Horizontal Alignment Of Highways Flashcards
corresponds roughly
to x and z coordinates
horizontal alignment (plan view)
corresponds to highway length (measured along some constant elevation) and y coordinates (elevation)
vertical alignment (profile view)
types of horizontal curves
- Circular curve
- Simple curve
- Compound Curve
- Reverse Curve
- Broken Back Curve
- transitional curve or Easement Curve
Sharpness of curve can be expressed by using two measurements:
a. Radius, R
b. Degree of curvature, D°
types of reverse curve
-with parallel tangents
-with non parallel tangents
types of transitional curve
- Clothoid
- symmetrical clothoid
For large radius (as in railroad practice), distances along the arc
are considered to be the same as along the chords.
Circular Curve
definition being followed in highway practice
arc definition
definition used for railroad practice
chord definition
Stationing In metric system, the length l used is ___ m for both arc and chord definitions. In English system, the length l used is ____ ft for both arc and chord definitions.
a. 20
b. 100
defined as a circular arc between two tangents.
Simple curve
point of intersection of two
tangents
PI
point of curvature or beginning of curve
PC
point of tangency or end of curve
PT
external angle of deflection between tangents
Δ
tangent distance
T
long chord connecting PC and PT
Lc
external distance
E
middle ordinate
M
length of curve from PC to PT R:
radius of curve
L
a two-arc simple curve having its centers on the same side of the common tangent.
Compound Curve
a two-arc simple curve having its centers on opposite sides. This type of curve is usually inadmissible on high-speed routes because superelevation at the point of reversal cannot be provided.
Reverse Curve
characterized by a short tangent
between two curves in the same direction. This type of curve should be avoided in design since most drivers do not expect succeeding curves to be in the same direction.
Broken Back Curve
a spiral that serves as a transition path as the vehicle enters or leaves a circular curve. This type of curve follows a natural path as the driver turns the steering wheel
transitional curve or Easement Curve
most well-known transition curve
Clothoid
T or F
clothoid enables a car driver to ride smoothly by turning the
steering wheel with a constant speed, defining a clothoidal spiral, a continuous and linear curvature variation.
True
a circular curve having two clothoids at each end serving as transition paths. The two clothoids have exactly the same properties.
symmetrical clothoid
3 Standard for the length of clothoid
- Based on travel time
- Based on rate of change of acceleration, p
- Based on aesthetics
