Saw this on FB and hadn't know.
Universal sign (it might be American 'universal' but it a good idea to have something universal, so let's go with) that a biker needs help is a helmet placed behind the bike, so if you see one, or you yourself need help - you know what to do.
https://www.bikermatch.co.uk/cgi-bin/thumbnail.aspx?width=1200&height=1200&source=/sites/bm/profiles/uploads/55314_bm_1_bm_9283392833.jpg&watermark=true&type=JPG
Not displaying right - why can't I ever do this!! GRRRR