2. Geometric Design of Highway, Transportation Engineering, CE 4006 (Same as CC 4006)

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For 4th Semester Polytechnic CE Students
Written by Garima Kanwar | Blog: Rajasthan Polytechnic

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Subject: Transportation Engineering, CE 4006 (Same as CC 4006)

Branch: Civil Engineering 🏗️
Semester: 4th Semester 📚

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2.1 Camber 🛣️

Definition:

• Camber is the slight incline or slope provided to the road surface from the center to the edges, helping water drain off. This prevents water from accumulating, which can cause safety hazards.

Purpose:

• Water Drainage: The primary function of camber is to allow rainwater to flow towards the sides of the road, avoiding pooling on the road surface. This is especially important in areas with frequent rainfall.
• Improving Durability: By preventing water accumulation, the road's surface is protected from premature deterioration caused by standing water.

Types:

1. Straight Camber: A linear slope from the center to the edges.
2. Parabolic Camber: A curve that starts gentle and becomes steeper as it reaches the edges.
3. Crowned Camber: A common design where the road center is higher than the edges, forming a "crown."

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2.2 Kerbs 🚧

Road Margin:

• The road margin refers to the area at the side of the road beyond the shoulder. It’s used for drainage and provides a buffer between the road and surrounding areas.

Road Formation:

• This is the total width of the road including the pavement, shoulders, and other features like side drains, and the road margin. It’s important to ensure enough space for vehicles and pedestrian movement.

Right of Way (ROW):

• Right of Way is the legal or planned space allocated for a road. It ensures that the necessary area is available for the construction and maintenance of road infrastructure. It often includes the road, sidewalks, and utilities.

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2.3 Design Speed 🚗

Design Speed is the speed at which a road is designed for vehicles to travel comfortably and safely. Several factors influence this, including:

1. Road Alignment: Curves and straight paths affect how fast vehicles can safely travel.
2. Surface Quality: A smooth surface allows for higher speeds, while a rough or damaged surface requires slower speeds.
3. Traffic Conditions: In dense traffic areas, lower speeds are recommended for safety.
4. Visibility: The clearer the sightlines, the higher the design speed can be.

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2.4 Gradient 📏

Definition:

• Gradient refers to the slope of the road, either uphill or downhill. It’s usually expressed as a percentage.

Types:

1. Upward Gradient: The road goes uphill, which can reduce vehicle speed due to the incline.
2. Downward Gradient: The road goes downhill, and vehicles may gain speed naturally.
3. Level Gradient: A flat road where no elevation changes occur.

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2.5 Sight Distance (SSD) 👀

Definition:

• Sight Distance is the distance a driver can see ahead, which is critical for safe stopping and overtaking.

Types:

1. Stopping Sight Distance (SSD): The minimum distance needed for a vehicle to come to a complete stop after noticing an obstacle.
2. Overtaking Sight Distance: The distance required to safely overtake another vehicle.

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2.6 Curves 🌪️

Necessity:

• Curves are necessary to make the road align with the terrain. In mountainous or hilly areas, curves allow the road to follow the natural landforms, avoiding direct uphill/downhill paths that may be impractical.

Types of Curves:

1. Horizontal Curves: Change the direction of the road along the horizontal plane (left or right turns).
2. Vertical Curves: Adjust the elevation, transitioning between uphill and downhill segments.
3. Transition Curves: These are gradual curves that help in easing the shift from a straight road to a sharp curve, improving comfort and safety.

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2.7 Extra Widening of Roads 🛤️

• Extra Widening is applied on curves to provide additional road width. This is especially important for large vehicles like trucks, which need more space to navigate sharp curves without crossing into the opposite lane.

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2.8 Super Elevation 🎢

Definition:

• Super Elevation is the banking or tilting of the road at curves to counteract the centrifugal force acting on vehicles as they move through the curve. This helps vehicles maintain stability and prevents them from skidding.

Method of Providing Super Elevation:

1. The super elevation is calculated using the formula:
   $e = \frac{V^2}{gR}$
   Where:

   • e = Super Elevation (the banking angle)
   • V = Speed of the vehicle
   • g = Gravitational force
   • R = Radius of the curve

2. IRC guidelines recommend a maximum super elevation of 7% for highways with high-speed traffic.

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Diagram Representation:

Here’s how we can imagine the layout of these features in a typical highway:

                _____________________
               |      Shoulder        |
               |----------------------|
  [Road Margin] |     [Carriageway]    | <- Super Elevation
  (Empty Space) |  /     /    /    /    | <- Curves (Horizontal)
               | /      /    /    /    |
               |/      /    /    /     |
               ------------------------
                    [Right of Way]
                      (ROW Area)

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Key Points to Remember:

1. Camber: Helps drain water off the road and prevent waterlogging.
2. Kerbs and Margins: Provide space for drainage and protect the road edges.
3. Design Speed: Depends on alignment, surface quality, and traffic density.
4. Gradient: Road slopes affect vehicle speed.
5. Sight Distance: Essential for safe stopping and overtaking.
6. Curves: Necessary for road alignment, but need extra attention for large vehicles.
7. Super Elevation: Reduces the risk of vehicle skidding on curves.

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