It works really well when it's the person right in front of you, because you can see that they aren't paying attention. Put a few cars between you and them, and

It works really well when it's the person right in front of you, because you can see that they aren't paying attention. Put a few cars between you and them, and all of a sudden you don't really know the reason they aren't moving and there is a significant delay before you can infer that they aren't paying attention. It would be so much better for their own car to recognize that they are in la la land and remind them asap.