I've never had motorist trouble (I've ridden a motorcycle for most of my life, afraid to get in anything with more than 2 wheels now) but I did once have a date, in Tokyo ... as I was entering a subway complex late at night in Shinjuku (an area crawling with yakuza, by the way) I saw someone drunk, collapsed, choking on his vomit and obviously dying. Since it was a bright area near the train terminal I
(1) Sent my anti-violence then-gf to get the cops (mistake # 1)
(2) Tried to help the guy up and clear his airways (mistake # 2)
Roused from his slumber, he punched me while I was trying to help him up (total surprise) and we got into it right then and there (very stupid) ... anyways gf brings over not the cops but one aged department store guard (the subway entrance connects to the mall complex) ... rentacop is fat, pretty uniform, no baton, radio, backup, nothing. I'm trying to explain to this very befuddled security guard what was happening while this other guy (6'1", 300lbs ... very fat. I'm 5'9" 160lbs for comparison) is STILL trying to beat me up, swinging windmills while he's covered in vomit ... yuk. Anyways I push him down, he hits his head against the marble wall (ouch) knocks himself out, and I make a quick exit with my shaken (and very useless date.)
Anyways after that I swore never to be a Samaritan again ... cha rite. Still am. But I say
(1) Stop at a distance, preferably out of their sight but with them in yours.
(2) Call the cops. Alert them to the difficulty, location, give them a through sitrep. You can always call AAA later.
(3) Determine from appearance as best you can if they're to be helped (senior citizens as pax and M1911 pointed out very rightly deserve help.)
With my luck, I'll probably get another puking nutcase trying to clock me out ... hope no one else here gets what I did.