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

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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.

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

Simple Curve Simple Curve – defined as a circular arc between two tangents.

Figure:

Where:

Simple Curve Simple Curve – defined as a circular arc between two tangents.

Figure:

Formulas:

Simple Curve-Problem Solving

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

Reverse Curve-Problem Solving

Reverse Curve

1. Reverse Curve with non-parallel tangents

Broken Back Curve

Broken Back Curve is characterized by short tangent between two curves in the same direction

Broken Back Curve – Prob. Solving

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

Types of Vertical Curve a.) Crest Curve

b.) Sag Curve

General Equation of Vertical Curve

Vertical Curve – Problem Solving

Sight Distances at Crest Vertical Curves

Stopping Sight Distance:

Sight Distances at Crest Vertical Curves

Passing Sight Distance:

Sight Distances at Crest Vertical Curves

S < L:

2 ( )

Sight Distances at Crest Vertical Curves

S > L:

2

Sight Distances at Sag Vertical Curves

S < L:

Sight Distances at Sag Vertical Curves

S > L:

Problem Solving –Vertical Curve

End