The reason that J was left out is clearly answered.
The reason U is left is not. My guess is simply that D.C. (and Detroit) planner Pierre L’Enfant was long gone before they got around to dedicating that street and less classicly minded heads prevailed.
I think you misread the column. There IS a U St. in Washington, D.C. “U St. SE” and “U St. NW” both exist. “V St. NW” exists for sure; didn’t look to see if the SE version is there. There are, however, no J St.s in any of the quadrants.
Cecil was mentioning U/V to comment upon the theory that J was dropped because of still occasional interchageable use of the two letters I and J. By mentioning that the plans called for use of U and V, it casts potential doubt upon this theory regarding J Street, because presumably the same rationale would have caused L’Enfant to avoid use of V or U. At least, that was my reading of what admittedly wasn’t the clearest writing…
But the point is that the original plans SHOW a U street. So, presumably L’Enfant WAS aware it would have a U street, and did not object. N’est-ce pas?
As a matter of practical fact, the use of both U and V in Latin is more popular in the modern era than is the use of J, so it is entirely reasonable that L’Enfant might have accepted the one but not the other.
The problem here is whether the plan mentioned was indeed original.
Recall Detroit. Many people say it followed his plan (Detroit was to be rebuilt after a major fire, so they called in a famous planner). Yet when you look up his actual work, it calls for not just a single circle with radiating streets, but several circles. He much admired the way certain important buildings and monuments are positioned at the end of streets, so that they can be admired head on from far away, instead of viewed only sideways in passing. Yet those extra circles disappeared as more practical considerations, such as easy traffic flow for hard to steer wagons dictated against more circles.
I’m guessing (i.e., won’t try to look up) that something like that happened in D.C.
The reason for having a U but not a J is probably because the U/V distinction was established earlier in modern languages than the I/J distinction. As Cecil pointed out, some dictionaries were still combining the latter pair as late as the early 19th century. I don’t think any dictionaries of that era did the same for U/V.
Note that this has nothing to do with how the letters were treated in classical writing or classical languages but rather in modern writing.
You also have to distinguish between actual ancient Roman use and modern Latin – which is what L’Enfant would have known. Well after his time there was still a general prejudice among the educated that Latin and Geek were somehow “real languages” in a way that modern languages were not.
Just wanted to add, back in mid-'80s (and maybe still today) the Washington Post Sunday Magazine included an around-town feature of short articles called “J Street,” after the nonexistent D.C. street.