Evidence has been mixed on whether speakers spontaneously and reliably produce prosodic cues that resolve syntactic ambiguities. And when speakers do produce such cues, it is unclear whether they do so "for" their addressees (the audience design hypothesis) or "for" themselves, as a by-product of planning and articulating utterances. Three experiments addressed these issues. In Experiments 1 and 3, speakers followed pictorial guides to spontaneously instruct addressees to move objects. Critical instructions (e.g., "Put the dog in the basket on the star") were syntactically ambiguous, and the referential situation supported either one or both interpretations. Speakers reliably produced disambiguating cues to syntactic ambiguity whether the situation was ambiguous or not. However, Experiment 2 suggested that most speakers were not yet aware of whether the situation was ambiguous by the time they began to speak, and so adapting to addressees' particular needs may not have been feasible in Experiment 1. Experiment 3 examined individual speakers' awareness of situational ambiguity and the extent to which they signaled structure, with or without addressees present. Speakers tended to produce prosodic cues to syntactic boundaries regardless of their addressees' needs in particular situations. Such cues did prove helpful to addressees, who correctly interpreted speakers' instructions virtually all the time. In fact, even when speakers produced syntactically ambiguous utterances in situations that supported both interpretations, eye-tracking data showed that 40% of the time addressees did not even consider the non-intended objects. We discuss the standards needed for a convincing test of the audience design hypothesis.