As my First Aid/CPR instructor once said, "Just because you see folks on cell phones, it doesn't mean they're calling 911. More likely, they're calling friends saying, "You're not going to believe what I'm seeing!!"."
Very true. But if I found myself in a situation where somebody had just dove in front of my car, obviously deliberately, and there were people with cell phones in sight, and my assessment of the situation (type of neighborhood, time of night, etc.) was that I was more likely to get mugged if I stopped and got out of the car, than to have any positive effect on the ultimate outcome for the diver, I’d realistically probably keep driving, and call police from my cell as soon as I got to a location where I felt safe stopping.