A wide range of reduction phenomena have been described in the literature as predictability effects, in which more predictable units (i.e. words, syllables, vowels) are reduced in duration or other acoustic dimensions relative to less predictable units. The goal of the current study was to critically evaluate these predictability effects on vowel duration in read speech to explore the extent to which they reflect a single underlying phenomenon. The results revealed shorter vowel duration for words with high phonotactic probability, for high-frequency words (in clear speech only), and for words in plain lab speech relative to clear speech. However, the results also revealed qualitatively different effects of three measures of contextual probability (cloze probability, written trigram probability, and spoken trigram probability). Greater spoken trigram probability predicted longer vowel duration, contrary to expectations, and this effect was limited to high-frequency words in first mentions and in plain speech. Cloze probability and written trigram probability exhibited even more complex interactions with other predictability measures. These results provide evidence for fundamental differences in these measures of predictability, suggesting that a more nuanced perspective on predictability effects and the mechanisms underlying them is necessary to account for the complexity of the empirical data.