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Alignment Design
I. Key points in this presentation factors influencing locating and con
figuration of horizontal alignment criteria for horizontal alignment de
sign factors influencing vertical alignme
nt design coordination of horizontal and vert
ical alignments 1 Thursday, April 20, 2023
Alignment Design
II. Introduction
1. Definition of alignment: Definitions from a dictionary In a highway design manual: a series of straight lines
called tangents connected by circular curves or transition or spiral curves in modern practice
2. Definition of alignment design: also geometric design, the configuration of horizontal, vertical and cross-sectional elements (first treated separately and finally coordinated to form a continuous whole facility)
III. Horizontal alignment design
1. Components of horizontal alignment Tangents (segments of straight lines) Circular/simple curves Spiral or transition curves
2 Thursday, April 20, 2023
Alignment Design
2. Horizontal curves Simple curves
Continuous arcs of constant radius which achieve the necessary highway deflection without an entering or existing transition
Compound curves
A series of two or more simple curves possessing a common tangent at any points of meeting which turn in the same general direction and have their centers on the same side of the line. Any two adjacent areas must, of course, have different radii if they are to be compounded pair rather than a single curve
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Alignment Design
2. Horizontal curves Spiral curvesA parabolic type of curve used as a transition from a tang
ent to a circular curve, or from a circular to another circular of different radii
Reverse curvesS–curves, are similar to compound curves except that thei
r centers are on opposite sides of the line and their direction of turning is opposite to each other
Broken-back curvesTwo closely spaced simple curves with deflection in the sa
me direction and a short intervening tangent
4 Thursday, April 20, 2023
Alignment Design3. Factors influencing the location and configuration
of horizontal alignment Physical controlsTopography, watercourses, geophysical conditions, l
and use, and man-made features Environment considerationsAffect on adjacent land use, community impacts, eco
logically sensitive areas EconomicsConstruction costs, right-of-way costs, utility impact
s, operating and maintenance costs SafetySight distance, consistency of alignment, human fact
or considerations Highway classification and design policies
Functional classification, level of service, design speed, design standards
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Alignment Design
4. Design criteria Design speed: principal factor
Consistency: consistent and as directional as possible preferable to long, flat curves no circular curves of different radii placed end to
end; no short tangents placed between two curves; Long, flat curves for small changes in direction no sudden changes from flat to sharp curves no long tangents followed by sharp curves
Length of curves:
Adjacent curves:
6 Thursday, April 20, 2023
Alignment Design4. Design criteria
High fills:
Bridges: at tangents positions
Miscellaneous5. Notations in reading alignment drawings
PC: point of curve BC: beginning of curve PT: point of tangent TS: tangent to spiral SC: spiral to circular curve CS: circular curve to spiral ST: spiral to tangent VPC: vertical point of curvature—the point at which a tang
ent grade ends and the vertical curve begins. VPI: vertical point of intersection—the point where the ext
ension of two tangent grades intersect VPT: vertical point of tangency—the point at which the ver
tical curve ends and the tangent grade begins
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Alignment Design
IV. Vertical alignment
1. Terms Elements of vertical curve Grade line/vertical alignment: a profile taken along
the road centerline, consisting of a series of straight lines connected by parabolic or circular vertical curves to which the straight grade are tangents.
Gradient: the rate of slope between two adjacent VPIs expressed as a percent
Convex curves Concave curves Plus grade/upgrade: the grade line increasing from
the horizontal Minus grade/downgrade: the grade line decreasing f
rom the horizontal8 Thursday, April 20, 2023
Alignment DesignIV. Grade line design
2. Principles for vertical alignment design Consistency A smooth grade line with gradual changes, consistent with
the type of highway and character of terrain Environmental Impacts Long Grades On a long ascending grade, it is preferable to place the stee
pest grade at the bottom and flatten the grade near the top. Intersections Maintain moderate grades through intersections to facilitat
e turning movements Broken-Back Curvature
Coordination with Natural/Man-Made Features
VPI Locations
9 Thursday, April 20, 2023
Alignment DesignV. Coordination of horizontal and vertical alignment In order to enhance highway safety and improve the facil
ity's operation, the following coordination of horizontal and vertical alignment must be achieved: Balance Curvature and grades in proper balance for safet
y, capacity, ease and uniformity of operations and a pleasing appearance
Coordination Vertical curvature superimposed upon horizontal
curvature (i.e., vertical and horizontal P.I.'s at approximately the same stations) generally results in a more pleasing appearance and reduces the number of sight distance restrictions
Crest Vertical Curves (convex curves) No sharp horizontal curvature at or near the top
of pronounced crest vertical curves
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Alignment DesignV. Coordination of horizontal and vertical alignment
Sag Vertical Curves (concave curves) No sharp horizontal curves at or near the low poi
nt of pronounced sag vertical curves or at the bottom of steep vertical grades
Passing Sight Distance Provision of long tangent sections to secure suffic
ient passing sight distance Intersections Horizontal and vertical alignment be as flat as pr
actical Divided Highways Provision of independent alignments for the two
1-way roadways on divided facilities with wide medians
Residential Areas
Aesthetics11 Thursday, April 20, 2023
Alignment Design
Definitions of alignment from a dictionary: an adjustment to a line; arrangement in a stra
ight line. the line or lines so formed. the proper adjustment of the components of a
n electronic circuit, machine, etc., for coordinated functioning: The front wheels of the car are out of alignment.
a state of agreement or cooperation among persons, groups, nations, etc., with a common cause or viewpoint.
a ground plan of a railroad or highway
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Simple curve elements
Simple curve in full superelevation
Reverse curves
Compound curve
Spiral transition curve in full superelevation
Superelevation transition for divided highways
Vertical curves
Convex and concave curves
Coordination of vertical and horizontal alignments
Coordination of vertical and horizontal alignments