I was guilty of that within the last year. I thought I had gotten in behind a bunch of slow moving morons and passed a couple of them before I saw the tiny little funeral procession signs in their cars. They were p!ssed at me, understandably, but I didn't realize they were in a funeral procession, either.
From my experience, there needs to be a better way of identifying them.
Locally, processions have the drivers turn on their headlights AND the emergency flashers.
But so what? When a 20 car procession is travelling down a busy city street or roadway... it just don’t work.
IIRC from my youth- the last car in the line had a magnetic sign attached to the left rear side that clearly stated this group was a funeral procession.