A multiplex or concurrency on a road network occurs where a single physical road carries two or more different highway, motorway or other road numbers.
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.
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).
The second fundamental flaw in the concurrency principle is that, in a free country, a community's future growth should not be - and probably cannot be - determined by the adequacy of its roadways at any given moment.
The overall ineffectiveness of the roadconcurrency policy is proven by the fact that Florida's total population has continued to rise sharply - up 23.5 percent in the 1990s alone - in spite of obvious shortfalls in the state's roads and other infrastructures.
Another major problem with concurrency is that key public bodies - including the state legislature - do not want to raise taxes; therefore, they may refuse to fund the road construction that has been planned to offset new developments.
Concurrency means that transportation facilities must be available to carry the traffic of a proposed development at County level of service standards, or else the proposed development cannot be approved.
Concurrency is determined by adding the traffic from the proposed development to the traffic already on the roads and then calculating the projected level of service.
Concurrency cannot be transferred to other parcels (because they may be in different concurrency zones or different land uses).
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