In two speech production experiments, we investigated the link between phonetic variation and the scope of advance planning at the word form encoding stage. We examined cases where a word has, in addition to the pronunciation of the word in isolation, a context-specific pronunciation variant that appears only when the following word includes specific sounds. To the extent that the speaker uses the variant specific to the following context, we can infer that the phonological content of the upcoming word is included in the current planning scope. We hypothesize that the time alignment between selection of the phonetic variant in the currently-being-encoded word and retrieval of segmental details of the upcoming word is variable from moment to moment depending on current task demands and the dynamics of lexical access for each word involved. The results showed that the use of a context-sensitive phonetic variant of /t/ (“flapping”) by English speakers reliably increased under conditions which favor advance planning. Our hypothesis was supported by evidence compatible with its three key predictions: an increase in flapping in phrases with a higher frequency following word, more flapping in a procedure with a response delay relative to a speeded response, and an attenuation of the following word frequency effect with delayed responses. This reveals that within speakers, the degree of advance planning varies continuously from moment to moment, reflecting (in part) the accessibility of form properties of individual words in the utterance.