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Design Flexibility and Exceptions

Under CSS, design flexibility allows road designers to tailor their design to a specific context. The concept of flexibility is being applied by state and local transportation agencies to existing standards as well as in the development of new guidelines and standards.



The decision of a transportation agency to deviate from applicable design criteria or standards is called a design exception. Exceptions are sometimes, but not always, needed for CSS projects. 3R design manuals, with their "reduced geometric criteria," can all but obviate the need for design exceptions.

Article Icon Article / Paper / Report Design Exceptions - CSS Quick Facts
The subject of design exceptions at the project level inevitably generates controversy. CSS skeptics have argued that accepting the premise of CSS inevitably means that more design exceptions will occur, or that they will be expected to more readily accept a design exception under pressure from a stakeholder or interest group. A commitment to CSS by an agency does not mean abandonment of design standards. Professional engineers are ethically and legally required to follow the accepted practices of the profession. A design exception is a documented decision to design a highway element or segment of highway to a design criterion or value that does not meet the minimum value that has been established for that highway or project. Examples may include the use of a narrower shoulder than design standards show, a curve with radius smaller than the minimum for the selected design speed, or a crest vertical curve that does not provide the minimum stopping sight distance for the selected design speed. FHWA has designated 13 controlling criteria as being of sufficient importance to warrant a formal design exception, summarized below.
Excerpt IconExcerpt Design Flexibility
"In general, the public more easily accepts the lack of flexibility in motorway design because of the purpose of these roads, mobility is gained at the expense of aesthetic treatment. Greater design flexibility was observed for urban and rural non-motorways that are typically responsive to site-specific conditions." more...
from  Geometric Design Practices for European Roads
Article Icon Article / Paper / Report Developing Street Standards That Allow Flexibility
"The City of Sacramento, California, recently updated its street design standards. This update was in response to a consistent message from residents that the previous standards did not result in livable neighborhoods, to protests from the development community that standards were too rigid..."
-- Gwen Owens, City of Sacramento
Book Icon Book Flexibility in Highway Design
A guide about designing highways that incorporate community values and are safe, efficient, and effective. It is written for highway engineers and project managers who want to learn more about flexibility available to them when designing roads and illustrates successful approaches used in other highway projects. The guide aims also at provoking innovative thinking for fully considering the scenic, historic, aesthetic, and other cultural values of communities, along with safety and mobility needs.
--  Federal Highway Administration
Excerpt IconExcerpt Philosophical Approach to Design Exceptions
Creativity or flexibility in highway design should not be equated with operating outside their geometric design policies. more...
from  NCHRP Report 480: A Guide to Best Practices for Achieving Context Sensitive Solutions
Article Icon Article / Paper / Report Design Exception Practices: A Synthesis of Highway Practice
"Only 20% of [State Transportation Agencies] STAs indicated that the advent of context-sensitive design has increased the number of design exceptions they prepare ... Almost all STAs view design exceptions as a value-adding process. Simply having a record of the decision process and its use in managing tort risk are the principle benefits cited by STAs..."
-- National Cooperative Highway Research Program (NCHRP)

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