. These approaches should account for:  a) the context of the roadway facility (including the surrounding environment, going beyond the urban/rural dichotomy), b) the function of the roadway for the various users, and c) system and community goals (e.g., connectivity, safety and operational performance, livability, sustainability, comprehensive plans).

Task 4. Prepare an interim report summarizing the results of Tasks 1 through 3 at least 1 month before the 2015 Summer Meeting of the Technical Committee on Geometric Design. Make a presentation to the technical committee on the project and subsequently meet with the project panel to decide which approach(es) to develop further.

Task 5. Upon panel approval, develop the selected approach(es); including a set of roadway classifications, methods for assigning a class to a particular roadway facility, and the range of design alternatives suitable for that class.

Task 6. Describe the impacts of the developed approach(es) on other uses of the current functional classification system.

Task 7. Prepare a final report that documents the entire research effort. The report should include an updated implementation plan for moving the research results into practice and an appendix with recommended modifications to the AASHTO Green Book.

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