Source: Carmanah Traffic Article

"On December 21, 2017, the Federal Highway Administration (FHWA) terminated the Interim Approval for Optional Use of Rectangular Rapid Flashing Beacons (IA-11). The FHWA’s position, as stated in their termination announcement, is that the RRFB is subject to current and pending patents and, as such, may not be included in the MUTCD or any interim approval. If you have not yet read the notice, it can be viewed on the following link: