The line should change in color or intensity as it gets further from you, so others can judge how soon your body will actually arrive at any point on the line.

When you see your line cross someone elses line there should be a way to resolve the 'conflict'. Maybe some sort of feedback to the device that if your line crosses an existing line then your line will show up as a dotted line (indicating second priority).

I always figured this would be best controled with thoughts rather than a manual interface. (I think we are getting to the point where technology can do stuff like that.)

I realize that it is not very useful in heavy crowds since people will not be see the ground, nor would it be useful when there are not too many people around. Therefore it should also be activated and deactivate by thought.

[I was reminded of this idea when I read the Sidewalk Encounter Protocol]