I reversed a race tshirt and applied large iron on letters to the back (obtained at a craft store) stating: RACE MSRG-THX.
I have found cars, bikers and walkers to be less threatening that appoach from behind or a sudden understanding of those that I pass. I still don't have a solution for head-on. A large enough sign to be read in time from the front would create excessive drag. The best that I can come up with is the wearing of a noticeable vest. Of course stopping until the conflict is gone before again proceeding, defuses much of the problem.
I have found screaming at someone to get the h--l out of my way along with the mention of the race sponsor or charity to be anti-productive and a threat to my health and welfare.