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.
Road classification systems define functions and present design criteria for different types of streets and roads. Get information and see interesting opinions on existing and alternative road classifications in terms of the way they impact CSS.
Fitting a transportation solution into the community and environmental contexts while at the same time satisfying transportation needs is one of the key - if not the key - fundamentals of Context Sensitive Solutions. When thinking of how to accomplish this, most designers and stakeholders refer to flexible design as a solution. Design elements such as lane and shoulder widths, horizontal and vertical curvature and sight distances immediately come to mind when one first begins to visualize how to fit a roadway into its context. What many overlook is that decisions to widen roadways or upgrade intersections consistently have more of an impact on a project's footprint. One of the most significant factors in such roadway expansion decisions are travel projection modeling assumptions.
CSS consider the function of streets and roads relative to their context in terms of access and mobility for all users. Accessibility refers to the "ease with which people can reach destinations", while mobility is "the ease of movement that people experience in moving place to place" - Institute of Transportation Engineers, Smart Growth Transportation Guidelines
It is the balance between accessibility and mobility that often needs to be achieved in CSS projects.
Roads are designed to achieve a desired travel speed of vehicles. The challenge of CSS is to plan and design the road layout and network so that the actual speed of vehicles is appropriate to the function of a road and its context. This is sometimes called speed management. Design assumptions about speeds are critical to CSS projects.
Roads are designed to provide different levels of service for different modes. With CSS, communities and engineers seek balance between the capacity of a road (to handle a specific volume of traffic during a given time period) and other goals and issues of the roads context. The desired Level of Service, for vehicles, pedestrians, and bicycles, of a road is a design choice.
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
NCHRP Report 480: A Guide to Best Practices for Achieving Context Sensitive Solutions
This guide demonstrates how state departments of transportation (DOTs) and other transportation agencies can incorporate context sensitivity into their transportation project development. This guide is applicable to a wide variety of projects that transportation agencies routinely encounter. While the guide is primarily written for transportation agency personnel who develop transportation projects, other stakeholders may find it useful in better understanding the project development process.
"Geometric design deals with features of the location, alignment, profile, cross section, and intersections for a range of highway types and classifications.... Central to geometric design is the application of design criteria, guidelines, and standards."