Introduction:
Transition curves are an essential component of horizontal curves in road design and are used to ensure smooth and safe vehicle movement as they transition from a straight path to a curved path. These curves allow for a gradual change in curvature and help in reducing the jerk experienced by the vehicles. The correct answer to the given question is option 'D', which states that transition curves are provided on the approach to horizontal curves to reduce jerk to allowable levels. Let's understand why this answer is correct in detail.

Explanation:
1. Definition of Jerk:
Jerk is the rate of change of acceleration with respect to time. In simple terms, it is the abruptness or suddenness of acceleration. High jerk values can lead to discomfort for passengers and can also cause damage to the vehicle and its occupants. Therefore, it is essential to keep jerk within allowable limits for safe and comfortable vehicle movement.

2. Need for Transition Curves:
Transition curves are introduced on the approach to horizontal curves to ensure a smooth and gradual change in curvature. Without transition curves, the sudden change in direction at the beginning of a curve can result in high jerk values, especially for vehicles traveling at high speeds. This abrupt change can be uncomfortable for passengers and may even lead to accidents due to loss of control.

3. Gradual Change in Curvature:
Transition curves allow for a gradual change in curvature, providing a smooth transition from a straight path to a curved path. By introducing a curve with a continuously changing radius, the change in direction becomes more gradual, reducing the jerk experienced by the vehicles. This gradual change in curvature helps in maintaining a comfortable ride and ensures the safety of the passengers.

4. Design Considerations:
Transition curves are designed to meet specific design criteria, including the allowable jerk levels. The design process involves determining the appropriate length and radius of the transition curve based on the desired speed, allowed jerk values, and other factors. By carefully designing the transition curve, the jerk can be reduced to allowable levels, ensuring a safe and comfortable ride for the vehicles.

5. Other Benefits:
Apart from reducing jerk, transition curves also offer other benefits:
- Minimizing the length of the horizontal curve: Transition curves allow for shorter horizontal curves, saving valuable space and reducing construction costs.
- Simplifying the laying-out and construction of the horizontal curve: Transition curves provide a smooth and well-defined path for construction, making it easier to lay out and construct the horizontal curve.

Conclusion:
Transition curves are an essential component of horizontal curves in road design. They are provided on the approach to horizontal curves to reduce jerk to allowable levels. By introducing a gradual change in curvature, transition curves ensure a smooth and safe transition from a straight path to a curved path, providing a comfortable ride for passengers and maintaining the safety of the vehicles. Additionally, transition curves also offer benefits such as minimizing the length of the horizontal curve and simplifying the laying-out and construction process.