Highway 17 is the coastal road featured in Half-Life 2, filled with tunnels and small coastal towns. It leads to Nova Prospekt.
The few man-made structures along the road are tunnels, bridges, houses, and towards the end Combine bunkers and forts. Tunnels should be arched at the top, straight along the sides, with a concrete texture along their inside. An env_lightglow at each end, facing inward, provides the sun glare.
Bridges should also be arched, along their bottom. There are pre-made props intended to be used as the underside of bridges, so these may be the wisest choice. Bridges should have fancy railings; as such, consider an npc clip brush along them to keep enemies from getting stuck.
Houses should be simple. At most, 4 rooms for a single-floor house is advisable. They should be wooden and plaster, with enough props so they appear usable. Some damage, such as missing roof or floor sections, is acceptable. Fireplaces are very common.
Bunkers should be fairly simple, using concrete materials and Combine props. They should be along coastal sections, but not the road itself. They should be kept in areas seeming near Nova Prospekt. Only Combine fences should be on the road itself, and should always be passable on foot. Alternatively, the player should have means to disable them.
The highway is based along the coastline, so the majority of the natural textures should be sands, grasses, and rocks. The "cleaner" water materials work best here. Vertical natural faces should be rocky, while flat surfaces close to the water take the sand. Grass should be kept away from the actual coastline. Because of how antlions spawn in Half-Life 2, the player may have learned to be wary of sand. Use this to your advantage. Since the Combine's arrival sea levels have fallen, so try to design your landforms and older buildings for an environment with a much higher tide.
Thumpers are often used within sandy areas, so consider placing these within large sand areas. They also create an ambiance in a level that suggests the presence of antlions. The ground shaking created by thumpers scares away and confuses antlions, who refuse to move within 1000 units of it. This makes thumpers a useful tool for creating rest points for players to reach. However, remember to have the thumper present for a reason, as the Combine would. Place thumpers near buildings and strategically or otherwise important locations.
A Highway 17 level can be either day or night, depending on the mapper's intentions. The lighting should match the time of day. There are no streetlights along the highway, however, so consider that when choosing the time of day.
Power lines are a reoccurring motif along the coast, either huge metal pylons carrying many wires or small wooden ones with only a few.
Terrain should be entirely displacements, so be wise about visibility. The coast curves considerably, so curved tunnels and roads can be expected. Rocky coastlines are acceptable as opposed to soft, sandy coasts.
Remember, the water is infested with leeches. Falling in can be an instant kill, but along lower sandy coasts should only damage the player slightly. If the player is provided the buggy, a situation can be set up where the player must jump a section of water, adding to the excitement of a level.
The Combine is a common enemy here, as they have taken many of the houses along the coast. They also have placed rollermines along the road at some points. A helicopter or gunship would be a fitting "boss" enemy. However, larger enemies such as striders would not fit in the atmosphere. Allies can be included if the mapper wishes for larger battles. And, of course, there are the antlions, which should be set to spawn in sandy, coastal portions.