On a CX flight, the pilots announced that the seat belt sign would be turned on __ minutes in the future for landing, so that people who want to use the lavatory should do so now.
You beat me to it! As others have mentioned, there is a dramatic difference between the use of the Fasten Seat Belt sign on US vs foreign carriers. For most foreign carriers, they turn it off even before cruising altitude is reached and leave it off until just before landing, unless there's a really good reason to turn it on. I realize that the legal environment in the US is such that perhaps more caution is needed for liability reasons, but even so, the situation can be improved upon.
By giving the passengers a 20-minute warning before the Fasten Seat Belt light will be turned on before landing, it gives passengers who need to use the lav a chance to so. As it stands now with most domestic US flight, the light comes on and descent starts without notice, at which point people that need to use the lav ignore it and actually use it as their cue to do so. If pilots made the use of the light more reasonable with the pre-descent announcement, it would be more likely that passengers would be able to comply.