Highway engineers, as designers, strive to meet the needs of highway users while maintaining the integrity of the environment. unique combinations of design controls and constraints that are often conflicting call for unique design solutions. A Policy on Geometric Design of Highways and Streets provides guidance based on established practices that are supplemented by recent research. This document is also intended as a comprehensive reference manual to assist in administrative, planning, and educational efforts pertaining to design formulation.
Design values are presented in this document in both metric and u.S. customary units and were developed independently within each system. The relationship between the metric and u.S. customary values is neither an exact (soft) conversion nor a completely rationalized (hard) conversion; and the use of brackets around u.S. Customary values does not indicate as in some AASHTO publications that these are soft conversions. The metric values are those that would have been used had the policy been presented exclusively in metric units; the u.S. customary values are those that would have been used if the policy had been presented exclusively in u.S. customary units.
Therefore, the user is advised to work entirely in one system and not attempt to convert directly between the two. The fact that new design values are presented herein does not imply that existing streets and highways are unsafe, nor does it mandate the initiation of improvement projects. This publication is not intended as a policy for resurfacing, restoration, or rehabilitation (3r) projects. For projects of this type, where major revisions to horizontal or vertical curvature are not necessary or practical, existing design values may be retained. Specific site investigations and crash history analyses often indicate that the existing design features are performing in a satisfactory manner.
The cost of full reconstruction for these facilities, particularly where major realignment is not needed, will often not be justified. resurfacing, restoration, and rehabilitation projects enable highway agencies to improve highway safety by selectively upgrading existing highway and roadside features without the cost of full reconstruction. When designing 3r projects, the designer should refer to TRB Special Report 214, Designing Safer Roads: Practices for Resurfacing, Restoration, and Rehabilitation, and related publications for guidance. The intent of this policy is to provide guidance to the designer by referencing a recommended range of values for critical dimensions.
Good highway design involves balancing safety, mobility, and preservation of scenic, aesthetic, historic, cultural, and environmental resources. This policy is therefore not intended to be a detailed design manual that could supersede the need for the application of sound principles by the knowledgeable design professional. Sufficient flexibility is permitted to encourage independent designs tailored to particular situations. Minimum values are either given or implied by the lower value in a given range of values.
The larger values within the ranges may be used where social, economic, and environmental impacts are not critical. Engineering judgment is exercised by highway agencies to select appropriate design values. The highway, vehicle, and individual users are all integral parts of transportation safety and efficiency. While this document primarily addresses geometric design issues, a properly equipped and maintained vehicle and reasonable and prudent performance by the user are also needed for safe and efficient operation of the transportation facility.
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