1. Select or evaluate appropriate values of:
a. Superelevation (e): The banking of the road to help counteract lateral
acceleration.
i. Defined as:
ii. Units: ft of rise per 100 ft of run (or %)
b. Side Friction (fs): Resistance between tires and pavement to help vehicles
navigate curves.
i. Low Speed Urban Design: Side friction alone is used to counter
centripetal acceleration on most curves, with superelevation used only on
the sharpest curves; emax= 4
ii. Rural and High-speed Urban, Interchange Ramps: Cannot be based
entirely on available side friction factor; emax= 12
2. Determine horizontal curve fundamentals, such as:
a. Radius of curvature (Rv): the path the vehicle takes
i. V = speed (ft/s or mph, converted accordingly)
ii. g = acceleration due to gravity (32.2 ft/s²)
iii. R = radius of curve (ft)
iv. e = superelevation (unitless or in % as needed)
v. fs = side friction factor (unitless)
b. Design speed (V): given, or in AASHTO standards, table 3.5
3. Typical Horizontal Curve:
a. Terms:
i. R (Radius): The radius of the curve measured to the centerline of the
road.
ii. Δ (Central Angle): The angle subtended by the curve, typically measured
in degrees. It indicates how much the road bends along the curve.
iii. T (Tangent Length): The distance from the beginning (PC) or ending (PT)
of the curve to the point where the road straightens out.
iv. M (Middle Ordinate): The perpendicular distance from the midpoint of the
curve to the tangent line (the line that touches the curve at one point).
v. E (External Distance): The distance from the center of the curve to the
middle ordinate.
vi. L (Length of Curve): The actual length of the horizontal curve.
vii. PC (Point of Curvature): The starting point of the curve, where the curve
begins to bend.
viii. PI (Point of Intersection): The point where two tangents meet.
ix. PT (Point of Tangency): The ending point of the curve, where the curve
straightens out.
b. Degree of Curve (D): The degree of the curve is calculated using:
i. D: Degree of the curve, in degrees.
ii. R: Radius of the curve, in feet.
c. Length of Curve (L): The length of the curve is found using:
i. Δ is the central angle of the curve in degrees.
ii. R is the radius of the curve.
iii. L is the length of the curve in feet.
d. Tangent Length (T): The tangent length is calculated by:
i. M is the middle ordinate.