This sounds a little like using turn signals (indicators) on a car. The law might mandate it, and it's certainly not a bad thing to do, but driving such that you are commited to relying upon others doing so correctly is fraught with danger.

The problem with this type of 'rule', is it requires that everyone voluntarily comply with it, before it will ever be safe to benefit from it.

To extend the analogy a little further, it also suffers in the same way as turn signals in that it is nearly impossible to automate, because it requires you know where you are going before you get to the decision point; but we all have to visit strange cities, and we are all subject to late breaking clues and changes of direction.

The problem with turn signals, is that just as soon as you get comfortable with relying upon them, you encounter the driver that sets his signal left and then turns right.