Except for traverse ways on or near an airport with an operative ground traffic
control service furnished by an airport traffic control tower or by the airport
management and coordinated with the air traffic control service, the standards
apply to traverse ways used or to be used for the passage of mobile objects only
after the heights of these traverse ways are increased by:
ᴏ 17 feet for an Interstate Highway that is part of the National System of
Military and Interstate Highways where overcrossings are designed for a
minimum of 17‐foot vertical distance.
ᴏ 15 feet for any other public roadway.
ᴏ 10 feet or the height of the highest mobile object that would normally
traverse the road, whichever is greater, for a private road.
ᴏ 23 feet for a railroad.
ᴏ For a waterway or any other traverse way not previously mentioned, an
amount equal to the height of the highest mobile object that would
normally traverse it.
When FAA identifies concerns, it may require avoidance or minimization, lighting, or
other measures to ensure airspace safety for military and civilian purposes. If potential
impacts to military operations are identified, the project must also undergo either formal
or informal review by the US Department of Defense Siting Clearinghouse to ensure the
proposed project is compatible with military missions. The Part 77 review process by law
requires that military interests be addressed if a proposed project may impact them.
The FAA and DoD have developed certain imaginary surfaces around runways that are
used to assess how structures and facilities are evaluated and whether they pose a
vertical obstruction in relation to the airspace around a runway based on their height.
The imaginary surfaces build upon one another and are designed to eliminate
obstructions to air navigation and operations, either natural or man‐made. The
dimension or size of an imaginary surface depends on the runway classification and is
different for helipads and helicopter airfields than for fixed wing runways.
National Environmental Policy Act
The National Environmental Policy Act (NEPA) of 1969 is a federal regulation that
established as U.S. policy the protection and enhancement of the environment and that
requires federal agencies to analyze and consider the potential environmental impacts of
their actions. The purpose of NEPA is to promote informed decision‐making by federal
agencies by making detailed information concerning significant environmental impacts
available to both agency leaders and the public.
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