Definition - What does Highway Drain mean?
Highway drains are part of a roadway drainage system. The highway drain isn't the drainage pit covered by a metal grate located along the road edge to route into the sewer system; that part of a highway drainage system is called a highway gully. Instead, the highway drain is the pipe that connects a highway gully to the nearest sewer line which carries the runoff and other sewage to the sewage treatment plant.
Trenchlesspedia explains Highway Drain
Highway drains are intended to keep the road surface as dry as possible. That isn't their only function though. By removing runoff and water from the highway, highway drains also help prevent flooding and keep the highway surface dry, providing an optimum service for vehicle drivers to operate on. They ensure water from the roadway enters the controlled environment of the surface sewer as soon as possible. They also ensure the longevity of the roads by keeping the road underlayment as dry as possible.