The reason why an overpass is designed with curves instead of being built straight can be due to several technical, economic, and practical factors, including:

1. Limited Terrain and Space Constraints

   • The area where the overpass is built may be restricted by existing structures such as buildings, roads, or other infrastructure. Curving the bridge helps avoid these obstacles without requiring extensive land clearance, thereby saving costs.

2. Ensuring Traffic Safety

   • If the overpass is too steep or has sharp turns, vehicles moving at high speeds may lose control. A curved design helps naturally reduce speed, increasing safety for road users.

3. Reducing the Gradient for Vehicles

   • A straight overpass may require a steeper incline to reach the necessary height within a short distance, making it difficult for heavy vehicles such as trucks and buses. A curved design extends the length, reducing the slope and allowing vehicles to move more easily.

4. Minimizing Impact on the Ground and Infrastructure

   • A straight bridge may exert a large amount of pressure on the ground and bridge foundations. A curved design distributes the load more evenly, reducing the risk of subsidence or structural cracks.

5. Aesthetic and Urban Design Considerations

   • Sometimes, overpasses are designed with curves to enhance architectural aesthetics and blend with the urban landscape. Some bridges have unique designs to fit the city's master plan or serve as symbolic landmarks.

6. Reducing Overall Costs

   • While a straight overpass may seem cheaper at first glance, considering land clearance costs, foundation adjustments, and traffic safety, a curved design may be the more cost-effective solution in the long run.

In conclusion, overpasses are not built straight but curved due to multiple factors, including technical considerations, spatial constraints, cost efficiency, and traffic safety. This design is carefully calculated to ensure optimal functionality.