http://www.stampcollectioninfo.com/american-rapid/
Why are colors used to identify rapid transit routes?
For example, in many American cities that have rapid transit systems, each route is identified by a color (Red Line, Green Line etc.). I actually like it that way. Nevertheless, I always wondered why exactly are colors used here.
In many cases it depends on the complexity of the system. For example: Washington, DC only has 5 lines and uses a different color for each of them. Now imagine if New York City used a similar system for their 26 lines, they'd have to ask Crayola for suggestions. Colored lines are also easier to remember and harder to confuse with bus lines for example.
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