Civil Engineering Reference
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(a)
(b)
Figure 18.3
Roadway mainline. (a) Mainline is the roadway centerline, but not necessar-
ily in the center of roadway. (b) Mainline is aligned on the top of the deck.
when roadway transits from one tangent to another the rate of curvature
changes and the maximal of curvature has certain limitation. This require-
ment of roadway makes arcs, spirals, and their combinations suitable and
very common for horizontal transitions.
An arc has a constant curvature. When used as a part or the whole tran-
sition as shown in Figures 18.1 and 18.4, the absolute value of its curvature
should meet the maximal curvature requirement and the changes to the
previous segment or to the next segment cannot be too sharp. For example,
Connecting
point
Connecting
point
Connecting
point
Arc
Connecting
point
Spiral
Spiral
Straight line
Straight line
Start point
Spiral
Connecting
point
Arc
Connecting
point
End
point
Figure 18.4
An example of a plane curve—components of a compound curve.
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