List of criteria for design exception in projects on NHS routes.
THE DESIGN EXCEPTION PROCESS
Despite the range of flexibility that exists with respect to virtually all the major
road design features, there are situations in which the application of even the
minimum criteria would result in unacceptably high costs or major impact on the
adjacent environment. For such instances when it is appropriate, the design exception
process allows for the use of criteria lower than those specified as minimum
acceptable values in the Green Book.
If the highway project is not on the NHS, the State does not need FHWA approval
for a design exception. Under the ISTEA, the State can request an exemption from
FHWA oversight on non-NHS projects.
For projects on NHS routes, FHWA requires that all exceptions from accepted
guidelines and policies be justified and documented in some manner and requires
formal approval for 13 specific controlling criteria. The process of justification
and documentation, although not required, can be followed by States with exemption
from FHWA oversight on non-NHS projects, as well. These criteria are as follows:
Design speed
Lane width
Shoulder width
Bridge width
Structural capacity
Horizontal alinement
Vertical alinement
Grade
Stopping sight distance
Cross slope
Superelevation
Vertical clearance
Horizontal clearance (not including clear zone)
For the most part, design exceptions to these 13 criteria can be easily identified
and defined. Two items, horizontal clearance and design speed, warrant further
explanation.
Horizontal Clearance
For clear zones, the criteria in the AASHTO Roadside Design Guideļ¾ should be
treated as guidance and not as a national standard requiring a design exception if
not numerically met.
Design Speed
Design speed is used to determine individual design elements, such as stopping
sight distance and horizontal curvature. Therefore, a design speed exception is an
exception to all the various design elements affected by it and should be justified
on that basis.
A few points to remember when evaluating design exceptions are as follows:
Consideration should be given to the effect of the variance on the
safety and operation of the facility and its compatibility with adjacent
sections of the roadway.
Consideration should be given to the functional classification of
the road, the amount and character of the traffic, the type of project,
and the accident history of the road.
The cost of attaining full standards and any resultant impact on
scenic, historic, or other environmental features should also be examined.
Finally, the following three issues should be considered. What is
the degree to which a guideline is being reduced? Will the exception
affect other guidelines? Are there any additional features being
introduced that would mitigate the deviation?
For preventive maintenance projects, no exceptions are needed for the retention of
existing features.
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