Quote Originally Posted by Meddle View Post
Early radio systems had a problem with being able to handle multiple conversations. 'Simplex' could only handle one transmitter. If another keyed up, the first could be interrupted. That could be a problem if Radio needed to put out a run. DFD central had similar issues and you can hear it in some of the recordings of major fires like the Briggs factory. Even later on once full duplex came into use, too many units trying to talk made a mess. During the Usher Oil fire [[late 70s sometime), Central can be heard repeatedly telling everyone to stay off the air.

I don't know if that was the basis for the DPD policy or not, but it makes sense to keep non essential traffic to am minimum.
Still called in and out of service in the '70's