The first instance of this phenomenon that I remember happened in the mid-80's during WFIL Philadelphia's "The Boss is Back" period when the rebooted Famous 56 would do "Expressway Checks" in weekday afternoon dayparts (not sure about nights or weekends), with a promo saying, "because you don't need a rush hour to have a traffic problem" or something similar.

When and where did radio start using regular non-rush hour traffic reports. I don't remember KYW for example starting this until the early '90s.

Or has this been discussed before on RD?