下载掌阅APP，畅读海量书库
立即打开
In the design of horizontal curves， it is necessary to establish the proper relationship between design speed， curvature and superelevation. Horizontal alignment must afford at least the minimum stopping sight distance for the design speed at all points on the roadway.
The major considerations in horizontal alignment design are： safety， grade， type of facility，design speed， topography and construction cost. In design， safety is always considered， either directly or indirectly. Topography controls both curve radius and design speed to a large extent. The design speed， in turn， controls sight distance， but sight distance must be considered concurrently with topography because it often demands a larger radius than the design speed. All these factors must be balanced to produce an alignment that is safe， economical， in harmony with the natural contour of the land and， at the same time， adequate for the design classification of the roadway or highway.
When a vehicle travels on a horizontal curve， it is forced radially outward by centrifugal force.This effect becomes more pronounced as the radius of the curve is shortened. This is counterbalanced by providing roadway superelevation and by the side friction between the vehicle tires and the surfacing. Safe travel at different speeds depends upon the radius of curvature， the side friction， and the rate of superelevation.
When the standard superelevation for a horizontal curve cannot be met， a design exception will be required. However， the highest practical superelevation should be selected for the horizontal curve design.
The minimum superelevation to be used is 1.5 percent on flat radius curves requiring superelevation ranging from 1.5 percent to 2.0 percent， the superelevation should be increased by 0.5 percent in each successive pair of lanes on the low side of the superelevation when more than two lanes are superelevated in the same direction.
It may be appropriate to provide adverse crown on flat radius curves （ less than 2 percent superelevation） to avoid water buildup on the low side of the superelevation when there are more than three lanes draining across the pavement （ This design treatment would require a design exception）. Another option is to construct a permeable surface course or a high macrotexture surface course since these surfaces appear to have the highest potential for reducing hydroplaning accidents.Also， grooving the pavement perpendicular to the traveled way may be considered as a corrective measure for severe localized hydroplaning problems.
（1）Axis of Rotation
For undivided highways， the axis of rotation for superelevation is usually the centerline of the traveled way. However， in special cases where curves are preceded by long， relatively level tangents， the plane of superelevation may be rotated about the inside edge of the pavement to improve perception of the curve. In flat terrain， drainage pockets caused by superelevation may be avoided by changing the axis of rotation from the centerline to the inside edge of the pavement.
（2）Superelevation Transition
The superelevation transition consists of the superelevation runoff （length of roadway needed to accomplish the change in outside-lane cross slope from zero to full superelevation or vice versa） and tangent runout （length of roadway needed to accomplish the change in outside-lane cross slope from the normal cross slope to zero or vice versa）. The definition of and method of deriving superelevation runoff and runout in this manual is the same as described in the AASHTO publication A Policy on Geometric Design of Highways and Streets， 2001.
The superelevation transition should be designed to satisfy the requirements of safety and comfort and be pleasing in appearance.
The changes in direction along a highway are basically accounted for by simple curves or compound curves. Excessive curvature or poor combinations of curvature generate accidents， limit capacity， cause economic losses in time and operating costs， and detract from a pleasing appearance. To avoid these poor design practices， the following general controls should be used.
（1）Curve Radii for Horizontal Curves
For specific design speeds Standards gives the minimum radius of open highway curves. Every effort should be made to exceed the minimum values. Minimum radii should be used only when the cost or other adverse effects of realizing a higher standard are inconsistent with the benefits. Where a longitudinal barrier is provided in the median， the above minimum radii may need to be increased or the adjacent shoulder widened to provide adequate horizontal stopping sight distance.
The suggested minimum radius for a freeway is 3，000 feet in rural areas and 1，600 feet in urban areas. For a land service highway， the preferred minimum radius is 1，600 feet and 1，000 feet for design speeds of 60 mph and 50 mph respectively.
Due to the higher center of gravity on large trucks， sharp curves on open highways may contribute to truck overturning. Overturning becomes critical on radii below approximately 700 feet.Where new or reconstructed curves on open highways with radii less than 700 feet must be provided，the design of these radii shall be based upon a design speed of at least 10 mph greater than the anticipated posted speed.
（2）Alignment Consistency
Sudden reductions in standards introduce the element of surprise to the driver and should be avoided. Where physical restrictions on curve radius cannot be overcome and it becomes necessary to introduce curvature of a lower standard than the design speed for the project， the design speed between successive curves shall change not more than 10 mph. Introduction of a curve for a design speed lower than the design speed of the project shall be avoided at the end of a long tangent or at other locations where high approach speeds may be anticipated.
（3）Stopping Sight Distance
Horizontal alignment should afford at least the desirable stopping sight distance for the design speed at all points of the highway. Where social， environmental or economic impacts do not permit the use of desirable values， lesser stopping sight distances may be used， but shall not be less than the minimum values.
（4）Curve Length and Central Angle
The following is applicable for freeways and rural arterial highways. Desirably， the minimum curve length for central angles less than 5 degrees should be 500 feet long， and the minimum length should be increased 100 feet for each 1 degree decrease in the central angle to avoid the appearance of a kink. For central angles smaller than 30 minutes， no curve is required. In no event shall sight distance or other safety considerations be sacrificed to meet the above requirement.
（5）Reversing Curves
The intervening tangent distance between reverse curves should， as a minimum， be sufficient to accommodate the superelevation transition. For design speeds of 50 mph and greater， longer tangent lengths are desirable.
（6）Broken Back Curves
A broken back curve consists of two curves in the same direction joined by a short tangent.Broken back curves are unsightly and violate driver expectancy. A reasonable additional expenditure may be warranted to avoid such curvature.
The intervening tangent distance between broken back curves should， as a minimum， be sufficient to accommodate the superelevation transition. For design speeds of 50 mph and greater，longer tangent lengths are desirable.
（7）Alignment at Bridges
Superelevation transitions on bridges almost always result in an unsightly appearance of the bridge and the bridge railing. Therefore， if at all possible， horizontal curves should begin and end a sufficient distance from the bridge so that no part of the superelevation transition extends onto the bridge. Alignment and safety considerations， however， are paramount and shall not be sacrificed to meet the above criteria.
horizontal curves 平曲线
superelevation n. 超高
side friction 侧向摩擦
superelevation transition 超高过渡
alignment consistency 线形的一致性