Wanted: A Taped Alarm Clock
I have observed a similar problem at academic workshops, where a speaker is supposed to have a limited amount of time to present his paper, with the remaining time for questions and discussions. It is a particularly serious problem at Chicago style workshops, where the theory is that everyone in the audience has read the paper so the speaker does not have to read it to them; his fifteen or twenty minutes are supposed to be spent talking about his paper. The more often speakers end up breaking the rules and using most of the time to present the paper the less likely those attending are to bother reading it in advance. The fewer read it in advance, the greater the incentive for the speaker to spend the time presenting it instead of discussing it.
I have a techno-social solution. It consists of an alarm clock that can have multiple settings, and will ring at each for a fixed length of time, say five seconds. At the start of the conference day, it is set to ring when each speaker is supposed to be finished. Once it has been set, the mechanism for changing the setting is ceremonially (but temporarily) disabled, perhaps by putting a strip of tape over the relevant button, and the alarm clock is put next to the podium. Now schedule drift can happen only by an explicit decision to change the rules, not by a gradual erosion.