The first view of the highway system is of a network of lines. To get from the airport to the hotel in the center of the city, a tourist map may show the principle roads into the city. That may be adequate, however, only to get the driver to the neighborhood. Getting to the actual entrance of the hotel may require a more intricate series of twists and turns.

The driver is not concerned in this case (except in the most general terms), with the condition of the road, its construction, or any of the other characteristics that may be of interest to, say, a highway construction crew.

To the driver, ultimately the road is simply a network of links with intersections connecting them together. “Take Highway 50 to 6th Street and turn right. Then ...

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