What are the primary functions of highway drainage systems?

The primary functions of highway drainage systems are centered around managing water effectively to protect the roadway infrastructure. The correct answer focuses on the essential role of removing surface water, which helps maintain safe driving conditions by minimizing hydroplaning risks and other hazards associated with water accumulation. Proper drainage systems also prevent soil erosion, which can undermine road stability, and pavement damage caused by water infiltration. When roads are designed with efficient drainage, it not only extends the life of the pavement but also reduces maintenance costs over time.

The other options do not align with the core purpose of drainage systems. While recreational areas and aesthetic enhancements contribute to the overall environment of highway systems, they do not directly relate to water management. Controlling traffic flow during heavy rain, while important, is not a function of the drainage system itself; it's more about traffic management strategies.