The CSS process empowers designers to use the flexibility found within design manuals.
Engineering judgment must be applied when developing designs for roadway improvement projects. The American Association of State Highway and Transportation Officials’ (AASHTO’s) guidance about providing for flexibility in design encourages agencies to incorporate the objectives of CSS principles such as substantive safety performance into project development. Flexibility in design (as discussed in AASHTO’s A Guide for Achieving Flexibility in Highway Design and the Federal Highway Administration’s [FHWA’s] Flexibility in Highway Design) helps agencies deliver solutions that balance safety and mobility for all users with the preservation and enhancement of community and environmental resources. That concept of “flexibility in design” is encouraged through use of an open, collaborative, and creative thinking process by which flexibility is exercised in (a) design approach, (b) use of standards and criteria, (c) implementation of solutions, and (d) consideration of the context of the area in which the project is being implemented, such as improvements to the street side or multi-modal considerations that provide integration of land use, transportation, and infrastructure needs.
Source: Integration of Safety in the Project Development Process and Beyond: A Context Sensitive Approach (ITE)
[Flexibility in Highway Design] encourages highway designers to expand their consideration in applying the Green Book criteria. It shows that having a process that is open, includes public involvement, and fosters creative thinking is an essential part of achieving good design. This Guide should be viewed as a useful tool to help highway designers, environmentalists, and the public move further along the path to sensitively designed highways and streets by identifying some possible approaches that fully consider aesthetic, historic, and scenic values, along with safety and mobility. It also recognizes that many designers have been sensitive to the protection of natural and human-made resources prior to ISTEA.
Source: Flexibility in Highway Design (FHWA)
While FHWA-approved standards apply to all projects on the NHS, we encourage flexibility and a context-sensitive approach which considers the full range of project needs and the impacts to the community and natural and human environment. Design exceptions are a useful tool that may be employed to achieve a balance of project needs and community values. State DOT or local authorities must evaluate, approve, and document design exceptions. Approving any design exception is a Federal Action, which requires reviewing and documenting their potential environmental impacts.
Source: Guidance on NHS Design Standards and Design Exceptions (FHWA)
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… Over the years, formal design exceptions review and approval protocols have served State DOTs and FHWA well by assuring appropriate effort and thought has been applied to the design problem and its solution. Documentation of design decisions and design exceptions can help address risk and risk management.
Source: Design Exceptions (FHWA)
Designers of streets in areas where pedestrian volumes are high should utilize flexibility in design, and consider shared space design principles. Shared space is a design approach that seeks to change the way streets operate by reducing the dominance of motor vehicles, primarily through lower speeds and encouraging drivers to behave more accommodatingly towards pedestrians.
Source: UK Department for Transport
Not only is it safer and more accommodating to cyclists and pedestrians, and amenable to Placemaking, research shows that compared to conventional intersection design, shared space provide greater vehicular capacity, and reduced delays for motorists.
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As part of the CSS framework, FHWA is encouraging DOTs to follow Performance Based Practical Design (PBPD) principles to achieve these goals and maximize their effectiveness per dollar spent.
A PBPD approach requires designers and decision makers to exercise engineering judgment and utilize the flexibility in current and future guidelines to develop innovative, engineered solutions that meet purpose and need of the project, rather than rely on maximum values or limits found in design specifications.
