Highway Geometrics- (Camber, Land Width, Carriage way, Sight Distance, Super Elevation, Gradient)

Highway Geometrics is a branch of highway engineering that focuses on the physical and geometrical elements of roadways. This includes:

• Land width

• Formation width

• Carriageway

• Side slopes

• Shoulders

• Kerbs

• Sight distance

• Superelevation

• Highway curves

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1. Land Width

Land width refers to the total width of land acquired by the government for the construction and maintenance of a roadway. No public buildings are permitted within this land boundary.

Factors affecting land width:

• Type of road: Higher-category roads like National Highways (NH) require more width than lower-category roads.

• Future traffic growth: Consideration is given to expected increases in traffic and economic or industrial development along the route.

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  Carriageway and formation width

2. Carriageway

The carriageway is the paved central portion of the road used by vehicles for travel. It is surfaced with materials like bituminous concrete to ensure durability and comfort for users.

• The width of the carriageway depends on the number of lanes, which is determined by the class of the road.
  Example: National Highways typically have multiple lanes, resulting in a wider carriageway.

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3. Camber

Camber is the transverse slope provided to the road surface to facilitate the drainage of rainwater, thereby improving the road’s performance.

It is denoted as:

• 1 in n or

• x% slope

Functions of Camber:

1. To maintain a safe friction between the road surface and tires.

2. To preserve the strength and durability of the road surface.

3. To protect the subgrade soil from water infiltration.

Types of Camber:

• (a) Straight Camber: Formed by two straight surfaces meeting at the crown (the central, highest point of the road). Rarely used due to edge discomfort.

• (b) Parabolic Camber: Features a continuous parabolic curve. Also uncommon due to steep edge slopes creating outward thrust.

• (c) Mixed Camber: Combines a parabolic center with straight edges, resolving the issues of both previous types. This is the most commonly used camber type in road construction.

Types of Camber

4. Gradient

The gradient is the longitudinal slope provided along the road to establish vertical alignment.

Types of Gradient:

• (a) Ruling Gradient: The design gradient, used under normal conditions.

• (b) Limiting Gradient: Used when terrain or conditions prevent use of the ruling gradient.

• (c) Exceptional Gradient: Used only in rare or emergency cases for short distances.

• (d) Minimum Gradient: Required in flat areas to ensure proper drainage.

A vehicle on ascending gradient

5. Sight Distance

Sight distance is the length of road visible to a driver at any moment. It is measured from a driver's eye height of 1.2 m, ensuring visibility of an obstacle of 0.15 m height, as standardized by the Indian Roads Congress (IRC).

Types of Sight Distances:

• (a) Stopping Sight Distance (SSD):
  The minimum distance required to bring a vehicle to a complete stop from the design speed without hitting an obstruction.
  → Also called the absolute minimum sight distance.
  → Provided at all road sections.

• (b) Overtaking Sight Distance (OSD):
  The distance needed for a vehicle traveling at design speed to safely overtake a slower-moving vehicle without colliding with oncoming traffic.
  → Usually provided at selected locations, not along the entire stretch of the road.

Sight Distance

6. Superelevation

Superelevation is the practice of raising the outer edge of a roadway relative to the inner edge along horizontal curves. It helps counteract centrifugal force exerted by moving vehicles, thus enhancing safety and comfort.

Super-elevation

Superelevation Formula:

$$e + f = \frac{v^2}{gR}$$

where,

• $e$ = Superelevation

• $\mathrm{f }$= Lateral friction factor

• $v$ = Design speed in m/sec

• $g$ = Acceleration due to gravity

• $R$ = Radius of the horizontal curve in meters