The closest I've come to a situation like that was when a couple of months ago I saw a middle-aged man lying on a street corner, with a younger man walking away from him. No one else was near by.
My first thought was that it might have been a mugging, then that he might have slipped, or had a heart attack, or maybe he was drunk or drugged.
So I took a good look at the younger man (in case I needed to give the police a description), then went over to help the man on the ground. I got close to him (but far enough away so that if he was crazy or tried to attack me I could get away), and asked if he was okay.
He said he was, but stayed sprawled on the ground.
He didn't look injured.
Or drunk.
Or drugged.
And he didn't look or sound crazy.
I asked him a few more times if he was all right, or needed help, but he assured me he was fine. Any further questions resulted in "don't go there".
So I turned and walked away (there wasn't really anything else I could do), but then after about a hunder yards I stepped behind a parked car where I could watch without being seen.
Another pedestrian came by, and walked past without stopping or saying anything.
A moment later a car came round the corner, pulled in, and one of the passengers got out to check up on the man. Basically, it looked like they got the same response as I did, and left shortly after.
To this day I have no idea what it was all about.
I wondered if maybe it was some sort of experiment to see how many people are willing to help a stranger in distress.
(My mum, when I told her, thought he might have been an epileptic, had a fit, and knew he would be alright after a bit of a rest).
As for motorists, I've just never seen a breakdown were anyone looked like they were actually in "trouble" (other than being broken down). Most were on motorways where there are emergancy telephones at the roadside, or cameras for keeping an eye on traffic flow, so they'd get help sooner or later, or were already being helped. If I saw someone who actually looked like they needed help, or had just crashed, I'd probably help, unless it somehow "felt bad" (in which case I'd call the police on my mobile).
(Just remebered I did witness a crash once, but it was not serious. Some idiot went round a corner too fast and hit a car waiting to turn right [ie across the flow of traffic, 'cos we drive on the left], and then the one behind went into him. (Not much damage caused to either). Then the idiot behind him changed lanes to avoid the pile-up (it was a dual carriageway), without looking to see if there was anyone there already (there was - me), and I had to go off the road to avoid hitting him. There was a petrol station just along from there, so I pulled it, informed the attendant, and then called the police to see if they wanted a report of what happened.