The correct answer is option 'C': Corner sight distance (CSD).

Corner sight distance (CSD) is the road alignment specification that provides a substantially clear line of sight for the driver of a vehicle approaching a corner or bend in the road. It refers to the distance required for a driver to safely navigate a corner or bend while maintaining adequate visibility of the road ahead. CSD is an important consideration in road design to ensure the safety of drivers and prevent accidents.

Importance of Corner Sight Distance:
Corner sight distance is crucial for several reasons:

1. Safety: The primary objective of CSD is to ensure the safety of drivers by providing them with sufficient visibility to anticipate and react to any potential hazards or obstacles on the road ahead.

2. Maneuverability: Adequate CSD allows drivers to safely negotiate corners without encroaching into opposing traffic lanes or colliding with roadside obstacles.

3. Speed Control: CSD also helps in regulating the speed of vehicles approaching corners, as drivers can assess the road conditions ahead and adjust their speed accordingly.

Factors Affecting Corner Sight Distance:
Several factors influence the determination of corner sight distance, including:

1. Design Speed: The design speed of the road affects the required CSD. Higher design speeds necessitate longer sight distances to allow for safe vehicle maneuvering.

2. Road Geometry: The horizontal and vertical alignment of the road, including the radius of the curve, superelevation, and gradient, impacts the sight distance requirements.

3. Roadside Obstructions: The presence of buildings, vegetation, signage, or other obstacles along the road can obstruct visibility and reduce the available CSD.

4. Driver Eye Height: The height of the driver's eye above the road surface is also considered in determining the required CSD.

Design Considerations:
To ensure adequate corner sight distance, several design considerations are taken into account:

1. Sight Distance Calculation: Various formulas and equations are used to calculate the required CSD based on the design speed, road geometry, and other factors.

2. Clear Zone: A clear zone is established along the roadside, free from any fixed objects or obstacles that may obstruct the driver's view.

3. Geometric Design: Road alignment and curve design are optimized to provide the necessary CSD while maintaining safe vehicle maneuverability.

4. Roadside Vegetation Management: Regular maintenance and trimming of roadside vegetation are essential to prevent obstruction of sight lines.

In conclusion, corner sight distance (CSD) is the road alignment specification that ensures a substantially clear line of sight for drivers approaching corners or bends. It is crucial for driver safety, vehicle maneuverability, and speed control. Determining the required CSD involves considering factors such as design speed, road geometry, roadside obstructions, and driver eye height. Proper design considerations and maintenance practices are essential to provide adequate CSD and enhance road safety.