|
Horizontal Alignment Design Suggestions
The following excerpt was taken from the 1994 edition of AASHTO's A
Policy on Geometric Design of Highways and Streets (pp. 223-226).
In addition to the specific design elements for horizontal alignment discussed
under previous headings, a number of design controls are recognized in practice. These
controls are not subject to empirical or formula derivation, but they are important for
efficient and smooth-flowing highways. Excessive curvature or poor combination of
curvature limit capacity, cause economic losses because of increased travel time and
operating costs, and detract from a pleasing appearance.
To avoid these evidences of poor design practices, the general controls in the
following paragraphs should be used where practicable:
- Alignment should be as directional as possible but should be consistent with the
topography and with preserving developed properties and community values. A flowing line
that conforms generally to the natural contours is preferable to one with long tangents
that slashes through terrain. . . .
- In alignment predicated on a given design speed, use of maximum curvature for that speed
should be avoided wherever possible. The designer should attempt to use generally flat
curves, retaining the maximum for the most critical conditions. In general, the central
angle of each curve should be as small as the physical conditions permit, so that the
highway will be as directional as possible. The central angle should be absorbed in the
longest possible curve. . . .
- Consistent alignment always should be sought. Sharp curves should not be introduced at
the ends of long tangents. Sudden changes from areas of flat curvature to areas of sharp
curvature should be avoided. Where sharp curvature must be introduced, it should be
approached, where possible, by successively sharper curves from the generally flat
curvature.
- For small deflection angles, curves should be sufficiently long to avoid the appearance
of a kink. Curves should be at least 150 m long for a central angle of 5° , and the
minimum length should be increased by 30 m for each 1° decrease (Ed: perhaps they mean
increase here) in the central angle. . . .
- Anything other than tangent or flat curvature should be avoided on high, long fills. In
the absence of cut slopes, shrubs, and trees above the roadway, it is difficult for
drivers to perceive the extent of curvature and adjust their operation to the conditions.
- Caution should be exercised in the use of compound circular curves. . . .
- Any abrupt reversal in alignment should be avoided. Such a change makes it difficult for
a driver to keep within his or her own lane. It is also difficult to superelevate both
curves adequately, and erratic operation may result. . . .
- The "broken-back" or "flat-back" arrangement of curves (having a
short tangent between two curves in the same direction) should be avoided except where
very unusual topographical or right-of-way conditions dictate otherwise. . . .
- To avoid the appearance of inconsistent distortion the horizontal alignment should be
coordinated carefully with the profile
design. . . .
|