1.1.2 Framework for Geometric Design

Historically, a range of “minimum” to “desirable” criteria for design values were provided with the “desirable” design criteria preferred.
American Association of State Highway and Transportation Officials (AASHTO) A Policy on Geometric Design of Highways and Streets
provides a framework by which projects should be assessed to determine appropriate design values rather than defaulting to a “minimum” or “desirable” value. This value is shown as the
“target” value
 throughout the remainder of the manual.
“Target” values
 are meant to serve as a starting point for selecting the specific design criteria. The final selected design criteria value may be more or in some cases less (a “minimum” value) than the “target” value. The designer should use their engineering judgement to select the final design criteria based on the functional classification, context classification and performance metrics of the project established during project scoping
Similarly, the RDM provides the following framework based on five factors:
  • Functional Classification
     – characterizes roadways by their position in the transportation network and the type of service it provides for vehicular traffic. See
    Chapter 3
    .
  • Context Classification System
     – characterizes roadways by their surrounding environment and their place in the community. See
    Chapter 3
    .
  • Modal Classification System
     – considers the needs of all transportation modes in design of the roadway. See
    Chapter 3
    .
  • Project Type of Construction
     – incorporates a design process to choose the appropriate design approach for each project. See
    Section 1.1.3
    .
  • Practical Design
     – an approach to finding and balancing the needs of all users for each project. See
    Chapter 2
    .
  • Performance-based Design
     – an approach for considering the effects of geometric design decisions. See
    Chapter 2
    .
The concept of
Performance-Based Practical Design (PBPD)
 is a decision-making approach that helps agencies better manage transportation investments and serve system-level needs and performance priorities with limited resources. See
Chapter 2
for a full discussion of PBPD.