Ever need a warning to tell you when one of your belts is backed up?
The basic idea:-
1. Place a  'Traffic Monitor' on the main belt and set it to show an alarm onscreen, when it senses 'Cargo Pass' (or items on belt).
2. Divert all traffic around the monitor itself, then place back onto main belt.
3. If the main belt backs up, items are then forced onto the monitor section of belt, and the alarm is activated.

Works best with fully upgraded Pile Sorters, but you can still use multiple lower tier sorters to do the same job.