In Saratoga Springs, New York, an officer pulled up beside me in T-intersection, as I was heading left and him right, and asked me how many times I had been pulled over for those plates, referring to my Danish number plates. At the time, I had not been pulled over once. But he insisted that I would get pulled over at some point on my trip.
I believed him at the time, as this had been my own suspicion as well. The foreign plates might call some attention, and a police officer unfamiliar with them or what to do in such a case (which may be most US police officers), might be inclined to pull me over.
But as I went through state after state, province after province, crossed the US/Canadian border four times, I had managed to reach southern California without getting pulled over. I began thinking it might not happen at all. A Canadian told me that there is a good chance it will not, because of the paper work required.
It is not that uncommon to see cars over here with European plates on the front. Indeed, a lot of people at the Citroën events I attended had left their cars' original European plates at the front of the vehicle, or had some made that matched the number of their locally issued plates.
In most US states and Canadian provinces, front facing plates are not required. And even some jurisdictions that issue the front plates, do not actually require them to be attached to the vehicle. However, the rear plate must be a proper plate. And that is how you can tell the difference between an authentic European registered car, and one simply 'styling' their vehicle.
And so, along the Californian 78 near Poway in Southern California, a police officer pulled me over. He wanted to know what was going on, and when I told my story and showed him all my papers, he sent me off. He admitted, however, that he had no idea what to do, as it was the first time he had encountered European plates.
In particular, it was my international driving permit that got me off the hook. So that was 25 DKK (~4 USD) well spent.