Chapter 3 - Design Controls
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This chapter
provides information with regard to design controls. Many factors contribute
to the roadway design criteria used by highway engineers. These factors
are based upon the physical characteristics of the vehicles (vehicle
types), the topography in which the road is set, operational safety
and speed of traffic on the road, and even driver behavior (speed, turns,
following distance, clear zones for emergencies). All of these
factors are important and should be balanced when selecting the appropriate
design criteria for a particular road or highway design. This chapter
addresses:
3.1. Functional Classifications for Freeways, Arterials, Collectors and Local Roads
3.1.1. Functional Roadway Classification
Design standards have been developed by the American
Association of State Highway and Transportation Officials (AASHTO)
for different functional systems of roadways. In order to qualify for
federal funding, the
Federal Highway Administration (FHWA) requires that each state categorize
state routes by functional classification. Detailed discussions on the
concept of functional classification and the characteristics of the various
functional systems can be found in the AASHTO
A Policy on Geometric Design of Highways and Streets (Green Book)
and FHWA
Functional Classification Guidelines. Additional information
specific to GDOT policies related to functional classification of roadways
is also available in the GDOT Plan
Development Process (PDP).
Roadway functional classification serves as the foundation of an access management program. Functional classification systems establish the planned function of different types of roadways and the priority placed on access as opposed to through traffic movement. Functional classification recognizes that design considerations vary for different classes of roads in accordance with the intended use.
Streets and highways are grouped into major
classes based on the type or kind of service they provide. The functional
classification process is based on the fact that roads are part of a travel
network and that “individual roads and streets do not serve travel
independently in any major way" (FHWA,
1989). The
three major functional systems are:
- freeways
- arterial highways
- collectors and local streets
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GDOT
Design Policy Manual ver. 2.0 Revised 11/17/2008