8-1.0 DEFINITIONS
1. Simple Curves. These are continuous arcs of constant radius that achieve the necessary highway deflection without an entering or exiting transition.
2. Compound Curves. These are a series of two or more simple curves with deflections in the same direction immediately adjacent to each other.
3. Reverse Curves. These are two simple curves with deflections in opposite directions that are joined by a relatively short tangent distance.
4. Broken-Back Curves. These are closely spaced horizontal curves with deflection angles in the same direction with an intervening, short tangent section.
5. Spiral Curves. A curve of continuously varying radius.
6. Superelevation (e). Superelevation is the amount of cross slope or “bank” provided on a horizontal curve to counterbalance, in combination with side friction, the centrifugal force of a vehicle traversing the curve.
7. Maximum Superelevation (emax). The maximum rate of superelevation (emax) is an
overall superelevation control used on a specific facility. Its selection depends on several factors including climatic conditions, terrain conditions, type of area (rural or urban), and highway functional classification.
8. Side Friction (f). The interaction between the tire and the pavement surface to counterbalance, in combination with the superelevation, the centrifugal force of a vehicle traversing a horizontal curve.
9. Maximum Side Friction (fmax). Limiting values selected by AASHTO for use in the design
of horizontal curves. The designated fmax values represent a threshold of driver
discomfort and not the point of impending skid.
10. Superelevation Transition Length. The superelevation transition length is the distance required to transition the roadway from a normal crown section to full superelevation. The superelevation transition length is the sum of the tangent runout (TR) and superelevation runoff (L) distances:
a. Tangent Runout (TR). Tangent runout is the distance needed to change from a normal crown section to a point where the adverse cross slope of the outside lane or lanes is removed (i.e., the outside lane(s) is level).
8-1(2) HORIZONTAL ALIGNMENT December 2006
b. Superelevation Runoff (L). Superelevation runoff is the distance needed to change the cross slope from the end of the tangent runout (adverse cross slope removed) to a section that is sloped at the design superelevation rate.
11. Axis of Rotation. The superelevation axis of rotation is the line about which the pavement is revolved to superelevate the roadway. This line will maintain the normal highway profile throughout the curve. The axis of rotation is generally located at the point of application of grade.
12. Crossover Line. The lane line between any two adjacent lanes of traffic.
13. Superelevation Rollover. Superelevation rollover is the algebraic difference (A) between the superelevated travel lane slope and the shoulder slope on the outside of a horizontal curve.
14. Normal Crown (NC). The typical cross section on a tangent section of roadway (i.e., no superelevation).
15. Remove Adverse Crown (RC). A superelevated roadway section that is sloped across the entire traveled way in the same direction and at a rate equal to the cross slope on the tangent section.
16. Relative Longitudinal Slope. In superelevation transition sections on two-lane facilities, the relative gradient between the profile grade and edge of traveled way.
17. Open Roadways. All urban facilities with a design speed greater than 45 mph and all rural facilities for all design speeds.
18. Low-Speed Urban Streets. All streets within an urbanized or small urban area with a design speed less than 45 mph.
19. Point of Application of Grade. The point on the cross section where the elevation of the calculated profile grade list is located.
December 2006 HORIZONTAL ALIGNMENT 8-2(1)
8-2.0 RURAL HIGHWAYS/HIGH-SPEED URBAN HIGHWAYS
This Section presents horizontal alignment criteria for all rural highways and for high-speed urban highways (V > 45 mph). See Section 8-3.0 for horizontal alignment criteria for low-speed urban streets (V < 45 mph).
8-2.01 General Controls
Much of the criteria for horizontal alignment seek to establish minimum design values that are based on specific limiting factors. These include side-friction factors, superelevation, longitudinal gradients for superelevation transition, and middle ordinate values for sight distance. In addition, the designer should adhere to several general controls for horizontal alignment. These are based on aesthetic and safety considerations. They include:
1. Horizontal alignment should be as directional as possible. Where feasible, minimum radii should be avoided. Flatter curvature with shorter tangents is generally preferable to sharp curves connected by long tangents.
2. Curves with small deflection angles should be long enough to avoid the appearance of a kink. For a central angle of 5o or less, the curve should be at least 500 ft long. On
freeways, the designer should try to provide a curve length, in feet, of at least 30 times the design speed in mph. On other major highways, try to provide a curve length 15 times the design speed.
3. Very small deflection angles may not require a horizontal curve; i.e., the roadway may be designed with an angular break. As a general guide, the designer may consider using an angle point when the deflection angle is less than 1o. The evaluation on the use of an
angle point will be based on urban/rural location, aesthetics, construction costs and the visibility of the kink.
4. Broken back curvature should be avoided.
5. Sharp horizontal curves should not be introduced near crest or sag vertical curves. The combination of horizontal and vertical curves can greatly reduce sight distance, and the likelihood of crashes is increased.
6. Horizontal curves and superelevation transitions should be avoided on bridges. These cause design, construction and operational problems when snow and ice are present. The designer should not, however, avoid placing a curve on a bridge if this results in sharp horizontal curves on the approaching roadway. Where a curve is necessary on a bridge, a simple curve should be used on the bridge, and any superelevation development should be placed on the approaching roadway.
7. Normally, simple circular curves will be used in design. However, spiral curves may be considered throughout the length of a curve to fit the roadway into a restricted roadside. Spiral transition curves should be considered in areas where high speeds are anticipated