I heard that Taiwan actually had a bus route 0, but whether that
bus route is still around or not, I'm not too sure. It also seems
to be the only example where the number 0 was used alone for a bus
route number anywhere in the world.
I once read in the Newman magazine a small column where they asked
SBS Transit how they get numbers for their bus routes. It was
explained that bus routes starting from one area try to use one
specific number system (example: 5X bus routes mostly start
from/terminate at Bishan), and SBS Transit further said that they
can use any number from 1 to 499 (fancy a route 444? 
).
As for the number 0 itself, I don't think it will ever be used,
even 100, 200, 300 years into the future (God knows how the bus
system will have evolved to then 
), when more parts of Singapore are developed
and more bus routes added. At the most, they might use letters more
liberally (examples that can be seen today: CT28, C3, and NR1), or
they might resort to using four digits (imagine bus routes with
numbers like 8888 or 1748 or 9413 or even worse, 4444 ). I still remember that many, many years back (early
1990s, I estimate), I dreamt of an old NAC Nissan U31SCN (not the
ones SMRT are using now) whose desto beared some strange route
"code" consisting of numbers and letters - can't remember exactly
the sequence, but it was something like this: "N30SR517". This
might be another possible alternative should SBS Transit or SMRT
run out of route numbers to use.
