Each day, Americans depend on the more than 4 million miles of Federal, State and local
road systems that make up our Nation's highway network. We drive these roads to get to work,
move goods and freight, and to appreciate the vastness and wide array of parks, forests,
cities, museums, beaches and cultures the United States has to offer.
The National Highway System, or NHS, makes up just a fraction of the Nation's total
road miles, yet it carries a significant percentage of all traffic. The NHS is a road system classification
consisting of the Interstate and other roadways of particular importance to the Nation's
economy, defense and mobility. The State and local road systems make up the vast majority
of remaining public streets and highways, which are also vital to our Nation.
Agencies at all levels of government are responsible for providing motorists a nearly uniform level
of consistency by building and maintaining our Nation's roads to a common set of geometric
design features. These design features establish the physical aspects of our roadways like
the sharpness of a curve or width of a lane.
You, as a local public agency, or LPA, are responsible for developing your Federal-aid
project consistent with the applicable geometric design features for the type of highway system
and type of project. Applying the appropriate geometric design features help to ensure the
safe accommodation of traffic and freight while serving our Nation's strategic and
economic priorities.
Geometric design requirements for roadways making up the NHS have been established by
the Federal Highway Administration, or FHWA, to preserve a high level of mobility and safety.
It is important for you to understand that the design requirements for projects affecting
the NHS apply regardless of funding source. Federal-aid projects on State or local roads
must adhere to the geometric design requirements established by your State.
The four primary geometric design requirements that may apply to your highway project are:
AASHTO's A Policy on Geometric Design of Highways and Streets, commonly referred to
as the Green Book. The Green Book is maintained by an organization of State government transportation
officials known as the American Association of State Highway and Transportation Officials,
or AASHTO;
AASHTO's A Policy on Design Standards-Interstate System;
Your State's approved 3R geometric design standards and;
Other State approved design standards.
To determine which design requirements apply to your highway project, you first should
consider the type of project and where it's located. Project types generally fall into
one of three categories, based on the scope of work: (1) new construction or reconstruction
projects, (2) resurfacing, restoration or rehabilitation projects, more commonly referred
to as 3R projects, and (3) preventive maintenance projects. Geometric Design requirements apply
to projects on one of the following four categories of roads:
an Interstate,
an NHS freeway,
a non-freeway NHS route, or
a non-NHS road.
All new construction and reconstruction projects on the NHS, as well as any 3R projects on
an NHS freeway, must be designed to meet the FHWA adopted edition of the AASHTO Green Book.
FHWA has also approved the use of State design manuals in lieu of the AASHTO Green Book in
some States. AASHTO's A Policy on Design Standards -- Interstate
System are the design requirements for all new construction, reconstruction or 3R-type
projects on the Interstate Highway System. A 3R-type project located on the NHS, but
not on the Interstate or an NHS freeway, may be required to be designed according to your
State's FHWA approved 3R design requirements. If your State does not have approved 3R design
requirements, then either the AASHTO Green Book or your State's approved design manual
requirements apply to your project.
Any preventive maintenance-type project located on the NHS must comply with your State's
design requirements established for preventive maintenance-type projects. You are responsible
for determining your State's preventive maintenance design requirements as they vary
in each State.
If you are responsible for any type of project located off the NHS that uses Federal-aid
funds, you must adhere to applicable laws and design requirements required by your State.
The design criteria found in the AASHTO Green Book provides substantial flexibility to address
specific conditions encountered on projects. Among the thousands of design features listed
in the AASHTO Green Book, FHWA recognizes the following 13 criteria having considerable
importance to the operational and safety performance of any highway:
Design Speed, Lane Width,
Shoulder Width, Bridge Width,
Horizontal Alignment, Vertical Alignment,
Grades, Stopping Sight Distance,
Cross Slope, Super-elevation,
Lateral Offset to Obstruction, Vertical Clearance,
Structural Capacity.
The FHWA calls these critical elements the "thirteen controlling criteria", and these
cannot be deviated from without first coordinating approval through your State DOT. If you determine
that conditions warrant an exception to the applicable design requirement, then follow
your State's procedures for requesting a design exception.
Design exception requests must consider all project conditions, including compatibility
with adjacent sections of road and the expected length of time before reconstruction of the
section. Your written request should detail the considerations and engineering judgment
used to analyze and justify the design exception. Include details of how you considered the
costs and impacts of meeting the design criteria, the risks involved of not meeting the criteria,
and any other relevant factors, such as traffic volumes or speeds or the extent to which the
design criteria cannot be met.
If you are considering a design exception, it is important to emphasize the mitigation
strategies you have considered to offset the risks of not meeting the design criteria.
For example, what mitigation strategies could you consider to address a sharp, substandard
curve ahead? In this situation, your goal is to consider features proven to help vehicles
stay on the road to mitigate the risk of building a substandard curve. In this example, you
could address this situation by including mitigation techniques such as the installation
of warning signs or speed plaques widening the roadway or shoulders, installing rumble
stripes, adding super-elevation or incorporating a skid resistant high-friction pavement surface.
Your completed design exception request should be forwarded to your State department of transportation,
or State DOT, which is responsible for facilitating the review and approval process for design
exceptions. Check your State DOT's procedures for design exceptions for projects located
off the NHS.
When a design exception is necessary, the key issue is to allow adequate time in your
project planning to get the design exception approved so that your construction is not
delayed.
Finally, remember that you are required to meet these geometric design requirements on
all Federal-aid projects. Doing so will help you avoid any unnecessary delay in receiving
construction authorization for your Federal-aid project.