This article argues that constraints regulating the distribution of metrical prominence must be able to reference fine-grained durational information. Evidence comes from an apparent segmental effect on stress in American English - ative: stress on - at- is more likely when it is preceded by an obstruent or a cluster (as in irrigative, integrative) than when it is preceded by a vowel or a sonorant consonant (as in palliative, speculative; Nanni 1977). I propose that this pattern should be understood as an effect of phonetically evaluated *L APSE: longer lapses are penalized more severely than shorter ones. Results from two studies of speaker preferences for stress placement in nonce - ative forms support this proposal.