  Turn-taking is a fundamental aspect of human communication where speakers
convey their intention to either hold, or yield, their turn through prosodic
cues. Using the recently proposed Voice Activity Projection model, we propose
an automatic evaluation approach to measure these aspects for conversational
speech synthesis. We investigate the ability of three commercial, and two
open-source, Text-To-Speech (TTS) systems ability to generate turn-taking cues
over simulated turns. By varying the stimuli, or controlling the prosody, we
analyze the models performances. We show that while commercial TTS largely
provide appropriate cues, they often produce ambiguous signals, and that
further improvements are possible. TTS, trained on read or spontaneous speech,
produce strong turn-hold but weak turn-yield cues. We argue that this approach,
that focus on functional aspects of interaction, provides a useful addition to
other important speech metrics, such as intelligibility and naturalness.
