The MUTCD allows for either ALT or ALTERNATE to be used, but I went with ALT for readability and conciseness.
All the bannered ALT routes I've been on (the ones that weren't US XXA) were signed as US XX ALT, at least on the reassurance shields. Wyoming, however, does something weird and sticks the banner
inside the shield for some reason (I don't expect anyone to code this in, I saw it when I was checking to see if ALT or ALTERNATE was more frequent in my experience and I thought it was funny).