As spoken dialogue systems become deployed in increasingly complex domains, they face rising demands on the naturalness of interaction. We focus on system responsiveness, aiming to mimic human-like dialogue flow control by predicting speaker changes as observed in real human-human conversations. We derive an instantaneous vector representation of pitch variation and show that it is amenable to standard acoustic modeling techniques. Using a small amount of automatically labeled data, we train models which significantly outperform current state-of-the-art pause-only systems, and replicate to within 1% absolute the performance of our previously published hand-crafted baseline. The new system additionally offers scope for run-time control over the precision or recall of locations at which to speak.