 | Parkways and park roads |  |
GLOSSARY:
Park Road - A scenic travel way of everchanging curvature within a
park, designed to lay lightly on land, for the purpose of
visitors' enjoyment of the scenic, natural and cultural features
being preserved and interpreted within or beyond the park
boundaries.
Parkway - An elongated park containing a scenic road connecting two
separated parks, distinguished by the following features:
- The roadway is of everchanging curvature with cuts and fills
kept to a minimum and side slopes are shaped to resemble the
adjacent natural topography.- The roadway lays lightly on the land to impart a sense of
effortless flight through the countryside.- The scenery is enjoyable and unsightly views and noise are
screened by vegetation or topography.- Grade separation structures insure limited access from
adjacent roads.- Indigenous plant species are used to blend the right-of-way
with the native landscape.- Safety is a primary design consideration to reduce accidents
to an absolute minimum.- Medians of varying width may separate roadways having independent
plans-and-profiles.- Vistas and views are directed toward features of interest
beyond the right-of-way.- Billboards are eliminated.
- Design speed increases gradually from the beginning to fit the
topography and decreases toward the end of the roadway.
Tangent - A straight segment of road; the straight line between
P.I.s; the distance between the end of one curve and the beginning
of the next curve (between P.C. and P.C.).
Circular Curve - An arc of constant radius common to two tangents.
Spiral - A curve of gradually decreasing radius beginning at a point
on the tangent and ending at a circular curve (an exception is
P.S.S.).
Transition Curve - A change in direction composed of a spiral, a
circular curve, and another spiral; terms "transition: and
"spiral" are often synonomous.
"P" Line - The line composed of tangents connecting Points of Intersection;
sometimes a Preliminary Location.
Chord - A line connecting the ends of a circular curve, or a vertical
curve, or a tenth-part of a spiral.
Superelevation - The rate of rise of the outside edge of pavement
to compensate for centrifugal force of a moving vehicle.
Assumed Design Speed (from Barnett) - "The maximum approximately
uniform speed that probably will be adopted by the faster group
of drivers but not, necessarily, by the small percentage of
reckless ones." (NOTE! The principal factor affecting the
choice of a design speed is the character of the terrain.)
Compound Curve - A change in direction composed of two or more
differing radii; a feature to be avoided in alignment.
Broken Back - Two circular curves in the same direction connected
by a short tangent.
Vertical Curve - A parabolic curve connecting two vertical tangents;
usually a curve on a profile.
Degree of Curve - The angle subtended by 100 feet of arc.
Radius - The distance from an arc to a common perpendicular point;
the ground distance from a compass point to the curve.
Location - That place on the ground where a road is built.
Alignment - The projected line along which a road is designed or
is built.
Subgrade - The surface on which imported material is to be placed;
the interface between residual soil and the subbase of the road.
Grade - Gradient along the profile; to move soil or material; an
elevation; to manipulate contours on paper; to sculpture the
ground surface.
P.I. - Point of intersection of the main tangents.
P.C. - Point of curvature at the beginning of a circular curve.
P.T. - Point of tangency at the end of a circular curve.
Excavation - The volume of material removed below the original
ground surface.
Embankment - The volume of material deposited on the original ground
surface or above the subgrade.
Borrow - The volume of material imported to account for shrinkage
of excavation.
Profile - The graphic vertical alignment; the line delineating the
existing ground surface.
Plan - The proposed horizontal alignment.
Grade Separation - A bridge separating roadways.
Interchange - A system of roads and ramps connected to a grade
separation structure.
Intersection - An at-grade junction of two or more roads.
AASHO (now AASHTO) - American Association of State Highway (and
Transportation) Officials.
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