Science Fair Project Encyclopedia
The practice is relatively common; where two roads must pass through a single geological feature, or crowded city streets, it is advantageous for road designers to merely divert them down the same road, saving on the cost of a new road.
In the United States, highways often form multiplexes in rural areas. As highways in the US are signed with a cardinal direction, it is possible for two highways signed with opposite directions to be running along the same piece of road; a wrong-way multiplex. Most of the time, multiplexes are simply marked by placing signs for both routes on the same post; occasionally a state will instead sign the road as TO the less major route. Several states don't officially have any multiplexes, instead officially ending routes on each side of one. In these states, multiplexes are typically poorly signed.
In the United Kingdom, it is common for major through routes to multiplex with others. Only one road number (typically that of the more heavily used route) is ever shown on road signs however; the other road is either bracketed on the sign, implying that the major route leads to a junction with the minor route (which it will do at the end of the multiplex), or left off altogether. For example, the A82 multiplexes with the A85 for 5 miles in western Scotland. Each route confirmation sign header denotes the road number as A82 (A85).
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