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Design Requirements of
Highways
CE 122 C2 Traffic & Highway Engineering College of CE-ENSE Mapua Institute of Technology (MIT)
Highway Design Coverage
• Horizontal alignment
• Vertical alignment
• Geotechnical Aspects
• Pavement Design
• Drainage Facilities
• Other Civil Structures
• Etc.
Design Speed
• Is the maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway may govern.
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• length of roadway ahead that is continuously visible to the
driver
• depends on the vehicle speed, vehicle size and driver PRT
• minimum sight distance available should be sufficiently long
enough to enable a vehicle traveling at the design speed to stop before reaching a stationary object in its path (AASHTO)
Sight Distance
Important elements in Sight Distance
• Stopping Sight Distance
• Decision Sight Distance
• Passing Sight Distance
• Intersection Sight Distance
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Source: A Policy on Geometric Design of Highways and Streets (The Green
Book). Washington, DC. American Association of State Highway and
Transportation Officials, 2001 4th Ed.
Stopping Sight Distance – SSD sum of the distance the vehicle travels from the driver’s first
possible sighting of the hazard to the instant the brakes are
touched, plus the distance required to stop after the initial brake
activation
Decision Sight Distance 6
Source: A Policy on
Geometric Design of
Highways and
Streets (The Green
Book). Washington,
DC. American
Association of State
Highway and
Transportation
Officials, 2001 4th
Ed.
distance required by the driver to detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered, recognize the hazard, select an appropriate speed or path, initiate and complete the required safety maneuver safely and efficiently
7
Source: A Policy on Geometric
Design of Highways and Streets
(The Green Book). Washington,
DC. American Association of State
Highway and Transportation
Officials, 2001 4th Ed.
Passing Sight Distance (PSD) for 2 lane rural road
Intersection Sight Distance - ISD
- should be sufficient to allow drivers on the dominant road to avoid a
collision between a vehicle starting across the road from a stopping position and a vehicle on the through-road traveling at the design speed and appearing after the crossing movement has begun
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Other Highway Design Requirement
Minimum Radius of Curvature - for any given design
speed
Superelevation
- To counteract the
centrifugal force acting
on the vehicle.
Other Highway Design Requirement
Design Volume - 30th highest peak hour
volume has been found to be the most reasonable hourly volume that provides the best result
Other Highway Design Requirement Number of Lanes
- Determination of number of lanes needed to meet the given
design volume under specified level of service (LOS)
Horizontal Alignment Circular Curve:
Sharpness of curve can be expressed by using 2 measurements: a.) Radius, R b.) Degrees of Curvature
Horizontal Alignment
Stationing:
Rewrite the following in station notations: a.) Stake located 10,682.325 m from beginning b.) Stake located 2,345.21 ft from the beginning
Compound Curve
Compound Curve – defined as 2-arc simple curve having its centers on the same side of the common tangent
Reverse Curve Reverse Curve –is a two-arc simple curve having its centers on opposite sides.
2 Types:
1. Reverse Curve with parallel tangents
2. Reverse Curve with non-parallel tangents
Broken Back Curve
Broken Back Curve is characterized by short tangent between two curves in the same direction
Easement Curve or Clothoid
Easement Curve or Clothoid – usually a spiral that serves as a transition path as the vehicle enters or leaves a circular curve.
Vertical Alignment
Properties of Vertical Alignment:
a.) The rate of change of grade on a vertical curve is constant.
b.) Tangents drawn from any two points on a vertical axis parabola always intersects midway between the point of tangency.
Vertical Alignment
Properties of Vertical Alignment:
c.) Vertical offsets from tangent to a parabola are proportional to the squares of the distances from the point of tangency.
d.) If a tangent to the parabola is drawn between main tangents, the horizontal projection of the intercept cut-off on this new tangent by the main tangents is equal to ½ of the horizontal projection of the long chord of parabola