Is the United States an optimal currency area (OCA)? Is Canada? Should we have a monetary union? A fun new paper argues that the answer is "no," "no," and "sort of." Specifically that you could combine and then remix the United States and Canada into three separate cross-border monetary unions, each of which would more closely approximate an OCA than the current United States or Canada. The resuling currency groups can be seen on the color-coded map above.
Now obviously in a practical sense it would be annoying for Delaware to be using a different currency from Maryland, Pennsylvania, and New Jersey or for Maine to be on a different currency than any of the states or provinces it borders. One also might argue that the use of state and provincial level analysis here is misleading in some ways. You might want to base a map on Metropolitan Statistical Areas instead.
But it's an interesting exercise and also a reminder than the North/South international border in North America is a bit odd. With most of Canada's population centers very close to the US border, the north-south commercial and cultural linkages within a given region (Vancouver with Seattle and Portland, Ontario with the Great Lakes region of the US, etc.) are often greater than the east-west linkages within the same country. The linguistic divide between the Anglophone majority and the Francophone minority is in many ways starker than anything separating Minnesota from Manitoba.