My ca. 1928 Rand McNally shows a highway numbered "99 1/2" in northwest Louisiana, and I haven't been able to learn anything more about it:
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fzoza.com%2F%7Ekniwt%2F99half.jpg&hash=c8278c018c4c916a0e28b8ee95ae0a70fa40cac7)
The 1927 edition available online (http://www.davidrumsey.com/luna/servlet/detail/RUMSEY~8~1~33762~1171478:Rand-McNally-junior-auto-road-map-L) appears to show nothing even close.
I'm curious about the history, reasoning, etc.
It looks like the legislature is to blame: http://www.google.com/search?tbs=bks%3A1&q=%22Route+991%2F2%22 As for reasoning, if you can find the detailed proceedings there may or may not be an answer there.