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Because the New England states are geographically small, but highly populated, there are many automobile trips that cross state borders. Interstate coordination of route numbers remains a good idea, and below are proposed signs. Only primary routes should get these signs; minor routes can keep their traditional state route markings.
Each sign is a hexagon, unlike any other standard highway sign. The pastel-colored "Y" (mnenomic "Yankee") in the background gives the effect of a cube, and makes the shape easier to recognize and remember. The "Y" might be different colors on different signs, perhaps as an indicator of what state the driver is currently in.
This plan does not call for building any new roads; rather, some highways that already exist would receive new signs. In many cases, state route numbers already cohere when crossing state lines, for example route 25 of Maine, New Hampshire and Vermont. When this happens, it makes sense to erect Y-signs displaying the current number, replacing the state route signs. By contrast, Connecticut route 25, which does not connect to the tri-state route 25, would continue to be signed as an ordinary state highway.
It is also possible to define new routes. To minimize confusion, the number selected for any new Y-route should differ from the numbers of all routes that already exist in New England. Still, Y-route numbers can be derived from other route numbers, as a Y-593 that branches off interstate 93. Most new Y-routes will have numbers above 200 because nearly all lower numbers have already been used.
There is nothing wrong with assigning a Y-number to a major intrastate route, particularly if it connects with several other Y-routes. (An analogy here is "interstate" 4, which lies entirely in Florida.)
The blank signs, which would not actually be installed, look like this:
Here are some signs with numbers, among them three different colors for Y-37.
The positions of white and pastel can be reversed:
If a rectangular sign is preferred, the corners can be filled in with paint, either black or pastel:
Optional is a state designation, much as (federal) interstate highway signs often carry the state name. These examples would be used for route Y-85, but only where it passes through New Hampshire.
Finally, this is how the sign can be rendered on a map: