License plate legibility

Every time I traveled to California, I would always look at the license plates and think to myself: "Yikes, this typeface is so ugly!"

Of course, it would be unfair just to say that something is ugly, so I tried to figure out what it was that I didn't like. Letter Q was definitely something that bothered me, but otherwise it was hard to pinpoint any particular flaw.

I would think about this on and off until one day realization hit me while I was waiting for my bus in Bellevue, WA. The bus was late, and I was bored, so I started reading people's license plates.

My vision is pretty good, but I had to wait for each car to get pretty close before I could decipher the letters and numbers on its license plate.
The reason for that is that a lot of the characters are very similar, and therefore hard to distinguish, especially when they are moving.
Thinking back to the typeface used on California license plates, I realized that the thing that bothered me most was that the glyphs that I expected to look alike were different.

While that may not make for the prettiest typeface, it is definitely very legible. Below are some examples of letter combinations (I used the states' respective patterns for the sake of keeping these realistic).
I used Penitentiary Gothic for the California license plate, and License Plate for the Washington one, both of which are replicas of the actual typefaces used on the license plates. You may notice that the weights don't quite match up: that is purely an artifact of using replica typefaces instead of photographing real license plates.

On the left is the California plate; on the right is the Washington one.
If you squint, you will notice how the first four characters on the right one become identical. In fact, the letter O and zero are already the same, making it impossible to tell which one is which, especially on a custom plate where there is no inherent character ordering.
In the California version, you still cannot tell which one is the O and which one is the zero, but they do look like two different characters. The zero is rounded, while the O has a squarish shape.

Now let's take a look at my favorite Q. While the version on the left is not pretty, it definitely cannot be confused with an O or a D. Unfortunately, the same cannot be said for the Q on the right.
Take a look at the D. The one on the left has slab serifs, which makes it look distinct, and prevents it from blending in with O and Q.
The subject of serifs is actually quite interesting in this context. There are only three letters in Penitentiary Gothic that have serifs: D, B and I. While this would not be a great choice in a regular typeface, in this case it does help a lot with legibility.

This image is slightly different from the one above since I also wanted to show the difference between B and eight, and between different digits.
As before, the image on the left has much more distinction between B, 8, 6, 9 and 3, whereas on the right they look very similar.

After having considered legibility, I feel like I have been unnecessarily harsh with my judgement of those California license plates. After all, the typeface that is used on them has a very different goal than a typeface that one would use for setting a book or making a sign. When viewed in the proper context, it is actually quite good, and definitely much better than the Washington state one.