Aligning Geometric Design Controls, Criteria and Elements with Roadway Context, Modal Priority and Functional Classification
Since 1984, the AASHTO “Green Book” (A Policy on the Geometric Design of Highways and Streets) and other roadway design criteria have been primarily based on a functional classification system of a hierarchical network composed of arterials, collector, and local roads. This classification is further assigned by an urban or rural designation. This system is described in detail in Highway Functional Classification Concepts, Criteria, and Procedures (FHWA-PL-13-026).
This traditional system of highway and street classification has been under increasing scrutiny and discussion because of its inability to reflect emerging design issues such as context-sensitive design, livable communities, practical design, and other innovative approaches. The following are some key concerns with the existing system:
Designation as simply urban or rural land use context is insufficient to adequately account for the full range of contexts that can exist along a highway or street.
Classification leads to recommended or limited design choices that may not be optimal for the particular facility and its adjacent land use contexts. These restrictions promote “designing to standards” rather than use of engineering judgment in carefully considering the safety, operational, and other impacts of design decisions on all travel modes using facility.
The public often questions the use of these classifications as the basis for design decisions.
The current system is focused on the needs of vehicle drivers and does not fully or effectively address the needs of other types of user modes (e.g., pedestrians, bicyclists, transit riders, freight vehicles). In particular, it does not help with design decisions that must balance benefits for one mode against disbenefits for another.
Project 15-52, Developing a Context-Sensitive Functional Classification System for More Flexibility in Geometric Design, was completed in 2018. NCHRP Research Report 855, An Expanded Functional Classification System for Highways and Streets, was published later in 2018 and the final product of this research. This report presents an expanded functional classification system for highways and streets that builds upon the current system to provide a better basis for the preliminary engineering of a design project, including developing the purpose and need. In particular, it provides additional contexts beyond urban and rural, facilitates accommodation of modes other than personal vehicles, and adds overlays for transit and freight.
The purpose of this proposed research is to further implement the work of NCHRP 15-52/Report 855 by developing recommended geometric design controls, criteria, and elements ranges and applications that are considered appropriate to the expanded context and modal functional classification matrix. This research will build on and advance previous and current geometric design research efforts for applying the design flexibility that currently exists in the Green Book to address the full range of system, context, and modal aspects of street and highway design in order to create a modal balance to achieve successful outcomes. These considerations will allow the planner/designer to identify potential areas of concern and determine the tradeoffs required to best accommodate all users and achieve the end operating conditions that are desired. Alternative designs should be developed and evaluated in order to deliver a design that is contextually appropriate and best achieves the balance of service, safety, convenience, and accessibility to all modes present or planned.
The objectives of the research are to identify
appropriate applications and ranges of design controls, criteria, and elements
across a range of functional classifications, contexts, and modal priorities
consistent with the recommendations of Research Report 855. This work will identify
the geometric design parameters for the types and designs of facilities needed
to serve the travel demands of all current and planned user modes across a
range of contexts within each functional classification. The results will be
presented in the form of design guidelines supporting each of the cells of the
table in Figure 25. Expanded FCS multimodal matrix by context and roadway type, from
Research Report NCHRP 855.
The research should include a literature review
of previous research and current practice, development of a work plan to
achieve the research objectives, collection of applicable best practices and
supporting information, assessment of the safety and operational effects of
various combinations of design controls, criteria, and elements, and
preparation of a final report. The final report should include proposed changes
to AASHTO documents, if the results support such changes.
In 2016 AASHTO approved a committee restructuring plan that addressed a number of elements including the need for the organization to provide more multimodal and context-focused guidance to the transportation profession along with guidance on use of flexibility in the design process.
This research topic was identified as a high priority by the AASHTO Technical Committee on Geometric Design, the TRB Committee on Geometric Design, and the TRB Committee on Operational Effects of Geometrics at their combined meeting in June 2018 from among a broad set of problems considered.
The research is critically needed to fill gaps in current roadway design guidance to address and serve the needs of all roadway facility users being served in the roadway right-of-way. The research results should be presented in a standalone document that can be used to supplement existing design policies and guidelines.
A literature search found extensive
research products and guidance on planning for all travel modes along roadways,
especially on an area-wide basis, but very little specific research and
guidance on comprehensive, context-sensitive multimodal geometric design guidance
for facilities or intersections.
Limited geometric design research and
guidance is available in this subject area. Several studies have been conducted
that dealt with the safety of the various non-vehicle roadway users, but little
has been done to correlate balance of design elements to optimize the relative
safety risks and operational levels of service between various motorized and
non-motorized roadway users across a range of contexts and functional
There are many documents that should be
considered as part of this research. Some of the more relevant documents
Association of State Highway and Transportation Officials. 2014. Guide for
the Development of Bicycle Facilities. Washington, DC: AASHTO.
Association of State Highway and Transportation Officials. 2011. Policy on
Geometric Design of Streets and Highways, Sixth Edition. Washington, DC: AASHTO.
Transportation Research Board. 2016. NCHRP Report 15-52: Developing a Context-Sensitive Functional
Classification System for More Flexibility in Geometric Design
Association of State Highway and Transportation Officials. 2004b. _Guide for
the Planning, Design and Operation of Pedestrian Facilities. _Washington,
Association of State Highways and Transportation Officials. 2010. _Highway
Safety Manual. _Washington, DC: AASHTO.
Association of State Highways and Transportation Officials. 2011. _Roadside
Design Guide. _Washington, DC: AASHTO.
Association of State Highways and Transportation Officials. 2004c. _A Guide
for Achieving Flexibility in Highway Design. _Washington, DC: AASHTO.
Highway Administration. 2016. _Achieving Multimodal Networks: Applying Design
Flexibility and Reducing Conflicts. _Washington, DC. FHWA.
Access Board. 2002. Americans with Disabilities Act Accessibility
Guidelines. Washington, DC. USAB.
Access Board. 2011. Proposed Guidelines for Pedestrian Facilities in the
Public Right-of-Way. Washington, DC. USAB.
Research Board. 2002. NCHRP Report 480: _A Guide to Best Practices for
Achieving Context Sensitive Solutions. _Washington, DC: TRB.
Research Board. 2002. NCHRP Report 480: _A Guide to Best Practices in
Achieving Context Sensitive Solutions. _Washington, DC: TRB.
American Association of State Highway and Transportation Officials, Guide
for Geometric Design of Transit Facilities on Highways and Streets, 2014
Federal Highway Administration, Separated Bike Lane Planning and Design
Federal Highway Administration, Livability in Transportation Guidebook,
Institute of Transportation Engineers, Designing Walkable Urban
Thoroughfares: A Context Sensitive Approach, 2010
National Association of City Transportation Officials, Urban Bikeway
Design Guide, 2014
National Association of City Transportation Officials, Urban Street
Design Guide, 2013
National Association of City Transportation Officials, Transit Street
Design Guide, 2016
Federal Highway Administration, 2013, Highway Functional Classification: Concepts,
Criteria and Procedures.
Institute of Transportation Engineers, 2016,_
Recommended Design Guidelines to Accommodate Pedestrians and Bicycles in
Research Board, 2016, NCHRP Report 737: Design
Guidance for High-Speed to Low-Speed Transitions Zones for Rural Highways
Research Board, 2008, NCHRP Report 613: Guidelines
for Selection of Speed Reduction Treatments at High-Speed Intersections
Research Board, 2016, Highway Capacity
Manual, Sixth Edition: A Guide for Multimodal Mobility Analysis
The results of this research could be directly incorporated into standard references, including the AASHTO Green Book (for geometric configurations) and the MUTCD (unique signing, pavement markings, and signalization needs) as related to the geometric design elements. In addition, the information could be used to update guidebooks such as AASHTO Pedestrian and Bicycle Accommodation guides.
The findings of this project will be directly applicable to the planning, design, and operation of roadways and intersections. Designers will have more comprehensive design guidance for these types of roadway facilities and the mobility and safety of all roadway users will be improved through better designs and integration of modes. The findings will be of interest to a number of TRB committees, AASHTO, State DOTs, and other roadway design agencies. They will also have the potential to provide new material for such authoritative documents as the _HCM _and various state standards and guidelines.
|Sponsoring Committee:||AKD10, Performance Effects on Geometric Design
|Research Period:||12 - 24 months|
|RNS Developer:||R. Marshall Elizer, Douglas Harwood|
|Source Info:||This problem statement was developed in connection with the June 2018 mid-year joint meeting of the AASHTO Technical Committee on Geometric Design, the TRB Committee on Geometric Design (AFB10), and the TRB Committee on Operational Effects of Geometrics (AHB65).|
|Index Terms:||Geometric design, Highway design, Design standards, Classification, Transportation modes, |
Pedestrians and Bicyclists
Operations and Traffic Management
Safety and Human Factors