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