(道路)在双向的道路口有一条单行道的；当车辆相遇时一方必须离开道路让另一方通过的。
(of roads) having a single lane for traffic in both directions; when vehicles meet one must pull off the road to let the other pass.
完整的道路和高速公路模型：单行道，双行道，交叉路口，长短弯道。
Complete road and highway sets: one lane, two lanes, intersections, long and short turns.
请注意，如果并非所有的道路都是单行道，那么我们可以在图中为每一条双行道画两条有向边，每条边指明一个方向。
Note that if not all of the streets are one-way, then we can obtain a digraph by drawing for each two-way road two directed edges, one in each direction.
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