MODULE 2: Horizontal alignment Flashcards
Or Alignment of road
The center line position of highway on the ground
Guides the good pass of the road through right alignment which is ECONOMICAL and easy to construct
HIGHWAY ALIGNMENT
TWO TYPES OF ALIGNMENT
- HORIZONTAL ALIGNMENT
- VERTICAL ALGINMENT
- includes combination of straight lines and curves
HORIZONTAL ALIGNMENT
- includes vertical curves
VERTICAL ALGINMENT
HORIZONTAL ALIGNMENT
Describes the location and orientation of highway in plan view (horizontal curves)
Consists of a series of straights (tangents), circular curves (simple, compound and reverse) and transition curves (spirals)
The basic elements for horizontal curves are
Radius of Curvature and Superelevation Rate
Proper designs results in high performance regarding
Speed
Safety
Efficiency
Comfort
*economy and increased capacity
Designs of horizontal alignment follows that the arc distance is the same as the chord distance
HORIZONTAL ALIGNMENT in RAILROADS
Distances along the arc is considered for the design of horizontal alignment
HORIZONTAL ALIGNMENT in HIGHWAYS
- defined by the radius, R and degree of curvature, D
- a short radius is a sharp curve
- make the curve risky and dangerous
- a small degree of curvature is a flat curve
- a short radius is a sharp curve
SHARPNESS OF HORIZONTAL CURVE
true or false:
the radius of the curve is inversely proportional to the sharpness the degree of curve is directly proportional to the sharpness the radius is inversely proportional to the degree of curve
true
true or false:
- a provision of an adequate safe stopping sight distance is important for any change in direction or alignment of road
true
- Usually unacceptable for high-speed roads because of superelevation at reversal point cannot be provided
REVERSE CURVES
For the other type of ______________, the approach is the same as for compound curves
REVERSE CURVES
Characterized by a short tangent between two curves in the same direction
Should avoided in design since most drivers do not expect succeeding curves to be in the same direction
BROKEN BACK CURVE
- also referred to as Easement curve
- A curve of varying radius at tangent point to a design circular radius
- Provided in between straight and circular path in order for the centrifugal force to change gradually
TRANSITION CURVES
OBJECTIVES of providing transition curve:
- Gradually introduce centrifugal force between tangent and point of curvature to avoid sudden jerk
- Increase comfort of passengers and ease of maneuver for drivers
- Introduce designed superelevation at desirable rate
- Fit road alignment
- Aesthetic enhancement and introduce extra widening at desirable rate
Types of Transition Curves
- Spiral or Clothoid
- Bernoulli’s Lemniscate
- Cubic Parabola
A curve at which radius of curve is inversely proportional to its length (arc)
Spiral or Clothoid
A curve at which radius of curve is inversely proportional to the length of the chord
Bernoulli’s Lemniscate
A curve at which radius of curve varies inversely as its abscissa (x)
Cubic Parabola
- a circular curve with two clothoids at each end serving as transition paths
Note: the two clothoids are identical
Symmetrical clothoid
Standard Length of clothoid based on:
- Based on travel time (sufficient reaction time)
- Based on rate of change of acceleration
- Based on aesthetics
Longitudinal gradient of superelevation runoff should be not exceed 1/200
Standard Length of clothoid based on:
- Based on travel time (sufficient reaction time)
- Based on rate of change of acceleration
- Based on aesthetics
Longitudinal gradient of superelevation runoff should be not exceed 1/200
If ∆R > ____ m, then spiral curves are necessary.
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