Apologies in advance for the slightly blog-like nature of this question.
Some of the comments in relation to this question here:
... made astute observations about the difference between words with intervocalic /t/ in Standard British and American Englishes (RP and General American).
It is well know that voiceless consonants cause pre-fortis clipping (a term coined by John Wells 1982) on preceding vowels. So the vowels in beat and bead are significantly different in length. Because /t/ is a fortis (unvoiced) consonant, it causes the vowel in beat to be clipped or shortened in the word beat. The word bead on the other hand ends in lenis (voiced) /d/, so there is no shortening of the vowel in this context.
Now if the /d/ at the end of bead is devoiced for some reason, we will still be able to tell these words apart. The lack of shortening of the vowel will tell our language brains that this consonant is the lenis /d/, and not the fortis /t/. This is very good news, because in actual fact, consonants which are normally voiced such as /b, d, g, v, ð, z, ʒ/ and /dʒ/ will become partially or fully devoiced when not surrounded on both sides by voiced sounds, so for example at the end of a word said in isolation. So in actual fact, the preceding vowel, not the consonant itself provides our brains with the crucial information to identify the consonant at the end of beat or bead as /d/ or /t/.
The Peter Shor point:
So the comment from Peter Shor, which precipitated this question was as follows:
The vowel length certainly plays a large part in decoding the phoneme. But I would think the voiced/unvoiced quality must also play some role in our decoding the phoneme. Otherwise, Americans wouldn't have any more trouble that Brits distinguishing bidder and bitter (and we do).
So, Peter's point here is that in Southern Standard British English bitter and bidder have distinctly different consonants. The first is fortis (unvoiced) and the second is lenis (voiced). And, unsurprisingly British English speakers can clearly distinguish between words like bitter and bidder.
However, in most examples of General American, the /t/ in bitter is realised by a voiced alveolar tap. This variety of English has an allophone (alternative pronunciation) of /t/ when it occurs between two vowels. So /t/ is normally realised by an unvoiced sound, but when it occurs between a vowel and another unstressed vowel, it becomes a voiced tap. Now the thing is that /d/ in American English also becomes a voiced tap in the same environment.
Now, in many, if not most instantiations of General American, the words bitter and bidder are homophones when they have a voiced tap (unlike the words bit and bid). In other words if listeners hear these words in isolation, they will not be able to tell them apart. So speakers of American English occasionally have some difficulty distinguishing these words.
Peter's point, then, is that if listeners weren't able to hear whether an actual /t/ or /d/ were voiced or not, then it seems at first blush that speakers of British and American English would have the same problem with pairs like bitter & bidder and writer & rider. But they don't.
Now just to make things more complicated here, most speakers of standard Canadian English also have a voiced tap for intervocalic /t/ and /d/ in these words too. But in Canadian English these words are not homophones. In other words speakers of Canadian English can easily tell these words apart even if said in isolation!
So the question is:
What is it that causes writer and rider to be distinguishable for speakers of standard British and Canadian Englishes?
Why are writer and rider indistinguishable in General American?
Of course, there are many varieties of American, British and Canadian English. Within these there is quite a lot of variation. I tried to put lots of careful hedges in the question (most, often and so forth). Of course many of the features described in American English may apply to speakers of Canadian and sometimes British Englishes and vice-versa. What I'm most interested in is what features help speakers distinguish these words and how this works in each case.