What is the plural of 'bus'? I have always thought it was 'buses'; but then someone sent me something that included the word 'busses', so I thought I'd better look it up. And my New Webster's Dictionary allows both.
However, I still felt that 'buses' is more common, so I checked in the Corpus of Contemporary American English. There are 5354 tokens of 'buses' and just 119 tokens of 'busses', which confirms that 'buses' is the usual plural, but 'busses' does sometimes occur.
The next question is: why would 'busses' occur? After all, it is not usual for 's' to be doubled in creating a plural.
The explanation for this is that 'bus' is rather an unusual word in English, as words that end in /s/ after a vowel are generally spelled with a double 'ss': 'miss', 'hiss', 'fuss', 'mess', 'toss', 'pass', 'dress', 'press', 'cross', 'grass', 'gloss', 'glass', 'class', 'floss', 'bliss', 'stress', 'address', 'abyss', 'across' etc. The only words that I can think of that have a final /s/ and are spelled with a single 's' are: 'this', 'thus', 'us', 'pus' and 'cos' (a kind of lettuce).
In contrast, most words that are spelled with a final 's' after a vowel are actually pronounced with /z/: 'is', 'was', 'has', 'does', 'as', 'his', 'hers', etc.
Furthermore, in the middle of a word, 's' between two vowels is often pronounced as /z/: 'these', 'those', 'phase', 'please', 'raise', 'rise', 'hose', 'lose', 'nose', 'fuse', 'muse', 'rouse', etc, though there are quite a few exceptions is which the 's' is pronounced as /s/: 'case', 'mouse', 'dose', etc. In fact, there are some words in which the medial 's' may be pronounced as /z/ if the word is a verb but as /s/ if it is a noun or adjective: 'use', 'house', 'close'.
In conclusion, it seems that 'busses' is actually not a bad way to spell the word, even if it is not very common, as it clearly indicates that the medial consonant is pronounced as /s/ not /z/.