Horizontal alignment in mountainous country is conditioned by drainage structures.

Explanation:
Horizontal alignment refers to the layout or arrangement of a road or highway in a horizontal plane. In a mountainous country, the topography and terrain pose unique challenges for road design and alignment. The choice of horizontal alignment is influenced by several factors, including the need to ensure the safety and efficiency of the road.

1. Drainage Structures:
One of the most important factors conditioning horizontal alignment in mountainous countries is the presence of drainage structures. Mountainous regions often experience heavy rainfall and the presence of natural water bodies such as rivers and streams. Therefore, proper drainage is crucial to prevent water from accumulating on the road surface, causing erosion, and compromising the stability and safety of the road. The alignment of the road needs to consider the natural drainage patterns and incorporate adequate structures such as culverts, bridges, and ditches to facilitate the flow of water.

Drainage structures ensure that water is efficiently channeled away from the road, preventing waterlogging and reducing the risk of landslides. The alignment of the road needs to take into account the location and elevation of these drainage structures to ensure their effectiveness. Proper alignment and integration of drainage structures contribute to the overall safety and durability of the road in mountainous regions.

In summary, the presence and proper alignment of drainage structures are critical factors that condition the horizontal alignment of roads in mountainous countries. By considering the natural drainage patterns and incorporating effective drainage structures, engineers can design roads that are safe, resilient, and sustainable in these challenging terrains.