I was giving first-aid to a guy with severe lacerations to his hand/arm and didn't want to leave him (partially panic on my behalf) - in a UK city, albeit a side-road, with shops and people walking past - but needed to call for an ambulance.
Nobody would stop and help:
"Help! I need you to call an ambulance."
and people just carried on walking.
In the end I was able to makeshift a bandage with some clothing and leave the guy sitting on the floor long enough to use the phone in the shop I was working at.
The ambulance, despite having the address and business name, drove past and I ended up running after it down the street ... overall an awful experience and very eye-opening.
My takeaway is that I'd give up much sooner seeking help from others, depressing as that is, and - contrary to my first-aid training - leave the patient and call an ambulance myself.
Aside: I wonder if there's a villager/townie effect too?