Hi,

In terms of the warning, you can log it as a normal 'Hazard' class with no selection, and just a short description of 'Traffic lights out' or something similar. The 'On Shoulder' warning has a 'Missing Sign' option, but that does not really apply in this case.
As far as traffic is concerned, Waze should automatically detect the holdup if there is one there, and reroute accordingly if it is unusually slower than normal.

If you log it from the road, Waze will only warn users travelling in the same direction as you, so not everybody will get a verbal cue or warning. The pin should be visible though.