An Imperial Highway is built above the level of the terrain, usually by less than a man's height but sometimes more, and reinforced with wooden palisades. There are ramps to allow carts to enter and leave a highway, and the roads will also have bridges and tunnels to allow people to cross from one side to the other without entering the highway.
The Imperial Highways are still held by ancient tradition to be land of the empire, outside the jurisdiction of any local power. Technically, a pursued man need only step foot upon a highway to be free of any local laws or crimes. Of course, in reality, this only tends to work if a force of the Dragon Army is patrolling the road at that precise moment. And once on the road, a traveler is subject to the whims of the nearest imperial commander, who occasionally takes the word of the locals if it will save him the time of a trial, or decides to teach some respect to a traveler who does not bow quickly enough.
Some brief investigation leads Xiao Fa to think that the Imperial Highway is also a very strong chi conduit. There's not a lot of chi flowing along it near the northern edge of the Hon'eth Arcade, however.