The following is a quote from a Wikipedia page on American English phonology and concerns flapping in American English:
The flapping of intervocalic /t/ and /d/ to alveolar tap [ɾ] before unstressed vowels (as in butter, party) and syllabic /l/ (bottle), as well as at the end of a word or morpheme before any vowel (what else, whatever). Thus, for most speakers, pairs such as ladder/latter, metal/medal, and coating/coding are pronounced the same. For many speakers, this merger is incomplete and does not occur after /aɪ/; these speakers tend to pronounce writer with [ʌɪ] and rider with [aɪ]. This is a form of Canadian raising but, unlike more extreme forms of that process, does not affect /aʊ/. In some areas and idiolects, a phonemic distinction between what elsewhere become homophones through this process is maintained by vowel lengthening in the vowel preceding the formerly voiced consonant, e.g., [ˈlæːɾɚ] for "ladder" as opposed to [ˈlæɾɚ] for "latter".
Q: Are there any studies on how many speakers have this vowel lengthening? Or can anyone confirm this based on anecdotal evidence?
It certainly makes sense that speakers might phonologise vowel lengthening before voiced plosives after the voicing distinction in alveolar plosives has been neutralised. But this is the first time I have read such a claim, so I would be glad if somebody can confirm or deny it.
Also see this previous question on a related topic, where question and answers did not specifically focus on the phonologisation of vowel length.