A raised edge at the side of the roadway is known as a curb.

The correct answer to the question is option 'A' - curb. A curb is a raised edge or barrier that is typically constructed at the side of a roadway. It serves several important functions in road design and traffic management. Let's explore these functions and the significance of curbs in more detail.

Functions of Curbs:
Curbs play a crucial role in maintaining the integrity and safety of roadways. Some of the key functions of curbs include:

1. Boundary Definition: Curbs provide a clear visual and physical separation between the roadway and adjacent areas such as sidewalks, bike lanes, parking areas, and green spaces. This helps to define the boundaries and prevent encroachment onto the roadway.

2. Drainage Control: Curbs act as a barrier that directs surface water runoff away from the road and towards designated drainage systems. They prevent water from entering the roadway, reducing the risk of flooding and preventing damage to the road surface and adjacent areas.

3. Traffic Safety: Curbs enhance road safety by providing a visual and physical barrier that discourages vehicles from leaving the roadway unintentionally. They help to prevent accidents and protect pedestrians and other road users by keeping vehicles within their designated lanes.

4. Pedestrian Safety: Curbs also serve as a safety feature for pedestrians. They provide a clear separation between the sidewalk and the roadway, reducing the risk of pedestrian-vehicle conflicts. Curbs also act as a tactile warning for visually impaired individuals, helping them navigate the roadways safely.

5. Street Furnishing Support: Curbs provide support for street furnishings such as streetlights, traffic signs, and traffic signals. They help to ensure the stability and proper positioning of these elements, enhancing their effectiveness and visibility.

Types of Curbs:
Curbs can be classified into various types based on their shape, material, and design. Some common types of curbs include:

1. Mountable Curbs: These curbs have a low height and a gentle slope, allowing vehicles to easily drive over them if necessary. They are often used in areas where occasional vehicle access to sidewalks or parking areas is required.

2. Barrier Curbs: Barrier curbs are taller and steeper, making it difficult for vehicles to drive over them. They are primarily used in locations where a clear separation between the roadway and adjacent areas is necessary, such as high-speed roadways or areas with heavy pedestrian traffic.

3. Roll Curb: This type of curb has a curved shape, allowing vehicles to smoothly transition from the roadway to the sidewalk. It provides a more aesthetically pleasing and accessible design.

4. Integral Curbs: Integral curbs are constructed as an integral part of the roadway pavement. They are typically made of the same material as the road surface, providing a seamless transition between the two.

In conclusion, a curb is a raised edge at the side of the roadway that serves multiple functions in road design and traffic management. It defines boundaries, controls drainage, enhances safety for both vehicles and pedestrians, and supports street furnishings. Different types of curbs are used based on their specific design requirements and the intended use of the roadway.